Mission Engineering Glossary
Key terms from the SMAD methodology, requirements engineering, and spacecraft mission design.
Attitude Control System
ACSThe subsystem responsible for maintaining and changing the orientation of a spacecraft. Includes sensors (star trackers, sun sensors, gyroscopes) and actuators (reaction wheels, thrusters, magnetic torquers).
Apogee
The point in an orbit farthest from the central body. For Earth orbits, the highest altitude above Earth's surface.
Bus
The structural and functional core of a spacecraft that houses and supports the payload and all subsystems. Includes power, thermal, communications, propulsion, and attitude control.
Budget (Mass/Power/Link)
An accounting of resource allocation across spacecraft subsystems with margins. Mass budgets track weight, power budgets track electrical consumption, and link budgets track signal strength through the communications chain.
See budget tools in Features →Concept of Operations
ConOpsA document describing how a system will be used from the user's perspective. Covers operational scenarios, mission timeline, ground segment operations, and contingency procedures.
Controlled Unclassified Information
CUIInformation that requires safeguarding or dissemination controls per law, regulation, or government-wide policy, but is not classified. Common in defense and government space programs.
Critical Design Review
CDRA phase gate review that evaluates the detailed design to ensure it satisfies the requirements and can proceed to fabrication, manufacturing, or coding. Typically occurs at the end of Phase C.
See phase gate use case →Delta-V
ΔVThe change in velocity required for an orbital maneuver. A fundamental measure of the propulsive effort needed to transfer between orbits, maintain station, or deorbit.
Downlink
Data transmission from a spacecraft to a ground station. Downlink budget analysis determines if the signal is strong enough to be received with acceptable bit error rate.
Eclipse
The period during an orbit when the spacecraft is in the shadow of the central body and cannot generate solar power. Eclipse duration drives battery sizing and thermal design.
End of Life
EOLThe planned end of a spacecraft's operational mission. EOL power and thermal analysis accounts for degradation of solar panels, batteries, and thermal coatings over the mission lifetime.
Geostationary Orbit
GEOA circular orbit at approximately 35,786 km altitude with zero inclination and an orbital period matching Earth's rotation. Spacecraft in GEO appear stationary relative to a ground observer.
Ground Segment
The collection of ground-based systems that support a space mission, including ground stations, mission control centers, data processing facilities, and communication networks.
Highly Elliptical Orbit
HEOAn orbit with high eccentricity, providing long dwell times over specific regions at apogee. Molniya and Tundra orbits are common HEO types for high-latitude coverage.
Inclination
The angle between the orbital plane and the equatorial plane. Determines the range of latitudes the spacecraft ground track covers. A 90-degree inclination is a polar orbit.
International Traffic in Arms Regulations
ITARUS regulations controlling the export and import of defense-related articles, services, and technical data. Most spacecraft and satellite technologies are ITAR-controlled.
ITAR compliance details →Link Budget
A detailed accounting of all gains and losses in a communications link from transmitter to receiver. Includes transmit power, antenna gains, path loss, atmospheric losses, and receiver sensitivity.
Low Earth Orbit
LEOAn orbit with an altitude between roughly 200 km and 2,000 km. LEO offers lower latency, higher resolution imaging, and lower launch costs, but requires more satellites for continuous coverage.
Margin
The difference between a resource allocation and the current best estimate. Positive margin means the design has room. Margin policies typically require 20-30% margin at PDR and 10-15% at CDR.
Mass Budget
A comprehensive accounting of the mass of every component, subsystem, and the total spacecraft. Includes dry mass, wet mass (with propellant), and contingency margins at each level.
Medium Earth Orbit
MEOAn orbit with altitude between LEO and GEO, typically 2,000 to 35,786 km. GPS and navigation constellations operate in MEO, balancing coverage area against signal latency.
Mission Assurance
Systematic activities ensuring a mission meets its requirements with acceptable risk. Covers reliability engineering, quality assurance, parts screening, and failure analysis.
Mission Lifetime
The planned operational duration of a space mission. Drives design choices for component reliability, radiation hardening, propellant budget, solar panel degradation, and battery cycle life.
Orbit Determination
ODThe process of estimating a spacecraft's orbit from tracking observations. Uses ground-based radar, GPS, or optical measurements to compute position and velocity.
Payload
The instrument or system that performs the mission's primary function. For Earth observation: cameras or sensors. For communications: transponders. For science: spectrometers, magnetometers, etc.
Perigee
The point in an orbit closest to the central body. For Earth orbits, the lowest altitude above Earth's surface.
Phase Gate
A review milestone where a program's readiness to proceed to the next development phase is evaluated. Standard gates include SRR, PDR, CDR, and TRR.
Phase gate review use case →Power Budget
An accounting of electrical power generation (solar panels, batteries) and consumption (subsystems, payload, heaters) across all mission modes including eclipse and contingency.
Preliminary Design Review
PDRA phase gate review that evaluates the preliminary design to confirm it meets requirements and the design approach is feasible. Typically occurs at the end of Phase B.
Requirements Traceability Matrix
RTMA document or tool that maps each requirement to its source (upstream traceability) and its verification method (downstream traceability). Ensures every requirement is tested and every test has a purpose.
RTM in Features →Revisit Time
The time interval between consecutive opportunities for a satellite to observe the same ground location. Key performance parameter for Earth observation constellations.
Risk Matrix
A grid plotting risk likelihood against consequence severity. Used to categorize and prioritize program risks. Typically 5x5 with color coding (green, yellow, red) for risk levels.
Risk Register
A structured list of identified program risks with likelihood scores, consequence scores, mitigation plans, owners, and status. Central artifact for program risk management.
Risk management features →Space Mission Analysis and Design
SMADThe standard methodology for spacecraft mission engineering, covering the full lifecycle from concept studies through operations. Defined in the textbook series by Wertz, Everett, and Puschell.
Sun-Synchronous Orbit
SSOA near-polar LEO orbit where the orbital plane precesses at the same rate as Earth's revolution around the Sun. Provides consistent lighting conditions for Earth observation.
Suspect Link
A traceability link flagged for review because the source or destination requirement has been modified. Suspect links indicate that downstream requirements may need updating.
Read: Why traceability breaks →System Requirements Review
SRRA phase gate review that evaluates whether the system requirements are complete, consistent, and testable before proceeding to preliminary design. Typically occurs at the end of Phase A.
Test Readiness Review
TRRA phase gate review confirming that the test program, test procedures, test facilities, and test articles are ready for formal testing. Occurs before environmental and acceptance testing.
Thermal Control System
TCSThe subsystem that maintains spacecraft temperatures within acceptable ranges. Uses passive methods (coatings, MLI, radiators) and active methods (heaters, heat pipes, louvers).
Uplink
Data or command transmission from a ground station to a spacecraft. Uplink budget analysis ensures commands are received reliably.
Verification
The process of confirming a requirement has been met. Methods include test (T), analysis (A), inspection (I), and demonstration (D). Each requirement should have an assigned verification method.
See these concepts in action
SMAD Portal implements these concepts as interactive tools. Start a pilot to see how they work with real mission data.