Glossary¶

Key terms, acronyms, and concepts used throughout this site. Terms are linked inline wherever they first appear on a page. If you encounter an unexplained term that isn't listed here, please consider contributing.

A¶

Absorptivity (α)¶

The fraction of incident solar radiation a surface absorbs rather than reflects. Together with emissivity it determines a surface's equilibrium temperature in sunlight: the ratio α/ε is the single most useful number in passive thermal design. Low α/ε surfaces (white paint, silver FEP tape) run cold and make good radiators; high α/ε surfaces (bare or polished metal) run hot. Solar absorptivity generally increases over a mission as coatings are degraded by UV and atomic oxygen. See Thermal.

Acceptance testing¶

Environmental testing performed on each flight article to demonstrate that it is free of workmanship defects, conducted at the levels expected in service rather than above them – typically maximum expected random vibration spectrum with no added margin, for one minute per axis. Contrast qualification testing, which proves the design has margin, and protoflight testing, which combines both on a single article.

Access (imaging)¶

The target-side counterpart of a pass: an interval during which a point on the ground lies within the instrument's reach, given the swath and any permitted off-nadir angle. An access is only useful if the scene is lit, so imaging planners filter by the Sun's elevation at the target and count lit accesses rather than geometric ones. Accesses and passes are separate events on separate timelines: a mission that images and downlinks in the same orbit has to reconcile the two against power, attitude and time.

ADCS¶

Attitude Determination and Control System. The subsystem responsible for measuring the spacecraft's orientation in space and applying torques to change or maintain it. Sensors (magnetometers, sun sensors, star trackers, IMUs) feed into an estimation algorithm; actuators (reaction wheels, magnetorquers) execute the resulting commands. See GNC.

AFSK¶

Audio Frequency Shift Keying. A simple modulation scheme in which digital data is encoded as audio tones that then modulate an FM carrier. Its virtue is that it works through any ordinary FM radio, which is why it is common on low-rate CubeSat beacons intended to be receivable by as many amateur stations as possible. Spectrally inefficient compared with BPSK or GMSK.

AIT¶

Assembly, Integration and Testing. The phase of spacecraft development in which hardware is physically assembled, subsystems are integrated together, and the combined system is tested. AIT comes after individual subsystem verification and before delivery for launch. See AIT.

Albedo¶

Sunlight reflected from a planet's surface and atmosphere back into space, and one of the three environmental heat inputs to a spacecraft in orbit. Modeled as a fraction of the solar constant; spacecraft thermal analysis conventionally uses an albedo factor of 0.30 nominal, 0.25 cold case and 0.35 hot case, though instantaneous values range from roughly 0.06 over ocean to 0.50 over cloud and ice. See Thermal.

AM0¶

Air Mass Zero. The solar spectrum outside the atmosphere, and the condition space solar cells are rated against – as opposed to AM1.5, the spectrum after passing through one and a half atmosphere thicknesses, which terrestrial cells are rated against. The two are not interchangeable, and the same cell quotes a lower efficiency at AM0: the extraterrestrial spectrum carries ultraviolet the cell converts poorly, over a larger total irradiance. Cell datasheets conventionally normalize AM0 to 1367 W/m², a few W/m² above the measured solar constant. See EPS – Cell technologies and efficiencies.

Apogee / Perigee¶

The highest and lowest points of an orbit relative to the body being orbited – apogee farthest from Earth, perigee closest. In a circular orbit the two coincide. For CubeSats the relevant consequence is usually orbital lifetime: perigee altitude governs how much atmospheric drag the spacecraft experiences and therefore how quickly the orbit decays.

AX.25¶

A data link layer protocol derived from the amateur X.25 standard, widely used in CubeSat UHF/VHF communications and compatible with amateur ground station networks such as SatNOGS.

Axial ratio¶

A measure of an antenna's polarization purity: the ratio of the major to the minor axis of the polarization ellipse traced out by the radiated field, in dB. 0 dB is perfect circular polarization and infinity is pure linear; everything real sits between. Practical crossed-dipole and turnstile antennas hold 1–3 dB near boresight and degrade steeply off-axis, which is why the polarization loss a pass actually suffers is worse than the nominal figure once the spacecraft is tumbling and the ground antenna is working at a low elevation angle.

B¶

Baseline¶

A formally agreed, frozen snapshot of a design – requirements, drawings, interfaces, software versions – captured at a project milestone such as PDR or CDR. Subsequent changes are made against the baseline and recorded, which is what makes it possible to say later what the spacecraft actually was at any point. Without baselines, test results cannot be reliably interpreted because nothing pins down what was tested.

Bayer filter / panchromatic¶

Two ways an imager handles color. A Bayer filter is a mosaic of red, green and blue filters over the pixels – a 2×2 pattern with two greens – from which a full-color image is interpolated, which is why a color sensor's effective resolution is lower than its pixel count suggests. Panchromatic means no filter at all: every pixel sees the whole visible band, giving more signal and the sharpest GSD the optics allow, and no spectral information. Earth observation instruments often carry both and sharpen the coarser color bands against the panchromatic channel. Where the imagery has to carry defined spectral bands, discrete filters replace the mosaic. See Payload – Imaging.

B-dot¶

The standard magnetic detumbling control law. It commands a magnetic dipole opposing the measured rate of change of the geomagnetic field in the body frame (in its common form m = −k·Ḃ), extracting rotational energy from the spacecraft. Its great virtue is that it needs only a magnetometer and magnetorquers – no attitude knowledge, no rate gyro, no orbit knowledge – which makes it the first thing that can run after deployment. See GNC – Control Algorithms.

Beacon¶

A short, low-rate, periodic transmission carrying basic health data – battery voltage, temperature, mode, an identifier. Deliberately designed to be receivable with minimal equipment, so that the spacecraft can be confirmed alive without a full pass. Publishing your beacon format and a decoder turns every SatNOGS station on Earth into a receiving station for your mission; many CubeSats have been first detected, and some diagnosed, entirely from beacons received by strangers.

Beamwidth (HPBW)¶

Half-Power Beamwidth. The angular width of an antenna's main lobe between the points 3 dB down from boresight, and the number that decides how accurately the antenna has to be pointed. It narrows as gain rises – roughly 70λ/D degrees for a circular aperture of diameter D – so a high-gain dish buys signal at the price of a tracking requirement, and pointing loss is conventionally estimated from it as about 12·(θ/HPBW)² dB for a pointing error θ. A 15 dBi UHF Yagi is forgiving at tens of degrees; a small S-band dish is a few degrees and needs a rotator that can keep up with an overhead pass.

Beta angle¶

The angle between the orbital plane and the vector from the Earth to the Sun. It determines what fraction of each orbit the spacecraft spends in eclipse. At high beta angles (close to ±90°) the spacecraft can be in continuous sunlight; near 0° it experiences the longest eclipses. Beta angle changes seasonally and is a key input to both power and thermal analysis.

Bit error rate (BER)¶

The fraction of received bits that are wrong, quoted as a power of ten – 10⁻⁵ is a common target for an uncoded telemetry downlink, 10⁻⁶ or better once FEC is applied. Its relationship to Eb/N0 is very steep near threshold, so a decibel or two of link margin moves BER by orders of magnitude, which is why margin is budgeted rather than trimmed to zero. For packetized links the more useful number is the frame error rate: at a BER of 10⁻⁵ a 2000-bit frame is lost about 2% of the time.

BMS¶

Battery Management System. Electronics that monitor and protect a battery pack, including cell voltage balancing, temperature monitoring, and over-voltage / over-current / under-voltage cutoffs. Often integrated with or closely coupled to the EPS.

BOL / EOL¶

Beginning of Life / End of Life. The two bounding conditions a spacecraft subsystem is sized against. Solar arrays are the classic case: cell efficiency falls over the mission from radiation damage and coverglass darkening, so an array sized to close the power budget at BOL will not close it at EOL. Always size generation for EOL and check that BOL does not overcharge anything.

BPSK¶

Binary Phase Shift Keying. A digital modulation scheme that encodes data by shifting the phase of a carrier signal between two values (0° and 180°). Common in CubeSat UHF downlinks for its simplicity and robustness at low SNR.

Burn wire¶

The most common CubeSat release mechanism: a synthetic line (Dyneema, Vectran or similar) under tension restrains a stowed deployable, and a resistive heater pressed against it melts or severs the line on command, letting springs deploy the structure. Cheap, light and easy to test many times. The critical caveat is that with no convection in vacuum the heater runs far hotter for the same power, so burn times measured in air are not valid – always characterize in vacuum. NASA's CubeSat 101 recommends two separate restraint lines to reduce the chance of a deployable coming loose prematurely – that improves retention, not release; release redundancy comes from independent heaters and drive circuits. See Inhibits and HDRM – Burn Wire–Based HDRM.

Bus (spacecraft)¶

Everything on a spacecraft that is not the payload – structure, power, avionics, communications, attitude control, thermal. The bus exists to keep the payload alive, powered, pointed and connected. Note the collision with the other engineering sense of "bus" (a shared electrical or data connection, as in an I²C bus); context normally disambiguates.

C¶

CCSDS¶

Consultative Committee for Space Data Systems. The international body that publishes the standards governing how spacecraft and ground systems exchange data – packet structures, space data link protocols, file delivery, time codes and more. Its "Blue Book" recommended standards are freely available at ccsds.org. Adopting CCSDS formats costs a little overhead and buys interoperability with existing ground software and third-party ground stations.

CDR¶

Critical Design Review. A formal milestone in the spacecraft development lifecycle, typically held after PDR, at which the design is considered mature enough to begin fabrication. At CDR the design should be fully defined, analysis complete, and risk understood. Margins are expected to have tightened from PDR levels (e.g. 20% mass margin at CDR vs. 30% at PDR).

CDS¶

CubeSat Design Specification. The original standard document defining the physical, electrical, and operational requirements for CubeSats and their deployers. Published by California Polytechnic State University (Cal Poly) and originally co-authored with Stanford. Sets the baseline 1U dimensions (100 × 100 × 113.5 mm) and mass limit (2 kg/U) that most CubeSat hardware and deployers conform to. Latest revision: CDS 14.1, February 2022.

C/N0¶

Carrier-to-noise-density ratio, in dB-Hz. Received carrier power divided by noise power spectral density, and the natural subtotal in a link budget: everything up to the receiver – EIRP, free-space path loss, pointing and polarization losses, G/T – determines it, and nothing about the modulation or the data rate does. Eb/N0 follows by subtracting the data rate in dB-Hz, SNR by subtracting the occupied bandwidth. Carrying C/N0 as its own line keeps the propagation half of a budget separate from the waveform half, so a change of modulation does not require redoing the path arithmetic.

