Glossary

STY-8 requires that once a term is defined here, the documentation uses that term rather than a synonym. If two pages disagree, this page wins.

Orbits and geometry

Altitude. Height above the Earth ellipsoid, in kilometers. Not the same as distance to a ground station; see slant range.

Apsidal rotation. Slow rotation of the orbit’s major axis within its plane, driven principally by Earth oblateness.

Constellation. A set of satellites designed and operated as one system.

Eccentricity. Dimensionless measure of how elongated an orbit is. Zero is circular.

ECEF. Earth-centered, Earth-fixed. A frame that rotates with the Earth, used for ground-relative geometry.

ECI. Earth-centered inertial. A non-rotating frame, used for propagation.

Epoch. The instant to which a state or element set refers. Stale epochs are a dominant error source.

Field of view. Angular width a payload can observe, in degrees. Determines the ground footprint together with altitude and pointing.

Inclination. Angle between the orbit plane and the equator, in degrees. Zero is equatorial, 90 is polar.

LLA. Latitude, longitude, altitude.

Nodal regression. Drift of the ascending node caused by Earth oblateness. The mechanism that makes sun-synchronous orbits possible.

Plane. One orbital ring. A constellation is built from several planes.

RAAN. Right ascension of the ascending node. The angle locating where an orbit crosses the equator going north.

Revisit time. Interval between successive observations of a given ground point. Usually quoted as a statistic, not a single value.

Shell. A group of planes sharing an altitude and inclination.

Slant range. Straight-line distance from satellite to ground station, in kilometers. Always greater than or equal to altitude, and much greater near the horizon.

True anomaly. Angular position of a satellite along its orbit, measured from perigee.

Walker Delta, Walker Star. Two standard patterns for distributing satellites across planes to spread coverage evenly.

Propagation

Cowell formulation. Numerical integration of the total acceleration directly. The basis of the numerical model.

Drag. Atmospheric force opposing motion, scaled by the ballistic term Cd times A over m. Dominant below roughly 500 km.

J2, J3, J4. Zonal harmonic coefficients describing Earth’s departure from a sphere. J2 is by far the largest.

Propagation. Advancing a satellite state through time.

SGP4/SDP4. The perturbation models that two-line element sets are fitted to. Valid only with TLE-derived states.

SRP. Solar radiation pressure. Force from sunlight, scaled by the radiation term Cr times A over m.

Third body. Gravitational influence of a body other than Earth, principally the Sun and Moon.

Radio frequency

Availability. Fraction of an average year for which a link meets its threshold, for example 99.9 percent. Link margins are quoted against a target.

C/N0. Carrier-to-noise density ratio, in dBHz. What the demodulator works with.

dB. Decibel. A logarithmic ratio, so gains and losses add rather than multiply.

EIRP. Effective isotropic radiated power, in dBW. Transmit power plus antenna gain minus feed loss.

Elevation mask. Minimum elevation angle at which a station will work a satellite, in degrees. Link budgets must be evaluated at the mask, not at zenith.

Free-space path loss. Loss from spreading of the wavefront over range, in dB. Geometric, not absorptive.

G/T. Receive figure of merit, in dB/K. Antenna gain over system noise temperature.

ISL. Inter-satellite link.

Margin. Decibels of headroom between the received C/N0 and the threshold the demodulator requires.

MODCOD. A paired modulation and coding scheme, each with its own required threshold.

Scintillation. Rapid signal fluctuation caused by atmospheric turbulence.

Site diversity. Using geographically separated stations so that rain fading is unlikely to affect both at once.

XPD. Cross-polarization discrimination, in dB.

Interoperability

CZML. Time-dynamic scene description consumed by the CesiumJS globe viewer. The visualization output format.

GeoJSON. Geographic vector format, used for coverage grids and station data.

OEM. CCSDS Orbit Ephemeris Message. A tabulated state-vector time series.

OMM. CCSDS Orbit Mean-Elements Message. Available in KVN, XML, and JSON.

SP3. Precise ephemeris format originating in the GNSS community.

TLE. Two-line element set. Compact mean elements distributed for catalogued objects, valid only with SGP4/SDP4.