Reference
Spacecraft engineering glossary
Plain-English definitions for the acronyms spacecraft engineers actually use — every term our calculators lean on, in one linkable place.
// one anchor per term — deep-link any definition as /tools/glossary#tle. the calculator pages link their first-use acronyms here.
Orbits & mission geometry
LEO — Low Earth Orbit
Orbits below roughly 2,000 km altitude, where nearly all CubeSats and constellations fly (most between 400 and 600 km). Residual atmospheric drag is significant across the whole band — it is why LEO satellites decay, and why the post-mission-disposal rules apply below 2,000 km.
MEO — Medium Earth Orbit
The band between LEO and the geostationary arc. Around 20,000 km it hosts the navigation constellations (the GPS belt) — and sits in the heart of the outer trapped-electron belt, one of the harshest radiation environments a spacecraft can occupy.
GEO — Geostationary Earth Orbit
The circular equatorial orbit near 35,800 km altitude whose period matches Earth's rotation, so the satellite hangs over one longitude. The long slant range costs a comms link about 26 dB more free-space path loss than a typical LEO pass.
GSO — Geostationary-Satellite Orbit
The regulatory name for the geostationary arc as protected spectrum real estate. ITU Radio Regulations Article 22 caps the interference (EPFD) that non-geostationary systems may throw at GSO networks, which were licensed first.
NGSO — Non-Geostationary-Satellite Orbit
Any satellite system not in the geostationary arc — in practice the LEO/MEO broadband constellations. NGSO systems sharing Ku- and Ka-band spectrum with GSO incumbents must demonstrate compliance with the ITU Article 22 EPFD limits.
SSO — Sun-Synchronous Orbit
A near-polar LEO whose orbit plane precesses (via the J2 oblateness perturbation) exactly once per year, holding a constant local solar time at the ascending node. Earth-observation missions favor it for consistent lighting; a dawn-dusk SSO barely sees eclipse.
RAAN — Right Ascension of the Ascending Node
The angle that locates where an orbit plane crosses the equator heading north, measured in the inertial frame. Earth's oblateness (J2) drifts RAAN steadily — the drift that makes Sun-synchronous orbits possible and that walks a satellite's eclipse season through the year.
Beta angle (β)
The angle between the Sun vector and the orbit plane — the Sun's elevation above the plane. It sets how long each orbit spends in eclipse: at high |β| eclipses shrink or vanish entirely; at β = 0 they are longest.
TLE — Two-Line Element set
The standard two-line text format that encodes a satellite's orbit state and drag term for the SGP4 propagator. Element sets are published continuously; freshness is the whole accuracy story — pass predictions degrade within days as a TLE ages.
SGP4 — Simplified General Perturbations 4
The analytic orbit propagator built to consume TLEs, modelling the dominant perturbations cheaply enough to predict pass windows in milliseconds. TLE plus SGP4 remains the operational standard for satellite tracking and pass prediction.
Delta-v (Δv)
The velocity change a maneuver requires, in m/s — the universal currency of orbital maneuvering. Every mission carries a delta-v budget, and the rocket equation converts it (exponentially) into propellant mass.
Isp — Specific impulse
A propulsion system's fuel efficiency: thrust per unit propellant weight flow, in seconds. Effective exhaust velocity is ve = Isp · g0, which is what enters the rocket equation.
Ballistic coefficient (BC)
BC = m/(Cd·A), in kg/m² — mass over drag coefficient times cross-sectional area. It is the one spacecraft-side number in orbital decay: high BC (heavy, compact) punches through the drag and stays up; low BC (light, draggy) comes down fast, which is exactly what a drag sail exploits.
Debris compliance & reentry
PMD — Post-Mission Disposal
Clearing a spacecraft from its operational orbit after end of mission — by natural decay, a disposal burn, added drag area, or a graveyard orbit. The FCC 5-year rule and ESA Zero Debris both regulate how fast LEO disposal must complete.