COCOM limits¶

Restrictions requiring commercial GNSS receivers to stop reporting a position above roughly 18 km altitude and 1,900 km/h, originally intended to prevent their use in guided missiles. Space-capable receivers have these limits removed or reconfigured, which is precisely what places them under dual-use export control regimes such as the Wassenaar Arrangement and, in the US, ITAR/EAR. A procurement lead-time risk worth identifying early.

Code rate¶

The ratio of information bits to transmitted bits in a FEC scheme, written k/n: rate 1/2 sends two channel bits per information bit, rate 7/8 sends eight per seven. The information rate is the channel rate times the code rate, so a 9600 bps channel at rate 1/2 delivers 4800 bps of information – before framing overhead, which is counted separately. A lower code rate almost always repays its throughput cost, because the bit error rate curve is steep near threshold, but the comparison has to be made at equal information rate rather than equal channel rate or the coding gain is double-counted.

Coherent / non-coherent detection¶

Whether a demodulator recovers and tracks the carrier's phase or works from its energy alone. Coherent detection multiplies the incoming signal against a locally regenerated carrier and reaches the waveform's theoretical sensitivity. Non-coherent detection – an envelope detector or a frequency discriminator – ignores phase, which makes it simple, cheap and tolerant of Doppler shift, and costs roughly 1 dB of Eb/N0 for binary FSK at threshold. Across schemes the gap is wider: a coherently demodulated GMSK receiver and a discriminator-based FSK receiver differ by about 4 dB. A modulation name therefore does not by itself fix a link budget – the demodulator at the far end is what sets the required Eb/N0.

Cold welding¶

The spontaneous bonding of two clean metal surfaces pressed together in vacuum. On Earth a thin oxide and adsorbed gas layer keeps metal surfaces apart; in vacuum that layer is absent and the surfaces can fuse. This is why the CDS requires aluminum CubeSat surfaces in contact with the dispenser rails to be hard anodized – a cold-welded CubeSat cannot leave its deployer. See Structure.

Commissioning¶

The phase immediately after LEOP in which each subsystem is switched on, checked out and calibrated in turn before the mission enters routine operations. Deliberately incremental: sensors verified before actuators are enabled, each actuator's polarity confirmed individually, payload characterized against its ground calibration. Rushing commissioning is how teams turn a recoverable anomaly into a lost mission.

Conformal coating¶

A thin polymer film applied over a populated board to protect it from moisture, contamination and handling damage, and to make it harder for a tin whisker to bridge two conductors. Acrylics and urethanes are the common choices; silicones tolerate wider temperature swings. Three cautions for spaceflight. It has to be a low-outgassing formulation or it becomes a contamination source in its own right. It has to go onto a clean board, because a coating applied over flux residue seals that residue in for the life of the mission. And it makes rework considerably harder, so mask connectors, test points and anything else you may need to reach. See Tools – Solder paste and flux.

CONOPS¶

Concept of Operations. A document or narrative describing how a mission is intended to work end-to-end, across all phases from launch to end-of-life. Covers nominal and off-nominal scenarios, how the ground segment interacts with the spacecraft, and what the operators will do and when.

COTS¶

Commercial Off-The-Shelf. Hardware or software purchased as a standard product rather than custom-designed for the mission. COTS components are the norm in CubeSat development – using tested, available parts reduces cost, schedule, and risk compared to building from scratch. Tradeoffs include radiation tolerance, size/power fit, and availability over the mission lifetime.

C-rate¶

A normalized measure of battery charge or discharge current, expressed relative to capacity. 1C discharges the full nominal capacity in one hour; 0.5C takes two hours; 2C takes thirty minutes. Cell datasheets specify maximum charge and discharge C-rates, and cycle life degrades as they are approached. Most CubeSat batteries operate well below 1C, which is one reason they last. The charge current is set by the charger until the pack reaches its float voltage, after which it tapers.

CSP¶

Cubesat Space Protocol. A small, MIT-licensed network-layer protocol stack written in C for communication between subsystems on a small spacecraft. Modeled on TCP/IP, with a lightweight header carrying both transport and network information, and drivers for CAN, UDP, USART and ZMQ. Originating at Aalborg University and GomSpace, it is the closest thing the CubeSat world has to a standard onboard middleware. See Flight Software.

D¶

Degraded mode¶

An operating state entered deliberately when a resource is known to be short but nothing has failed – reduced duty cycles, a slower beacon cadence, payload off – as distinct from safe mode, which is a fault response. The distinction matters because the exits differ: a degraded mode is left when the resource recovers, a safe mode when the fault is understood and cleared. A power budget should close in the degraded case as well as the nominal one, since the degraded beacon is the one the ground will actually be listening for during a bad period.

Delta-v¶

The change in velocity a spacecraft can achieve with its propulsion system, in m/s – the standard currency of orbital maneuvering, since it maps directly onto what maneuvers are possible. Most CubeSats have no propulsion and therefore no delta-v budget at all; those that do use it for orbit raising, phasing, collision avoidance or controlled deorbit.

Deorbit¶

Removing a spacecraft from orbit at end of life, either passively (letting atmospheric drag decay the orbit) or actively (propulsion, a drag sail or another deorbit device). Increasingly a regulatory requirement rather than good practice: the FCC's 5-year rule requires disposal within five years of mission completion for satellites under its jurisdiction, against a long-standing 25-year international guideline. At or around 400 km natural decay meets a five-year limit on its own; beyond 500 km there is no guarantee, and a passive CubeSat in the commonly available 500–600 km sun-synchronous band should expect to demonstrate compliance rather than assume it. See Qualification and Launch – Space debris mitigation.

Deployer¶

Also dispenser or pod. The spring-loaded container that holds one or more CubeSats during launch and ejects them once on orbit. The deployer defines the mechanical envelope the CubeSat must fit, provides the rails it slides along, and actuates the deployment switches that tell the spacecraft it is free. Common families include the Cal Poly P-POD, ISISPACE's ISIPOD, Exolaunch's EXOpod, and the ISS-based J-SSOD and NanoRacks deployers. The deployer's user manual takes precedence over the CDS wherever the two disagree. See Structure.

Deployment switch¶

A switch held actuated by the deployer walls while the CubeSat is stowed, which releases on ejection and thereby tells the spacecraft it is in orbit. The CDS requires at least one, requires it to electrically disconnect the power system from powered functions while actuated, and requires that if it toggles and returns, the satellite resets to a pre-launch state including its transmission and deployable timers. Also the T-zero for the mandated quiet periods. See Inhibits and HDRM.

Depth of field¶

The range of object distances that stay acceptably sharp at a fixed focus, bounded by how large a defocused point may grow before it is noticed – the circle of confusion, conventionally taken as one or two pixel pitches on a digital sensor. It widens with the f-number and with subject distance, and narrows with focal length. Focusing at the hyperfocal distance, the nearest setting whose far limit still reaches infinity, maximizes it for a scene running to the horizon. Irrelevant for nadir Earth observation, where everything is effectively at infinity, and central to any camera looking at the spacecraft itself: a boom-mounted inspection camera works at a few hundred millimeters, where depth of field is a design constraint rather than a detail.

Derating¶

Deliberately operating a component below its maximum rated voltage, current, power or temperature, to increase reliability and margin. Standard practice in spacecraft design, and particularly important when flying COTS parts whose ratings assume a benign terrestrial environment and a short life.

Detumbling¶

Reducing the spacecraft's body rotation rates after deployment, when it typically emerges from the deployer tumbling at a few degrees per second. Nothing else works until this is done: solar generation is poor, star trackers cannot lock, and communications are intermittent. Almost always performed by a B-dot controller driving magnetorquers, autonomously and without ground intervention.

Diffraction limit¶

The finest detail an optical system can form at all, set by the aperture diameter and not by the detector. The Rayleigh criterion puts the angular resolution at θ ≈ 1.22 λ/D for wavelength λ and aperture D, which projected from orbit becomes a ground distance. It is routinely confused with GSD, which is a sampling interval: an instrument is either pixel-limited, with pixels coarser than the optics can resolve, or diffraction-limited, with the optics blurring across several pixels. The Q sampling factor, Q = λ·N ÷ pixel pitch for f-number N, expresses the balance in one number – below about 1 the detector limits, above about 2 the optics do. Knowing which binds tells you whether to spend on focal length or on aperture. See Payload – Imaging.

DITL¶

Day-In-The-Life testing. A full rehearsal in which the integrated spacecraft is run through a realistic 24-hour operational timeline – eclipse cycles, ground station passes, payload operations, housekeeping – with the real ground segment on the other end. Routinely finds problems no other test does, because it is the first time every element runs together at realistic timescales. See AIT – Mission Simulation.

DoD (battery)¶

Depth of Discharge. The fraction of a battery's total capacity that has been discharged relative to its fully charged state. Cycling a lithium battery to high DoD (e.g. >80%) significantly reduces its cycle life. CubeSat designs typically target a maximum DoD of 20–30% to keep the battery healthy across thousands of orbits. Its complement is state of charge.

Doppler shift¶

The change in received frequency caused by relative motion between spacecraft and ground station. A LEO satellite closes and opens range fast enough to shift a 437 MHz carrier by roughly ±10 kHz across a pass, sweeping through zero at closest approach – where the rate of change is greatest, precisely when the signal is strongest. Correction is required on the downlink and, just as importantly, pre-compensation is required on the uplink; forgetting the latter is a classic reason a spacecraft that is being heard perfectly will not accept commands.

Downlink / Uplink¶

Downlink is the direction from spacecraft to ground – telemetry, beacons and payload data. Uplink is ground to spacecraft – commands and software updates. The two are strongly asymmetric on a CubeSat: the ground transmitter can be far more powerful than the spacecraft's, so uplink usually closes more easily, while downlink capacity is the binding constraint on how much data a mission can return.

Duplex (half / full)¶

Whether a radio can transmit and receive at the same time. Full duplex uses separate transmit and receive frequencies with enough filtering and isolation to run both at once – normal on the ground, uncommon in the space segment, where the transmitter would desensitize its own receiver a few centimeters away. Half duplex alternates: the spacecraft listens, then talks. Nearly every CubeSat link is half duplex on the spacecraft side even when the ground station is full duplex, which is why command sessions are turn-based, why a transmitting spacecraft cannot be interrupted mid-downlink, and why every command protocol needs a timeout rather than an abort.

Duty cycle¶

The fraction of time a component is actively operating. Central to CubeSat design because very few payloads or transmitters can run continuously on the available power: average power equals peak power times duty cycle, so the duty cycle the power budget permits is a hard mission parameter determining how much data can be collected at all. Note that peak draw still matters independently – a load that is fine on average can still brown out the bus. For a packet radio the duty cycle is measured in time on air rather than in transmitter on-time.

E¶

Earth IR¶

Also outgoing longwave radiation (OLR). The infrared energy radiated by the Earth itself, which a spacecraft in orbit intercepts as a heat input. Modeled as a roughly 255 K blackbody giving about 241 W/m², with thermal analysis values typically spanning 214–267 W/m². Unlike albedo, Earth IR continues during eclipse, and is therefore what keeps the cold case from being colder than it is.

Eb/N0¶

Energy per bit to noise power spectral density ratio, in dB. The normalized figure of merit for a digital link, and the quantity modulation and coding performance is quoted against, because unlike SNR it is independent of bandwidth and data rate, and it follows from C/N0 by subtracting the data rate in dB-Hz. The two relate as Eb/N0 = SNR × (bandwidth / data rate), so halving the data rate buys 3 dB. Uncoded BPSK needs roughly 9.6 dB for a bit error rate of 10⁻⁵; coding gain lowers that requirement and implementation loss raises it. See Comms – Link Budget.

ECEF¶

Earth-Centered, Earth-Fixed frame. A coordinate frame centered at Earth's center of mass that rotates with the Earth. Convenient for describing ground station locations but not inertial – spacecraft state vectors are usually expressed in ECI, not ECEF.

ECI¶

Earth-Centered Inertial frame. A coordinate frame centered at Earth's center of mass with axes that do not rotate with the Earth. The X-axis points toward the vernal equinox, Z toward the North Pole. Used to describe spacecraft position and velocity in a non-rotating frame. See GNC.

Eclipse fraction¶

The proportion of an orbit spent in Earth's shadow and therefore receiving no solar power. Around 37–40% for low-beta-angle LEO orbits across the 300–600 km band – roughly 35 minutes of a 90–100 minute orbit, almost independent of altitude – falling as the beta angle rises and reaching zero above a critical beta angle of roughly 68–70°. A key input to power budget sizing. See EPS – Power Requirements and Budgets and beta angle.

EDAC¶

Error Detection And Correction. Memory protection that appends check bits to each stored word so that single-bit errors can be corrected and double-bit errors detected. Standard mitigation against single-event upsets in spacecraft memory, usually paired with scrubbing so errors are corrected before a second upset makes them uncorrectable.

EGSE¶

Electrical Ground Support Equipment. The bench-side hardware and software used to power, command and monitor a spacecraft during assembly and test: supplies, umbilicals, breakout boards, RF loopback paths and the test console. Best built around the same command-and-telemetry software that will be used for flight operations, so the operations tooling is exercised throughout the AIT campaign rather than written at the end.

EIRP¶

Effective Isotropically Radiated Power, in dBW or dBm. Transmitter power plus antenna gain minus feedline and connector losses – the whole transmit side of a link budget collapsed into one number, expressed as the power an isotropic radiator would need to produce the same field in the direction of interest. It is also the quantity spectrum regulators and IARU coordination actually constrain. A 1 W (30 dBm) CubeSat UHF transmitter into a near-omnidirectional antenna is around 30 dBm EIRP; a ground station running 50 W into a 15 dBi Yagi is around 62 dBm before feedline loss.

Elevation angle¶

The angle between the local horizon and the spacecraft as seen from the ground station – 0° at the horizon, 90° directly overhead. It sets slant range and therefore free-space path loss, governs how much atmosphere and how much terrain, building and tree line the signal has to cross, and decides whether a pass is usable at all. Stations normally define a minimum usable elevation, commonly 5–10°, and count only the time above it. High-elevation passes are short and strong; low ones are long, weak and noisy.

Emissivity (ε)¶

The efficiency with which a surface radiates heat in the infrared, relative to a perfect blackbody (ε = 1). High emissivity is what makes a radiator work. Paired with absorptivity as the α/ε ratio that governs equilibrium temperature. Note that α is measured in the solar spectrum and ε in the infrared, which is why a surface can be highly reflective to sunlight and highly emissive in the IR at the same time.

Engineering model¶

A functionally equivalent, non-flight copy of the spacecraft or a subsystem, used for software development, procedure rehearsal, destructive testing and troubleshooting. Frequently the single best investment a CubeSat team can make: it unblocks software work from hardware availability, and it means every procedure has been rehearsed before it touches the flight article. Distinct from a flatsat, which is a bench layout rather than a built spacecraft.

Ephemeris¶

A table or model giving a spacecraft's position and velocity as a function of time. On a CubeSat it is usually not stored as a table but generated onboard by propagating a TLE with SGP4, or taken directly from a GNSS receiver. Needed by anything that has to know where the spacecraft is: attitude determination, pointing, payload scheduling and pass planning.

Epoch¶

The reference instant at which a set of orbital elements or a state vector is valid. Orbit predictions degrade with time since epoch – a TLE is accurate to roughly a kilometer near its epoch and tens of kilometers a week or two later – which is why operational tracking requires regularly refreshed element sets.

EPS¶

Electrical Power System. The subsystem responsible for generating (solar panels), storing (batteries), conditioning, and distributing electrical power to all other subsystems. See EPS.

ESD¶

Electrostatic discharge. The transfer of charge between two objects at different potentials, and the most common way to destroy a semiconductor on a workbench. What makes it insidious is that most of the damage is latent rather than immediate: the part passes every test, flies, and fails weeks later with nothing to diagnose. Control is a system rather than a purchase – a grounded work surface and wrist strap whose resistances fall inside a defined band, ESD-safe tools and packaging, humidity within a range, and insulators kept away from exposed parts. "ESD-safe" on a product listing means nothing without the numbers behind it. See Tools – ESD control.

ESPA ring¶

EELV Secondary Payload Adapter. A ring-shaped structural adapter mounted between a launch vehicle's upper stage and its primary payload, carrying secondary payloads on ports around its circumference – six 38 cm ports each supporting up to 257 kg in the standard configuration, with larger ESPA Grande variants. The standard way spacecraft above CubeSat scale fly as secondary payloads; CubeSat dispensers are often themselves mounted on an ESPA port.

Eutectic¶

An alloy composition that melts and freezes at a single temperature instead of across a range. Sn63Pb37 is the tin-lead eutectic at 183 °C, and the reason it remains the default for hand soldering is that it has no plastic range: the joint is liquid or solid and nothing in between, so one disturbed while cooling looks visibly wrong rather than quietly weak. Near-eutectic and lead-free alloys pass through a pasty phase on the way down, which hides exactly that defect. See Tools – Solder alloy and tin whiskers.

F¶

FCC¶

Federal Communications Commission. The United States regulator for radio spectrum, including satellite communications. Relevant well beyond the US because a great many missions are licensed by or communicate through the US, which brings them under FCC rules – notably its orbital debris requirements. Its counterparts elsewhere include Ofcom (UK), BNetzA (Germany) and OFCOM (Switzerland).

FDIR¶

Fault Detection, Isolation and Recovery. The onboard machinery that notices something has gone wrong, works out which element is responsible, and takes action to restore a working state – without waiting for a ground contact that may be hours away. Usually built as an escalation ladder from retry, through subsystem reset and power cycle, to safe mode and full spacecraft reset. See Flight Software.

FEC¶

Forward Error Correction. Adding structured redundancy to transmitted data so the receiver can detect and correct errors without asking for a retransmission – essential when the round trip is a satellite pass. Convolutional, Reed-Solomon, LDPC and turbo codes are common, often concatenated, and the redundancy is quantified by the code rate. The resulting coding gain of several dB is frequently the difference between a link that closes and one that does not.

Field of view (FOV)¶

The angular extent an instrument sees, following from the sensor dimension d and the focal length f as FOV = 2·arctan(d/2f). Projected at the orbit altitude it gives the swath; the same quantity for a single pixel is the instantaneous field of view (IFOV), whose ground projection is the GSD. External field of view is also a systems-level resource rather than a payload property: the instrument, the antennas, the radiators, the solar cells and the star tracker all need unobstructed sky, and on the instrument side the usable field is bounded by the lens's image circle as much as by the sensor, and the allocation is worth settling early. See Payload – Mechanical Integration.

Finite element analysis (FEA)¶

A numerical method for predicting how a structure responds to loads, by subdividing it into many small elements and solving the resulting system of equations. Used on CubeSats mainly for modal analysis (finding natural frequencies) and for demonstrating positive margin of safety under launch loads. Also referred to as FEM (finite element method/modeling). See Structure – Structural Analysis and FEM.

Flatsat¶

A spacecraft's full electronics set laid out flat on a bench, wired as flown but fully accessible for probing, reprogramming and fault injection. The primary integration and software development environment for most CubeSat teams, used long before and long after the flight hardware is assembled. See AIT.

Flight heritage¶

Evidence that a component or design has already operated successfully in space. Heritage reduces perceived risk and is a major factor in COTS component selection, but it is specific: heritage applies to a particular configuration, in a particular orbit, for a particular duration. A part with heritage in LEO for six months tells you little about a three-year mission at a different altitude.

Float voltage and charge termination¶

How a lithium charge cycle ends. Constant-current charging raises the cell voltage until it reaches the float or constant-voltage setpoint – around 4.20 V per cell for most lithium-ion chemistries, 3.60 V for LFP – after which the charger holds that voltage and the current tapers, the charge terminating when it falls below a set threshold. Two consequences on a spacecraft: the charger needs input headroom above the float voltage times the series count, or it never reaches termination and the pack sits permanently part-charged while looking healthy; and floating deliberately below the maximum, 4.10 V rather than 4.20 V, costs a few percent of capacity and buys substantially more cycle life – usually the better trade across thousands of orbits. See EPS – Converters.

Flux (soldering)¶

The chemistry that strips oxide from the surfaces being joined and keeps them clean while the solder wets them – without it, solder beads up instead of flowing. IPC J-STD-004 classifies it in four characters: base chemistry (RO rosin, RE resin, OR organic, IN inorganic), activity (L, M or H) and halide content (0 or 1), so ROL0 is a low-activity halide-free rosin. The space addendum to J-STD-001 narrows the field to rosin and resin bases at L0 or L1, which excludes the water-soluble and organic-acid chemistries. Distinct from the radiometric and thermal senses of the word used elsewhere on this site; see solar constant. See Tools – Solder paste and flux.

f-number¶

The ratio of a lens's focal length to its aperture diameter, written f/2.8 or N = f/D. It governs how much light the optics collect – irradiance at the focal plane goes as 1/N² – and, with the wavelength and the pixel pitch, where the instrument sits against its diffraction limit. A fast (low) f-number shortens the exposure needed for a given scene and so reduces image smear, which is why it matters more on a spacecraft crossing the ground at 7 km/s than it does on a tripod. It also sets the depth of field, which matters only for a camera looking at the spacecraft rather than at the Earth.