ODAR — Orbital Debris Assessment Report
The debris-mitigation assessment regulators expect alongside a mission filing: the disposal plan, the predicted post-mission orbital lifetime, and the reentry casualty risk, each checked against the applicable limit.
ODMSP — Orbital Debris Mitigation Standard Practices
The US Government's debris-mitigation baseline. Among its requirements: the expected casualties from an uncontrolled reentry must stay below 1 in 10,000 — the same threshold NASA-STD-8719.14 carries.
DAS — Debris Assessment Software
NASA's reference tool for orbital-debris compliance — the program regulators expect a filing's lifetime and demise numbers to come from. First-order screens tell you where you stand; a DAS run at the actual disposal epoch produces the filing number.
ORSAT — Object Reentry Survival Analysis Tool
NASA's high-fidelity reentry-survival code: per-component aerothermal ablation, break-up altitude, and fragment-by-fragment survival through the trajectory. The certified step beyond a binary per-material demise screen.
DCA — Debris Casualty Area
The summed ground-hazard area of everything that survives reentry: DCA = Σ (0.6 + √A_i)², where the 0.6 m comes from the standard 0.36 m² human ground-projected cross-section added around each fragment. DCA times the footprint population density gives the expected casualties.
Radiation environment
TID — Total Ionizing Dose
The accumulated ionizing radiation a part absorbs over the mission, measured in krad(Si). Parts degrade slowly as TID accumulates — thresholds shift, leakage grows — so electronics are rated to a TID tolerance and shielded to stay under it.
SEE — Single-Event Effect
Any disruption caused by one energetic particle passing through a device — a bit flip (SEU), a latch-up (SEL), or a transient. Unlike total ionizing dose, which accumulates over years, an SEE is instantaneous and can strike on day one.
SEU — Single-Event Upset
A single particle depositing enough charge in a memory node to flip a bit — the most common single-event effect. Recoverable by scrubbing and error correction, but the rate drives how much protection a design needs.
SEL — Single-Event Latch-up
A particle-triggered parasitic short in a CMOS device that persists until power is cycled — potentially destructive, and the reason latch-up-immune parts or current-limited power rails matter in orbit.
LET — Linear Energy Transfer
How much energy an ionizing particle deposits per unit path length, in MeV·cm²/mg. Device single-event cross-sections are measured against LET, and convolving the orbit's LET spectrum with that cross-section gives the upset rate.
GCR — Galactic Cosmic Rays
The background flux of high-energy ions arriving from outside the solar system. Impractical to shield against, GCRs set the single-event-effect floor for any orbit — heaviest at high altitude and inclination, and at solar minimum.
EDAC — Error Detection and Correction
Memory protection that detects and corrects bit errors, usually paired with regular scrubbing so single upsets are corrected before they accumulate into uncorrectable multi-bit errors.
MTBU — Mean Time Between Upsets
The reciprocal of a device's upset rate — how long, on average, between single-event upsets. The number that turns a rate into an engineering decision: an MTBU of half a day means EDAC and scrubbing are non-negotiable.
COTS — Commercial Off-The-Shelf
Standard commercial parts rather than radiation-hardened ones. Most CubeSats fly COTS electronics — a typical unshielded COTS tolerance is around 10 krad(Si) of total ionizing dose, which is why shielding thickness and orbit choice matter.
RF & links
EIRP — Effective Isotropic Radiated Power
Transmit power plus antenna gain (minus feed losses), in dBW — what the link "looks like" radiated from an ideal isotropic antenna. The opening term of every link budget.
FSPL — Free-Space Path Loss
The geometric spreading loss over the link distance: FSPL = 20·log₁₀(4πR/λ). The dominant loss in any space link — around 180 dB for a LEO pass, over 205 dB from GEO.
C/N₀ — Carrier-to-Noise-Density Ratio
Carrier power over noise power density, in dB-Hz — a property of the radio link itself, independent of data rate. The link equation collapses to C/N₀ = EIRP − losses + G/T + 228.6.