Frame error rate (FER)¶

The fraction of received frames discarded rather than decoded, and the number that actually describes a packetized link, since a frame fails whole: one uncorrected bit costs all of it. It follows from the bit error rate and the frame length – at a BER of 10⁻⁵ a 2000-bit frame fails about 2% of the time, at 10⁻⁴ about 18%. Because the dependence on length is that steep, shortening frames is a legitimate way to hold a marginal link together, paid for in framing overhead.

Framing overhead¶

Everything transmitted that is neither payload data nor FEC parity: preamble and sync words, frame headers and addressing, length fields, checksums, trailers, and the flag or idle bits between frames. Distinct from coding overhead, which the code rate already accounts for, and routinely left out of data budgets – on short frames it can exceed a third of everything sent. It is the gap between a link's information rate and the bytes a mission actually delivers in a pass.

Free-space path loss¶

Also FSPL. The fall-off in power density with distance from an isotropic radiator, (4πd/λ)² – strictly a spreading term rather than a loss, since nothing absorbs the energy. In dB it is 92.45 + 20 log₁₀(d in km) + 20 log₁₀(f in GHz), which at 437 MHz gives about 139 dB directly overhead at 500 km and about 145 dB at a 1000 km slant range. Almost always the largest single term in a link budget. It grows with frequency at fixed antenna gain, so moving to S-band or X-band pays only when real aperture comes with it.

FSK¶

Frequency Shift Keying. Modulation that encodes data by switching the carrier between discrete frequencies – in its binary form the direct digital relative of AFSK. Its constant envelope lets a power amplifier run at saturation, where it is most efficient, which matters on a spacecraft where DC power is the scarce resource. The price is sensitivity: coherent binary FSK needs about 3 dB more Eb/N0 than BPSK for the same bit error rate, and non-coherent detection about 4 dB. Gaussian-filtered variants, including GMSK, give back some spectral efficiency.

G¶

Geolocation accuracy¶

How well a point in an image can be tied to a point on the ground. It follows from pointing knowledge rather than pointing accuracy: the error at the ground is roughly the knowledge error in radians times the slant range, so 0.1° from 500 km is about 870 m at nadir, with further contributions from orbit determination, timing, the geometric calibration of the instrument frame, and terrain relief when looking off-nadir. It can be improved after the fact by registering images against known ground features, which is how a mission with modest ADCS still delivers usable products – provided attitude and time were recorded honestly. See Payload – Calibration and Validation.

GEVS¶

General Environmental Verification Standard, NASA document GSFC-STD-7000. Defines generalized environmental test levels and verification requirements for spacecraft hardware – random vibration, shock, thermal vacuum, EMC and more. Widely used as the default qualification envelope by CubeSat teams that do not yet know their launch vehicle. The component-level random vibration qualification level is 14.1 Grms; acceptance levels sit 3 dB lower in PSD (≈10 Grms). See Structure and AIT.

Gimbal lock¶

A loss of one rotational degree of freedom that occurs when two of three rotation axes align, a known singularity in Euler angle representations. Avoided in flight software by using quaternions for attitude representation.

GMSK¶

Gaussian Minimum Shift Keying. A continuous-phase frequency modulation scheme with a Gaussian filter applied to reduce spectral bandwidth. Its modulation index is 0.5, the minimum that keeps the tones orthogonal. Used in CubeSat communications for its spectral efficiency and compatibility with amateur radio infrastructure.

GNC¶

Guidance, Navigation, and Control. See GNC. Sometimes used interchangeably with ADCS in the CubeSat context, though strictly GNC is broader (includes navigation and trajectory) while ADCS focuses on attitude.

GNSS¶

Global Navigation Satellite System. Umbrella term for satellite navigation systems including GPS (US), GLONASS (Russia), Galileo (EU), and BeiDou (China). CubeSats in LEO can use GNSS receivers for onboard position and time determination, though high-altitude or fast-maneuvering spacecraft may require special receiver modes.

Golden image¶

A minimal, known-good software image held in write-protected memory that can never be overwritten in flight. It does not need to be capable of running the mission – only of booting, charging the battery, and listening for commands – so that a corrupted or faulty software update is always recoverable. Usually paired with a boot counter that falls back to the golden image automatically after repeated failed boots. See Flight Software.

Gravity gradient stabilization¶

A passive attitude stabilization technique exploiting the torque that a gravity field exerts on an elongated body, tending to align its long axis with the local vertical. Requires no power and no actuators, typically achieves a few degrees of nadir pointing, provides no yaw control, and is bistable – the spacecraft will settle happily either way up. Often enhanced with a deployed boom and tip mass, and usually paired with magnetic hysteresis damping.

Green propellant¶

A low-toxicity chemical propellant – ionic-liquid monopropellants such as LMP-103S and AF-M315E (ASCENT), ADN-based blends, or stored water split by electrolysis – used in place of hydrazine, which is corrosive, toxic and demands protective-suit handling that few CubeSat programs or launch sites will accommodate. Under range safety rules a less hazardous propellant lowers the hazard classification and therefore the number of barriers required, which is what makes chemical propulsion feasible on a rideshare CubeSat. See Propulsion.

Grms¶

Root-mean-square acceleration, in units of g. A single number summarizing the overall energy of a random vibration environment, obtained by integrating the acceleration power spectral density across the test frequency band and taking the square root. Useful for comparing environments at a glance, but it says nothing about where in frequency the energy sits – two very different PSDs can share a Grms value.

Ground sample distance (GSD)¶

The ground footprint of a single pixel, and the usual headline number for an imaging payload. At nadir it is GSD = p·h/f, for pixel pitch p, altitude h and focal length f – set by the detector and the focal length, not by the aperture, which is the most common error in CubeSat imager specifications. GSD is a sampling interval rather than a resolution: what can actually be distinguished depends also on the diffraction limit, on image smear and on scene contrast, and is generally two GSD or more. It degrades away from nadir as the footprint stretches; see off-nadir. See Payload – Imaging.

Ground track¶

The path traced on the Earth's surface directly beneath a spacecraft's orbit; the point immediately below the spacecraft is the sub-satellite point. Because the Earth rotates underneath, successive orbits shift westward, which is what determines revisit time – how long a target waits between opportunities – and how often a given ground station gets a pass. Revisit also depends on how far to either side of the track the spacecraft can see: a wide swath or a tolerance for off-nadir viewing shortens it considerably.

GSaaS¶

Ground-Station-as-a-Service. Commercial ground network capacity sold per pass or per minute, rather than built and operated by the mission. Providers include KSAT, ATLAS, Leaf Space, SSC and AWS Ground Station. Attractive for missions needing S-band or above, where building comparable capability is expensive, and for short missions where capital cost cannot be amortized. Generally does not serve amateur UHF/VHF bands. See Ground Segment.

G/T¶

Gain-to-noise-temperature ratio, in dB/K. The standard figure of merit for a receiving station, combining antenna gain and system noise temperature into a single number that can be dropped straight into a link budget. Higher is better. Gain is bought with aperture and therefore paid for in beamwidth, which is what turns a receiving figure of merit into a tracking requirement. Representative values for commercial ground networks range from about 11 dB/K for small S-band apertures to about 36 dB/K for large X-band dishes.

H¶

HAL¶

Hardware Abstraction Layer. The software layer that isolates all register-level hardware access behind interfaces, so application logic never touches hardware directly. Beyond portability, its value is testability: application code written against a HAL can be compiled and run on a development machine against simulated hardware, turning bugs that would otherwise surface slowly on a flatsat into ones caught by an automated test suite.

Harness¶

The wiring assembly connecting a spacecraft's subsystems – wires, connectors, shielding, strain relief and labeling. A disproportionate source of integration problems, largely because it is often improvised rather than designed. A harness that accreted during integration cannot be verified or rebuilt identically; one that was drawn, documented and built to a schedule can be.

HDRM¶

Hold Down and Release Mechanism. A mechanical device that keeps deployable structures (antennas, solar panels, booms) stowed during launch and releases them on command in orbit. See Inhibits and HDRM.

Heat pipe¶

A sealed passive device that transports heat with a very high effective thermal conductivity, using a working fluid that evaporates at the hot end, travels as vapor to the cold end, condenses, and returns via a capillary wick. Standard on larger spacecraft; flat-plate and conformable micro variants have been demonstrated on 6U CubeSats. No moving parts and no power required, but orientation-sensitive during ground testing.

HIL¶

Hardware-In-the-Loop. A test configuration in which real flight hardware runs against a simulated environment – synthetic sensor data, modeled orbital dynamics and actuator responses – so that control loops and mode logic can be exercised across many orbits without leaving the lab. See AIT.

Hot case / cold case¶

The two bounding thermal scenarios a spacecraft design is analyzed against. The hot case combines maximum solar flux, high albedo and Earth IR, worst-case attitude, maximum internal dissipation and degraded (end-of-life) surface properties. The cold case combines minimum environmental inputs, maximum eclipse and minimum internal dissipation – typically safe mode, when the spacecraft has shed exactly the loads whose waste heat it needs. See Thermal.

Hyperspectral imaging¶

Imaging in hundreds of contiguous, narrow spectral bands, producing a full spectrum for every pixel rather than a few discrete color channels. Scientifically rich and a severe data-volume problem for a small spacecraft: a single scene can exceed a CubeSat's entire daily downlink capacity, which is why onboard filtering and standards such as CCSDS 123.0-B-2 matter. Contrast multispectral imaging, which uses a handful of bands chosen for a specific application.

I¶

IARU¶

International Amateur Radio Union. The body that coordinates amateur satellite frequency use, reviewing proposed missions to avoid interference with existing satellites and issuing a coordinated frequency assignment. The process is free and well documented but takes months, and passing through it is expected practice for any CubeSat using amateur bands. Distinct from, and usually a precursor to, national licensing and ITU notification.

ICD¶

Interface Control Document. A document that defines the interface between two subsystems or between the spacecraft and an external system (e.g. the launch vehicle). Specifies mechanical, electrical, and data connections, pin-outs, signal levels, protocols, and environmental boundaries. ICDs are the contracts between subsystem teams.

Image circle¶

The diameter of the illuminated, usably sharp region a lens projects at its focal plane. A lens whose image circle is smaller than the sensor's diagonal vignettes – dark, soft corners – so the compatibility check when pairing a lens with a sensor is image circle against sensor diagonal, not the mount, which only guarantees that the two parts screw together. C-mount and S-mount lenses are usually specified for a sensor format rather than by diameter, and a lens sold for a 1/2.3" format will not cover a 1/1.1" one.

Image smear¶

Blur produced by relative motion between instrument and scene during an exposure. Two sources matter in orbit: the ground track motion, about 7 km/s at LEO altitudes and therefore roughly 7 m of ground travel per millisecond of exposure, and the spacecraft's own attitude rate, which multiplies by the slant range. Holding smear under one GSD is the usual design rule, and it caps the exposure – which is how an imaging payload ends up driving both the f-number and the ADCS stability requirement. See Payload – Imaging.

Implementation loss¶

The gap between a modem's theoretical performance and what it actually achieves – imperfect filtering, timing and carrier recovery, quantization, phase noise and amplifier non-linearity, each costing a fraction of a dB. Conventionally budgeted as 1–3 dB added to the required Eb/N0. Worth carrying explicitly, because a link budget built on textbook uncoded thresholds and one built on a datasheet's measured sensitivity are not the same calculation: coding gain and implementation loss move the answer in opposite directions, and omitting both is not the same as letting them cancel.

IMU¶

Inertial Measurement Unit. A sensor package combining accelerometers and gyroscopes to measure linear acceleration and angular velocity. Used in ADCS for short-term attitude propagation. IMUs drift over time and are typically fused with absolute sensors (magnetometers, sun sensors, star trackers) for long-term accuracy.

Inclination¶

The angle between an orbital plane and the Earth's equator, in degrees. It determines the range of latitudes the spacecraft passes over: a 0° orbit stays over the equator, a 51.6° orbit (the ISS inclination, and therefore that of ISS-deployed CubeSats) reaches ±51.6°, and a polar or sun-synchronous orbit near 98° covers the whole planet. Inclination is set by the launch and is expensive to change.

Inhibit¶

A physical device interrupting the power path between an energy source and a hazard, preventing that hazard from occurring. The CDS requires at least three independent inhibits against inadvertent RF transmission and three against inadvertent deployable release. Crucially, a timer does not count as an inhibit – software delays are separately required but do not contribute to the count. Independence is what reviewers check: three switches driven by one mechanism are one inhibit. See Inhibits and HDRM.

ITU¶

International Telecommunication Union. The UN specialized agency that coordinates spectrum use globally. Frequency assignments for space missions require ITU notification and coordination, typically handled at the national level through regulators such as the FCC (US) or Ofcom (UK).

J¶

J2¶

The dominant term in the spherical harmonic expansion of Earth's gravity field, about 1.0826 × 10⁻³, produced by the equatorial bulge. It is the reason orbits are not closed ellipses: J2 makes the RAAN precess and the argument of perigee rotate, at rates set by altitude and inclination. A 500 km orbit at 51.6° sees its node regress roughly 5° per day; near 97.4° the precession instead runs prograde at 0.9856° per day and matches the Earth's motion about the Sun, which is the definition of a sun-synchronous orbit. J2 is built into SGP4, so anyone propagating a TLE is already using it.

K¶

Kalman filter¶

A recursive estimator that combines a dynamic model with noisy measurements to produce a best estimate of a system's state, together with an estimate of its own uncertainty. In spacecraft attitude determination the standard variant is the multiplicative extended Kalman filter (MEKF), which propagates attitude using gyro data, corrects it when absolute observations (sun, magnetic field, stars) arrive, and simultaneously estimates gyro bias – which is why a good filter with a cheap MEMS gyro outperforms an expensive gyro used open-loop.

L¶

LCL¶

Latching Current Limiter. A protected power switch that limits output current to a set value for a defined period and then latches off, staying off until explicitly commanded to reset. The standard spacecraft approach to fault containment on a distributed power bus: it isolates a faulty load without the one-shot permanence of a fuse. An eFuse is the same function as an integrated part – current limit, fault latch and enable pin in one device – and is what most CubeSat EPS designs actually use, often with a programmable trip level and a fault flag back to the OBC. See EPS – Power Switching and Protection.

LEO¶

Low Earth Orbit. Roughly defined as orbits with altitudes between ~200 km and ~2000 km. The vast majority of CubeSats fly in LEO. Orbital periods are typically 90–120 minutes, eclipse fractions up to 37–40% at low beta angles, and radiation environments are relatively benign compared to higher orbits (though the South Atlantic Anomaly can be significant for some missions).

LEOP¶

Launch and Early Orbit Phase. The period immediately after deployment, covering first acquisition of signal, detumbling, initial health assessment and establishing routine contact. The highest-workload and highest-risk phase of a CubeSat mission, and the one where automation is least useful and rehearsal most valuable. Complicated for rideshare CubeSats by the fact that the spacecraft may not yet be individually identified in the catalog.

Link budget¶

An accounting of all gains and losses along a communications path, from transmitter to receiver. The result is a link margin – how many dB of headroom exist above the minimum required SNR. Positive link margin means the link should close; the required margin depends on how much uncertainty is in the system. See Comms – Link Budget.

Link margin¶

The difference (in dB) between the received signal level and the minimum signal level required to achieve the target bit error rate. A link margin of 0 dB means the link is exactly at threshold; margins of 3–6 dB are typical for well-designed CubeSat links.

LNA¶

Low-Noise Amplifier. The first amplifier in a receive chain, whose noise figure dominates the noise performance of the whole system. Mounting it at the antenna, ahead of the feedline, is the single highest-value improvement to a marginal ground station: feedline loss before the LNA adds directly to system noise figure, while the same loss after it is nearly irrelevant.

Load shedding¶

Turning off non-essential loads when generation or state of charge falls below a threshold, so that the essential ones – receiver, OBC, battery heaters – keep running. The ladder is defined in advance, as a sequence of thresholds with hysteresis rather than a single cliff, and it is what makes safe mode sustainable instead of merely a label. It is also the mechanism that breaks a brownout loop: a boot configuration allowed to draw more than the array can supply collapses the bus and starts again. See EPS – Startup and brownout behavior.

Local time of descending node (LTDN)¶

The mean solar time at the point where a satellite crosses the equator heading south, and the parameter that fixes the illumination of a sun-synchronous orbit. It follows from the RAAN at launch and is then held by the orbit's precession. A 10:30 LTDN is a common Earth observation choice, giving moderate shadows for terrain interpretation; a 06:00 or 18:00 dawn–dusk orbit maximizes sunlight on the array and minimizes eclipse. Missions that quote the ascending crossing instead use the local time of the ascending node (LTAN); the two differ by twelve hours.

LoRa¶

A proprietary chirp spread spectrum modulation from Semtech, ubiquitous terrestrially in sub-GHz ISM bands and flown on a number of CubeSats and PocketQubes. Its appeal is cost and sensitivity: a COTS transceiver module costs a few euros and receives down to roughly −136 dBm at the highest spreading factor, closing a link that would otherwise demand far more antenna. The costs are a very low data rate, a closed specification that sits awkwardly with amateur-service rules on documented emissions, and shrinking Doppler tolerance as the channel bandwidth narrows.

LVLH¶

Local Vertical, Local Horizontal frame. An orbital reference frame centered on the spacecraft, with one axis pointing toward Earth's center (local vertical) and another along the velocity vector (local horizontal). Commonly used to define attitude modes such as nadir-pointing or velocity-pointing.

M¶

Magnetorquer¶

An attitude actuator that generates torque by producing a magnetic dipole which interacts with the Earth's magnetic field, following τ = m × B. Simple, robust, power-efficient and with no moving parts or consumables. The cross product means no torque can be produced about the field direction, so magnetorquers are underactuated at any instant and rely on the field direction changing around the orbit – which makes magnetic-only control slow but workable. Implemented as air-core coils, rods wound on a ferromagnetic core, or traces embedded in a PCB. See GNC.

Margin of safety¶

A normalized measure of how much structural capability remains beyond what the applied load requires: MS = allowable / (factor of safety × applied) − 1. A margin of safety must be positive; zero means the structure is exactly at its limit with no headroom. Reported separately against yield and ultimate allowables. Distinct from the margins tracked in mass and power budgets, which are bookkeeping reserves rather than structural quantities.

MLI¶

Multi-Layer Insulation. Thin layers of reflective foil (typically aluminized Mylar or Kapton) separated by spacer netting, used to reduce radiative heat transfer between a spacecraft and its environment. Common in thermal control but requires careful design around conductive pathways and grounding.

Modulation index (h)¶

In frequency modulation, the peak-to-peak frequency deviation divided by the symbol rate. It sets the trade between occupied bandwidth and sensitivity: h = 1 is the classic wideband FSK choice, easy to recover with a discriminator and wasteful of spectrum, while h = 0.5 is minimum shift keying – the narrowest deviation that keeps the tones orthogonal, and the basis of GMSK. The receiver's filter has to match the index the transmitter actually uses; a mismatch usually costs more than the difference between the two schemes on paper.

Momentum desaturation¶

Also momentum dumping. The process of removing accumulated angular momentum from reaction wheels by exerting an external torque on the spacecraft – on a CubeSat, using magnetorquers against the Earth's magnetic field. Necessary because persistent disturbance torques continuously spin the wheels up; once a wheel saturates it can produce no further torque and control authority about that axis is lost. A reaction wheel system without a working desaturation path will eventually stop working.

MPPT¶

Maximum Power Point Tracking. A power electronics technique that continuously adjusts the electrical operating point of a solar panel array to extract maximum available power. The maximum power point shifts with illumination intensity and temperature. MPPT is standard on most modern CubeSat EPS boards.

N¶

Nadir¶

The direction from a spacecraft straight down toward the center of the body it orbits – for a CubeSat, the center of the Earth. Nadir-pointing holds a chosen body axis along this direction, which is the standard attitude for Earth observation payloads and downward-facing antennas. The opposite direction is zenith. An instrument's off-nadir angle is how far its boresight is tilted away from nadir: tilting reaches targets the ground track does not pass over and shortens revisit, at the cost of a longer slant path, a coarser and distorted GSD, and an oblique view that is harder to georeference.

Nanosatellite¶

A satellite with a mass between roughly 1 and 10 kg. Most CubeSats from 1U to 6U fall in this class. Neighboring classes are picosatellites (0.1–1 kg, including PocketQubes), microsatellites (10–100 kg) and minisatellites (100–500 kg), though the boundaries are conventions rather than standards.

Nodal model¶

Also lumped-parameter model. The standard method of spacecraft thermal analysis: the spacecraft is divided into a number of isothermal nodes, each with a thermal capacitance, linked by conductive and radiative couplings and driven by environmental and internal heat loads. Model fidelity ranges from a single node (useful for a first feasibility estimate) through 6–20 nodes (typical for a CubeSat) to hundreds for gradient-critical payloads. See Thermal – Thermal Modeling and Simulation.