Eb/N₀ — Energy per Bit over Noise Density
What the demodulator actually cares about: Eb/N₀ = C/N₀ − 10·log₁₀(data rate). Spend the same C/N₀ on more bits per second and each bit gets less energy — the fundamental rate-versus-margin trade.
G/T — Gain-to-Noise-Temperature
The receive system's figure of merit, in dB/K: antenna gain over system noise temperature. A bigger dish or a quieter front end both raise G/T, and every dB of it is a dB of link margin.
EPFD — Equivalent Power Flux-Density
The regulatory measure of how much interference a non-geostationary constellation throws at protected geostationary networks, capped by ITU Radio Regulations Article 22. NGSO filings must demonstrate compliance with the limits.
EDT — Electrodynamic Tether
A long conductor deployed from a satellite: moving through Earth's magnetic field it develops a motional EMF, drives a current closed through the ionosphere, and the resulting J×B force acts as propellantless drag (or thrust). Studied for both power generation and deorbit.
AOS / LOS — Acquisition / Loss of Signal
The instants a satellite rises above and drops below a ground station's usable horizon (the elevation mask) — the boundaries of a pass. Every downlink byte and every command has to fit between them.
Power & bus sizing
DoD — Depth of Discharge
The fraction of a battery's capacity drawn each cycle. A LEO battery sees roughly 15 eclipse cycles a day — tens of thousands over a mission — so DoD is held deliberately low to preserve cycle life.
BOL / EOL — Beginning of Life / End of Life
The bookends of performance accounting: solar arrays and batteries are sized so the degraded end-of-life performance still meets requirements, not the fresh beginning-of-life numbers. Array sizing typically carries an explicit BOL-to-EOL degradation factor.
ADCS — Attitude Determination and Control System
The subsystem that senses and commands spacecraft orientation — star trackers, sun sensors, and gyros for determination; reaction wheels, magnetorquers, and thrusters for control. Sized against the disturbance torques of the orbit environment.
MGA — Mass Growth Allowance
The maturity-based contingency added to each subsystem's estimated mass — the less mature the design, the more it gets. Rolled up, it is what keeps a paper design from busting the launch-mass cap when the hardware turns real.
MLI — Multi-Layer Insulation
The metallized-film blanket stack that radiatively decouples a spacecraft from its environment — the gold-foil look of most satellites. It sets the effective emissivity and absorptivity of the surfaces it covers in the thermal balance.
SRP — Solar Radiation Pressure
The momentum flux of sunlight (about 4.5×10⁻⁶ N/m² at Earth) pushing on illuminated surfaces. Tiny, but a persistent disturbance torque whenever the pressure centroid is offset from the center of mass.
PDR — Preliminary Design Review
The gate where a design's architecture and budgets are frozen enough to proceed to detailed design. Margin philosophy keys off it — for example, holding roughly 20% mass margin before PDR.
Flight software & avionics
OBC — On-Board Computer
The spacecraft's flight computer: it runs the flight software, executes commands, collects telemetry, and arbitrates faults. HawkLogic's ICARUS OS is a CubeSat OBC platform.
RTOS — Real-Time Operating System
An operating system built for deterministic timing: bounded scheduling latency, priority-driven tasks, and predictable response to interrupts. Flight software runs on an RTOS (or bare metal) because "usually fast enough" is not a guarantee.
FDIR — Fault Detection, Isolation, and Recovery
The onboard logic that notices something is wrong, contains it, and restores a safe configuration without waiting for the ground — essential when the next contact window may be hours away.
MPU — Memory Protection Unit
The microcontroller-class hardware that fences memory regions per task, so a misbehaving task faults immediately instead of silently corrupting its neighbours. Lighter than a full MMU, and standard practice in flight-grade software design.
CCSDS — Consultative Committee for Space Data Systems
The standards body whose protocols (Space Packet Protocol, CFDP file transfer, and more) define how spacecraft and ground systems talk. "CCSDS-compliant TC/TM" means commands and telemetry follow those interoperable formats.
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