Noise figure¶

NF, in dB. How much a component or chain degrades the SNR passing through it, referenced to a 290 K source: NF = 10 log₁₀(1 + Te/290), with Te the equivalent noise temperature. 1 dB corresponds to 75 K, 0.5 dB to 35 K, 3 dB to 289 K. Friis' formula makes the first stage dominant, and every dB of loss ahead of it – feedline, relays, filters – adds directly to the total, which is the argument for mounting the LNA at the antenna. See system noise temperature.

O¶

OAP¶

Orbit Average Power. The time-weighted average power consumed (or generated) over a complete orbit, accounting for different operational modes, duty cycles, and eclipse/sunlight fractions. The headline figure a power budget must balance: average generation ≥ average consumption. See EPS – Power Requirements and Budgets.

OBC¶

On-Board Computer. The central processing unit of the spacecraft, responsible for executing flight software, managing mode transitions, handling telecommands, collecting telemetry, and coordinating subsystem activity. See OBC.

Orbit¶

A closed (or near-closed) path followed by a spacecraft around a celestial body under the influence of gravity. For CubeSats, this almost always means an orbit around Earth. Key parameters that define an orbit include its altitude (how far above Earth's surface), inclination (the angle between the orbital plane and the equator), eccentricity (how circular vs. elliptical the path is), and period (the time to complete one revolution – typically 90–120 minutes in LEO).

The orbital parameters determine almost everything else about the mission environment: how long each pass lasts over a ground station, what fraction of each orbit is spent in eclipse, how intense the radiation environment is, how quickly the orbit decays from atmospheric drag, and what beta angle the spacecraft sees at different times of year. See LEO, beta angle, eclipse fraction, and TLE.

OTV¶

Orbital Transfer Vehicle, also OMV (orbital maneuvering vehicle). A propulsive "space tug" that carries payloads from a rideshare drop-off orbit to a different altitude, inclination or phasing. Flown examples include D-Orbit's ION, Rocket Lab's Photon, Impulse Space's Mira and Momentus's Vigoride. An increasingly practical answer when the available rideshare orbit is wrong for the mission.

Outgassing¶

The release of trapped gases and volatile compounds from materials exposed to vacuum. The escaping molecules condense on the coldest nearby surfaces – typically optics, radiators and solar cells – degrading their performance. Non-metals are the main offenders: adhesives, tapes, cable ties, conformal coatings, printed parts and potting compounds. Materials are screened using ASTM E595, which reports TML (total mass loss) and CVCM (collected volatile condensable material) after 24 hours at 125 °C in vacuum; the commonly applied screening criteria are CVCM ≤ 0.10% and TML ≤ 1.00% – strictly, TML less water vapor recovery; see Structure – Outgassing. The NASA Goddard Outgassing Database publishes results for thousands of materials.

P¶

Pass¶

A single opportunity to communicate with a satellite, from the moment it rises above the station's usable horizon until it sets. A typical LEO CubeSat gives a mid-latitude ground station roughly four to six usable passes a day, each lasting 5–12 minutes – on the order of 30–60 minutes of daily contact, which is the fundamental constraint on how much data a mission can return. Pass quality varies enormously with maximum elevation. The imaging counterpart, over a target rather than a station, is an access.

Passivation¶

Permanently de-energizing a spacecraft at end of life – discharging batteries, venting or depleting any stored pressure, and disabling transmitters – so that it cannot later explode or fragment and generate debris. Increasingly a licensing requirement rather than good practice, and it needs to be a commandable function that has actually been tested.

Payload user guide (PUG)¶

The document issued by a launch provider defining what a payload must do to fly on their vehicle: mechanical envelope, mass and center-of-mass limits, environmental levels, electrical interfaces, safety requirements and delivery schedule. Together with the deployer manual, it supersedes the CDS wherever they conflict. Read it before finalizing any external geometry.

PC/104¶

An embedded computing form factor, originally for industrial PCs, whose stacking-connector concept was adapted by the CubeSat community into a de facto internal board standard – roughly 90 × 96 mm boards joined by a 104-pin stacking header. The mechanical convention is widely shared; the pinout is not standardized, so boards from different vendors may mate physically and still be electrically incompatible. See Structure – Mounting and Mechanical Interfaces.

PCM¶

Phase Change Material. A substance used to buffer thermal transients by absorbing latent heat as it melts and releasing it as it solidifies, holding a near-constant temperature meanwhile. Paraffins melting between 20 and 60 °C store roughly 140–280 kJ/kg. Useful for payloads that dissipate hard in short bursts; the cost is the mass of the containment housing.

PDR¶

Preliminary Design Review. A formal milestone in the spacecraft development lifecycle, typically held after concept design when the architecture is established and preliminary analysis exists but detailed design is not yet complete. At PDR the design should be feasible and requirements-compliant, with conservative margins (e.g. 30% on mass, power). CDR follows once the design matures.

Pixel pitch¶

The center-to-center spacing of pixels on a sensor, in micrometers, and the parameter most optical calculations start from: with focal length and altitude it sets the GSD, with the pixel count it sets the sensor's physical dimensions and therefore the field of view, and with wavelength and f-number it decides where the instrument falls against its diffraction limit. Smaller pixels buy finer sampling and cost signal, since each collects less light and so needs a longer exposure or faster optics – which is why resolution gained by shrinking pixels eventually runs into image smear.

PocketQube¶

A satellite form factor smaller than a CubeSat, based on a 5 cm cube (1P) rather than a 10 cm one. Cheaper to build and launch, correspondingly more constrained in power, volume and communications. Shares most of the engineering considerations described on this site, at a harder scale.

Pointing accuracy / pointing knowledge¶

Two distinct requirements that are frequently conflated. Pointing accuracy (or control) is how precisely the spacecraft actually points at the time. Pointing knowledge is how precisely you can determine, afterwards, where it was pointing – which can be reconstructed on the ground from recorded sensor data. Knowledge is much cheaper than accuracy: an imaging mission that georeferences its images in post-processing may need tight knowledge but only loose control, since geolocation accuracy follows from knowledge. Specifying accuracy where knowledge would do is a common way to make an ADCS needlessly expensive.

Polarization loss¶

The mismatch between the polarization a transmitting antenna radiates and the one a receiving antenna accepts. Circular to linear costs a nominal 3 dB whatever the orientation, which is at least a predictable price. Linear to linear is worse in practice: the loss goes as 20 log₁₀(cos θ) in the relative angle, so a tumbling spacecraft produces deep fades, and Faraday rotation in the ionosphere turns the plane unpredictably at VHF and UHF in any case. Circular polarization at the ground station, ideally with switchable handedness, is the standard answer; how close it gets to the ideal is bounded by axial ratio. See Ground Segment.

P-POD¶

Poly-Picosatellite Orbital Deployer. The original CubeSat deployer, developed at California Polytechnic State University alongside the CDS. It holds up to 3U of CubeSats and ejects them with a spring. Largely superseded by newer commercial deployers, but its interface conventions – rails, deployment switches, the 3U tube – shaped the standard that everything since has followed.

Protoflight¶

A test philosophy in which the actual flight hardware is tested at qualification levels but for reduced duration, rather than qualifying a separate dedicated test article. Standard practice for CubeSats, where building a second full spacecraft purely to break it is rarely affordable. The tradeoff is that the flight article accumulates some fatigue life during testing. See AIT.

PUS¶

Packet Utilization Standard, ECSS-E-ST-70-41C. The European standard defining a service-based model for spacecraft telemetry and telecommand: standardized services for command verification, housekeeping reporting, event reporting, onboard scheduling, parameter management and more. Comprehensive to the point of being heavy for a 1U, but explicitly designed to be tailored – adopting a handful of its services is a common middle path.

Q¶

QPSK¶

Quadrature Phase Shift Keying. Modulation using four carrier phase states, carrying two bits per symbol and so reaching twice the spectral efficiency of BPSK at the same theoretical Eb/N0 – the bit stream is simply split across two orthogonal carriers. The price is paid in hardware: tighter amplifier linearity, and carrier recovery that has to resolve a four-way phase ambiguity. Offset QPSK (OQPSK) removes the 180° transitions and the envelope excursions that come with them, and is the usual choice where the amplifier runs near saturation. Common on CubeSat S-band and X-band downlinks.

Qualification testing¶

Environmental testing performed to demonstrate that a design has margin, by testing at levels above those expected in service – typically maximum expected random vibration spectrum +3 dB for two minutes per axis, and a larger number of thermal cycles. Conducted on a dedicated qualification model, unless a protoflight approach is used. Contrast acceptance testing.

Quasi-static load¶

The steady, low-frequency acceleration a spacecraft experiences during launch, expressed as a constant g-load along each axis. Taken from the launch vehicle's user guide and applied in structural analysis as a simple static load case. For CubeSats it is usually less demanding than random vibration, which tends to be the sizing case.

Quaternion¶

A four-element mathematical representation of orientation in 3D space, widely used in spacecraft ADCS for its numerical stability and freedom from the gimbal lock singularities that affect Euler angles. A unit quaternion q = [q₀, q₁, q₂, q₃] encodes a rotation axis and angle in a form convenient for onboard computation.

R¶

RAAN¶

Right Ascension of the Ascending Node. One of the six classical orbital elements: the angle, measured in an inertial frame, to the point where the orbit crosses the equatorial plane traveling north. Together with inclination it defines the orientation of the orbital plane in space. RAAN precesses over time due to Earth's oblateness – an effect deliberately exploited by sun-synchronous orbits.

Radiator¶

A surface designed to reject heat to space, characterized by high infrared emissivity and low solar absorptivity, with a clear view of deep space and a good conduction path to whatever it is cooling. Scarce on CubeSats, because the external faces are usually already committed to solar cells.

Radiometric calibration¶

Establishing the relationship between an instrument's raw output counts and the physical quantity being measured, using reference sources of known output – integrating spheres for imagers, radioactive sources of known energy for particle and gamma-ray detectors, blackbodies for thermal instruments. Without it, instrument output is a number rather than a measurement. Ground calibration shifts during launch and drifts in orbit, so it must be verified and repeated on orbit.

Rail¶

The four load-bearing edges running along a CubeSat's Z axis, which slide against the matching rails inside the deployer. The CDS requires them to be at least 8.5 mm wide, with a surface roughness below 1.6 µm and edges rounded to at least 1 mm radius, and requires aluminum rails to be hard anodized against cold welding. At least 75% of the rail length should contact the deployer. Rails are simultaneously the most tolerance-critical and the most easily damaged surfaces on the spacecraft.

Random vibration¶

A broadband, statistically described vibration environment – the dominant structural load case for most CubeSat hardware. Specified as an acceleration power spectral density (PSD) in g²/Hz across a frequency band, and summarized by an overall Grms value. GEVS gives 14.1 Grms for 2 minutes per axis as the component qualification level. See Structure.

RBF pin¶

Remove-Before-Flight pin. A physical insert that cuts all power to the spacecraft while in place, fitted with a large brightly colored streamer so it cannot be overlooked. Removed before launch – before deployer integration if the deployer has no access port. The CDS requires it to cut all power and to protrude no more than 6.5 mm from the rail surface when fully inserted.

Reaction wheel¶

A spinning flywheel used as an attitude actuator. By accelerating or decelerating the wheel, angular momentum is exchanged with the spacecraft body, producing a controlled torque without expelling propellant. Reaction wheels can become saturated (spin at their maximum rate) and must be periodically desaturated using magnetorquers or thrusters.

Red plague¶

Galvanic corrosion of silver-plated copper conductors. Moisture reaching the copper through a break in the plating drives a reaction that consumes the copper and leaves a reddish-brown cuprous oxide, and it travels along the conductor under the insulation – so by the time it shows at a termination, the damage runs further than it looks. It is why the space addendum to J-STD-001 requires a red plague control plan, and why NASA prohibits aqueous cleaning of silver-plated copper: water is the one thing the reaction needs. See Standards – Workmanship.

Reflow¶

Soldering by melting solder paste that has already been deposited, rather than by feeding wire into a joint. The board is taken through a temperature profile – preheat, soak, a peak above the alloy's melting point, then a controlled cool – in an oven, on a hotplate or under hot air. The profile belongs to the paste and the board rather than to a generic curve: the paste datasheet gives the peak temperature and the time above liquidus, and a board with more thermal mass than the datasheet's reference needs a slower ramp to reach them. See Tools – Solder paste and flux.

Requirements traceability¶

The practice of linking every requirement to the parent objective it derives from, the design element that satisfies it, and the verification activity that proves it. Usually maintained as a matrix – a spreadsheet is perfectly adequate on a CubeSat. Its practical value shows up twice: when a late change forces you to identify what it breaks, and when the V&V program needs to demonstrate that every requirement has been closed out.

Rework¶

Reprocessing a nonconforming item so that it conforms completely – a lifted part reseated, a bridge cleared, a wrong component replaced. NASA's workmanship standards draw a line between this and repair, which only reduces the effect of a nonconformance and leaves the item still outside its drawing. The distinction matters on flight hardware because a repair needs disposition and documentation where rework does not, and because every thermal excursion spends some of the board's remaining life. See Tools – Soldering and rework.

Rideshare¶

Flying as a secondary payload alongside a primary mission, sharing the cost of a launch. The dominant route to orbit for CubeSats, and the origin of most of their constraints: the primary's orbit is the orbit you get, and a set of "do no harm" requirements – restricting transmitters, deployments and hazardous materials – is imposed to protect the primary payload. See Qualification and Launch.

Rolling shutter / global shutter¶

Two ways a sensor reads out a frame. A global shutter exposes every pixel over the same interval. A rolling shutter exposes row after row, so the bottom of the frame is captured milliseconds after the top – and on a spacecraft moving under the scene, that skews the image geometrically by the ground distance covered during readout. Rolling-shutter parts are cheaper and dominate COTS camera modules, and are usable where readout is fast and the geometry is corrected on the ground; a global shutter earns its cost where the imagery has to be metrically accurate. See Payload – Imaging.

Rotator¶

The azimuth-elevation drive that keeps a directional ground antenna on a spacecraft through a pass. What it has to deliver is set by the beamwidth: residual tracking error must stay a modest fraction of it, and a high pass sweeps through the zenith fast enough that slew rate rather than accuracy becomes the limit, demanding several degrees per second in azimuth. Wide-beam stations avoid the problem entirely, which is much of why so many amateur CubeSat stations run a fixed turnstile and no rotator at all. See Ground Segment.

RTOS¶

Real-Time Operating System. A small operating system providing pre-emptive task scheduling with priorities and bounded, predictable response times – the property a general-purpose OS does not guarantee. FreeRTOS, Zephyr and RTEMS are the common choices in CubeSat flight software, with RTEMS carrying the most spaceflight heritage. See Flight Software.

S¶

Safe mode¶

The fallback operating state a spacecraft enters autonomously when something goes wrong – as opposed to a degraded mode, which is a planned response to a short resource rather than to a fault. Non-essential loads are shed, attitude control reduced to something power-positive or passive, battery charging, receiver listening and beacon transmitting. Safe mode must be reachable from every other mode, must be entered without ground intervention, and must be sustainable indefinitely – it is the state the mission is recovered from. See Flight Software – Modes.

SatNOGS¶

Satellite Networked Open Ground Station. An open-source, community-driven global network of ground stations that receive and share satellite signals, including CubeSat beacons. Operated by the Libre Space Foundation. A de facto standard for beacon reception in academic and open-source missions. satnogs.org.

Scrubbing¶

Periodically reading through memory (or an SRAM-based FPGA's configuration) and rewriting corrected values, so that accumulated single-event upsets are removed before a second upset in the same word makes them uncorrectable. Usually paired with EDAC.

SDR¶

Software-Defined Radio. A radio in which demodulation, filtering and decoding are performed in software on digitized samples rather than in fixed analog hardware. This makes one piece of hardware able to receive almost any waveform, which is why SDRs dominate CubeSat ground stations. Common devices range from the very low-cost RTL-SDR (the SatNOGS reference radio) through Airspy and Adalm-Pluto to full-duplex USRP hardware.

SEFI¶

Single-Event Functional Interrupt. A radiation-induced fault that puts a device into a non-functional state – a hung processor, a corrupted control register, a peripheral that stops responding – without physically damaging it. Cleared by a reset, which is the main reason spacecraft carry layered watchdogs.

SEL¶

Single-Event Latch-up. A radiation-induced fault in which an energetic particle triggers a parasitic thyristor structure in a CMOS device, creating a low-impedance path between supply and ground. Current rises sharply and the device is destroyed unless power is removed promptly. The standard mitigation on CubeSats is a current-limiting switch that trips and power-cycles the affected channel. See EPS and OBC.

SEU¶

Single-Event Upset. A bit flip in a memory cell or register caused by a single energetic particle. Non-destructive but silently corrupting: the device keeps working, with wrong data. Mitigated by EDAC, scrubbing and TMR.

SGP4¶

Simplified General Perturbations model 4. The analytical orbit propagator that TLEs are designed to be used with. TLE element values are fitted to SGP4's specific internal assumptions, so feeding them into a general-purpose Keplerian propagator produces plausible-looking but wrong results. SGP4 outputs coordinates in the TEME frame rather than J2000, another common source of quiet errors.

Slant range¶

The straight-line distance from ground station to spacecraft, and what free-space path loss and Doppler shift actually depend on – not altitude, which the two coincide with only at the zenith. For a 500 km orbit the slant range runs from 500 km overhead to about 2570 km at the horizon: a factor of five, or some 14 dB of extra path loss, and roughly 1700 km at 10° elevation. A link budget is normally closed at the minimum usable elevation rather than at the zenith.

Slew¶

A commanded reorientation of the spacecraft from one attitude to another. Slew rate – how fast the spacecraft can turn – determines how much of a ground station pass or imaging opportunity can actually be used, and is set by available actuator torque against the spacecraft's moment of inertia.

SMA¶

Shape Memory Alloy. A nickel-titanium alloy that changes crystal phase when heated, producing a strong, repeatable mechanical stroke. Used in release mechanisms (release nuts, pin pullers, frangibolts) as a cleaner alternative to burn wires: no debris, no melting residue, high actuation force, and many devices are resettable on the ground – which greatly improves how thoroughly they can be tested. Costs more and draws more power, and actuation timing depends on starting temperature.

SNR¶

Signal-to-Noise Ratio. The ratio of signal power to noise power at a receiver input, usually expressed in dB. A key figure in link budget analysis – the received SNR must exceed the minimum required for the chosen modulation and coding scheme. It depends on the occupied bandwidth, which is why a link budget usually carries C/N0 as the bandwidth-independent subtotal instead.

Solar constant¶

The solar radiant flux at 1 AU, about 1361 W/m² by current measurement. Older engineering references and most thermal-analysis practice work from 1367–1367.5 W/m², and space solar cell datasheets normalize AM0 to 1367 W/m²; the difference is under half a percent and sits well inside the seasonal swing, which is roughly ±3.5% with Earth's orbital distance. The ends of that swing – approximately 1422 W/m² at perihelion and 1318 W/m² at aphelion – are the recommended hot and cold case values in thermal analysis. See Thermal – Thermal Environment in Orbit.

Solar string¶

Cells wired in series so their voltages add – the unit a solar array is actually designed in, with a module being the physical assembly of one or more strings bonded to a panel. Strings are sized against the hot Vmp rather than the nominal, because a string that falls below what the charger needs delivers nothing at all rather than a little less. Bypass diodes across cells or sub-strings let current route around a shadowed cell instead of reverse-biasing it; blocking diodes stop the battery draining back through a shadowed or failed string. Since a series string is limited by its weakest cell, partial shadowing on a tumbling spacecraft costs more than the shadowed area suggests. See EPS – Panel electrical design.

Solder paste¶

Solder powder suspended in flux, typically 88–91% metal by weight, printed through a stencil or dispensed and then reflowed. Graded by particle size under IPC J-STD-005, from Type 3 at 25–45 µm to Type 6 at 5–15 µm – so a smaller type number means a coarser powder, which catches people out. Choose by the smallest stencil aperture rather than by fineness: finer powder carries more surface area per unit mass, oxidizes faster and has a shorter usable life. Stored refrigerated and brought to room temperature sealed. See Tools – Solder paste and flux.

Specific impulse (Isp)¶

The efficiency of a propulsion system, in seconds: the impulse delivered per unit weight of propellant, or equivalently exhaust velocity divided by g₀. It sets the exchange rate between delta-v and propellant mass through the rocket equation. Cold gas sits around 40–110 s, chemical monopropellants 150–310 s, electric propulsion from a few hundred to several thousand seconds – but electric systems buy that Isp with electrical power, which is the binding constraint on a CubeSat. See Propulsion – Delta-v: The Currency.

SPOF¶

Single Point of Failure. Any element whose failure alone ends the mission. CubeSats have many – the battery, the EPS, the OBC, the radio, the antenna deployment, the bus linking OBC to EPS – and the useful exercise is not eliminating them, which mass and volume rarely permit, but knowing where they are and ensuring each is as simple, well-tested and recoverable as possible.

Spreading factor¶

In direct-sequence spread spectrum generally, the number of chips transmitted per data bit, which sets the processing gain. In LoRa specifically, a parameter from SF7 to SF12 setting how many chips carry each symbol. Each step up doubles the chip count and the symbol duration, roughly halves the data rate and buys about 2.5 dB of sensitivity – at 125 kHz bandwidth, SF7 gives around 5.5 kbps and SF12 around 300 bps. A high spreading factor is what makes a marginal link close, at the cost of throughput, of tolerance to Doppler shift, and of time on air.

SSO¶

Sun-Synchronous Orbit. A near-polar orbit whose plane precesses at the same rate as the Earth orbits the Sun, so that the local solar time of each pass stays constant – the parameter a mission actually specifies being the local time of descending node. Valuable for imaging missions because lighting conditions are repeatable, and in the dawn–dusk variant it keeps the spacecraft in near-continuous sunlight – which transforms the power budget while making the thermal hot case harder.

Standoff¶

A threaded spacer that separates and supports stacked circuit boards inside a spacecraft, carrying both the mechanical load and, frequently, the electrical ground path between boards. Standoff material is a deliberate thermal design choice: metal standoffs couple a board to the structure as a heat sink, polymer ones isolate it. Where the structure is hard anodized and therefore non-conductive, the ground return must be designed explicitly rather than assumed through the standoffs.

Star tracker¶

An attitude sensor that images the star field and matches the observed pattern against an onboard catalog to determine absolute orientation, typically to a few arcseconds. The most accurate attitude sensor available, and the most demanding: it needs real onboard compute, careful baffle design against stray light from the Sun, Earth and Moon, and it loses lock above a few degrees per second of body rate – so it cannot help during detumbling.

State of charge (SoC)¶

The energy remaining in a battery as a fraction of its full capacity, and the complement of DoD. Onboard it is usually estimated by coulomb counting – integrating current in and out – because a lithium cell's voltage is nearly flat across the middle of its range and so a poor indicator on its own; the count drifts and wants re-anchoring at a full charge. SoC thresholds are the usual trigger for entering and leaving safe mode, and they need hysteresis so the spacecraft does not oscillate at the boundary. See EPS – Coupling with other subsystems.

Sun sensor¶

An attitude sensor measuring the direction to the Sun, and the cheapest useful attitude sensor available. Ranges from cosine detectors (a bare photodiode, 2–5° accuracy – often just a solar panel's current) through quadrant detectors (0.2–0.5°) to digital sun cameras (0.1° down to 0.01°). Needs several units for full sky coverage, can be confused by Earth albedo, and is useless in eclipse.

Swath¶

The width of the strip on the ground an imager covers in one pass, set by the across-track field of view and the altitude. For a fixed sensor it trades directly against GSD: the focal length that buys fine detail narrows the strip. Swath is what turns resolution into coverage, because a narrow swath needs the ground track to fall much closer to a target before it is in view at all, and so lengthens revisit.

System noise temperature¶

Tsys, in kelvin. The total noise a receiving system delivers, referred to a common reference plane – usually the antenna port – and the denominator of the G/T figure of merit. It sums antenna noise (galactic background, atmosphere, and warm Earth picked up by sidelobes) with feedline loss and receiver noise figure. The practical consequence at amateur UHF is that external noise dominates: a few hundred kelvin is normal for a ground station, so swapping a 1 dB LNA for a 0.5 dB one buys much less than the datasheet difference suggests. Siting and sidelobe control usually matter more.

T¶

Telecommand¶

An instruction sent from the ground to a spacecraft. Because the spacecraft cannot be reached physically and contact is intermittent, telecommands are validated at several levels – packet integrity, command validity, argument range, and whether the command makes sense in the current state – and irreversible ones are typically guarded by an arm-then-fire sequence with an independent timeout. See Flight Software.

Thermal strap¶

A flexible conductive link that carries heat between two points while allowing them to move relative to one another, decoupling thermal and mechanical design. Typically copper or aluminum braid, or pyrolytic graphite sheet (PGS), which offers higher conductivity than either metal in the same geometry.

TID¶

Total Ionizing Dose. The cumulative radiation dose a component absorbs over a mission, measured in rad or gray (1 krad(Si) = 10 Gy). TID causes gradual parameter drift and eventual failure in semiconductors. A typical LEO CubeSat mission accumulates on the order of a few krad(Si) behind normal shielding, which is why carefully screened COTS parts are viable where they would not be in a higher-radiation orbit.

Time on air¶

How long a packet actually occupies the channel, which for LoRa is considerably more than payload bits divided by bit rate. Preamble, explicit header, CRC and symbol-level rounding all add to it, and the symbol duration doubles with every step of spreading factor, so a short packet at SF12 can sit on air for seconds. Low-data-rate optimization, mandatory at the highest spreading factors, lengthens it further. Time on air is what a regulatory or self-imposed duty cycle limit is measured against, and what sets a beacon's energy cost per transmission.

Tin whisker¶

A single-crystal filament of tin that grows spontaneously out of a pure tin surface, with no applied voltage, over months to years. Whiskers reach millimeters and can short adjacent conductors – and in vacuum a whisker short can strike a plasma arc carrying far more current than the filament itself could sustain, which has taken commercial satellites out of service. NASA's conclusion after decades of investigation is that the only certain avoidance is not to use pure tin plating; alloying the tin with lead suppresses the growth, which is why spaceflight practice stayed with tin-lead while the rest of the industry moved to lead-free. Conformal coating is a mitigation, not a cure. See Tools – Solder alloy and tin whiskers.

TLE¶

Two-Line Element set. A standardized format for encoding a satellite's orbital parameters at a given epoch, used as input to propagators such as SGP4. TLEs are published by the US Space Surveillance Network (via Space-Track) and used widely for tracking and pass prediction. See GNC – Orbit Representation / TLEs.

TMR¶

Triple Modular Redundancy. Implementing a function three times in parallel and taking a majority vote on the output, so that a fault in any single instance is outvoted. Common inside radiation-tolerant FPGA designs, at roughly three times the resource cost.

Trade study¶

A structured comparison of design options against criteria that are chosen and weighted before scoring. The lasting value is less the numerical result than the recorded rationale, which is what allows a team – often a different team – to understand a design later rather than reverse-engineer it. Worth the effort for decisions that are hard to reverse (form factor, bus architecture, attitude control approach) and not for ones that are cheap to change.

Triple-junction solar cell¶

A photovoltaic cell stacking three semiconductor junctions of different bandgaps, so each converts a different portion of the solar spectrum. Built from Group III-V materials (typically GaInP/GaAs/Ge) rather than silicon, these reach around 30% beginning-of-life efficiency against roughly 20% for single-junction silicon, and are the flight standard for CubeSat solar arrays. Often abbreviated TJ. See EPS – Solar Power Generation.

TRL¶

Technology Readiness Level. A scale from 1 to 9 used to assess the maturity of a technology, from basic principles (TRL 1) through lab demonstration (TRL 4–5), relevant environment testing (TRL 6–7), to flight-proven (TRL 9). Originally developed by NASA. Commonly used in CubeSat procurement and mission risk assessment.

TT&C¶

Telemetry, Tracking, and Command. The fundamental communication function between a spacecraft and its ground segment. Telemetry is housekeeping data downlinked from the spacecraft; tracking determines its position and velocity; command is the uplink of instructions. See Comms.

Tuna can¶

The additional cylindrical volume permitted beyond the nominal CubeSat envelope on certain form factors, so named for its shape and size. Commonly used to house a deployable antenna, a propulsion nozzle or a radiator that will not fit inside the rails. Availability and dimensions vary by deployer – confirm against the payload user guide rather than assuming.

TVAC¶

Thermal Vacuum testing. Environmental testing in a vacuum chamber with a controlled thermal environment – the only realistic way to verify spacecraft thermal behavior, since removing convection fundamentally changes how heat moves. Two distinct tests are run under this heading: thermal cycling, which demonstrates survival and function across the temperature range and exposes workmanship defects, and thermal balance, which reaches steady state at a predicted case so the nodal model can be correlated against measurement. See AIT.

U¶

U (CubeSat unit)¶

The standard size increment in the CubeSat form factor system. 1U is defined as 100 × 100 × 113.5 mm with a mass limit of 2 kg. Larger form factors (1.5U, 2U, 3U, 6U, 12U, 16U…) scale linearly in one or more dimensions. Defined in the CDS.

UHF / VHF¶

Ultra-High Frequency / Very High Frequency. Radio frequency bands commonly used in CubeSat communications. VHF covers 30–300 MHz (uplinks often around 145–146 MHz); UHF covers 300 MHz–3 GHz (downlinks often around 437 MHz). Both bands are popular with amateur missions due to favorable propagation, lower required transmit power, and the availability of the SatNOGS network.

Under-voltage lockout (UVLO)¶

A protection that disconnects the load when a cell falls below a set voltage, preventing the deep discharge that can leave a lithium cell unsafe to recharge. Implemented per cell, in hardware, with hysteresis – and with automatic recovery, because a spacecraft that locks out and then waits for a ground command cannot be commanded. It is also the protection that passivation has to override deliberately at end of mission. See EPS – Protections.

V¶

V&V¶

Verification and Validation. Two complementary processes. Verification confirms that the system was built correctly (meets requirements, as demonstrated by analysis, demonstration, inspection or test – ECSS additionally counts similarity to an already-qualified item as a form of analysis). Validation confirms that the right system was built (meets the mission need). See Systems Engineering.

W¶

Watchdog¶

A timer that resets or power-cycles a device unless it is periodically "kicked" by software that is proving itself healthy. The single most important reliability mechanism on a CubeSat, usually implemented as a hierarchy: an internal MCU watchdog, an external timer IC, and an EPS-level watchdog that cuts power to the whole processor. Kicking the watchdog from a bare timer interrupt defeats its purpose, since it will keep firing while the rest of the system is deadlocked. See OBC.

Workmanship standard¶

A document defining what a hand-built assembly has to look like to be accepted – wetting and fillet shape on a solder joint, strand damage on a crimp, coverage on a conformal coat – as opposed to a design standard, which says what to build. For spaceflight electronics the governing document is IPC J-STD-001 with its space addendum, with IPC-A-610 as the photographic acceptability counterpart and NASA-STD-8739.6 covering NASA implementation. A CubeSat flying as a secondary payload is rarely held to any of them, but they are what a trained inspector applies, and they are specific in a way no tutorial is. See Standards – Workmanship.

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