Glossary

Updated July 13, 2026

Plain-language definitions of the orbital-mechanics terms used across the Orbit Visualizer and this documentation. Terms link to the guide that covers them in depth.

Terms

AOS (acquisition of signal): The moment a satellite rises above a ground site's elevation mask and a pass begins. See ground sites & access.
Apoapsis (apogee): The farthest point of an orbit from the body it circles — called apogee for Earth orbits.
Argument of periapsis (ω): The angle within the orbital plane from the ascending node to periapsis. See classical orbital elements.
B* (B-star): The SGP4 drag-like coefficient carried in a TLE that scales atmospheric drag. See how to read a TLE.
Conjunction: A close approach between two orbiting objects, screened over a time window. See conjunction screening.
ΔV (delta-v): The change in velocity an impulsive maneuver imparts, in metres per second. See maneuvers.
ECEF: An Earth-centered, Earth-fixed frame that rotates with the planet. See time & reference frames.
Eccentricity (e): How far an orbit departs from a circle: 0 is circular, values near 1 are highly elongated. See classical orbital elements.
Elevation mask: The minimum elevation above the horizon that counts as a pass at a ground site. See ground sites & access.
EME2000 (J2000): The app's inertial scene frame, fixed to Earth's mean equator and equinox at J2000.0. See time & reference frames.
Epoch: The instant at which a set of elements or a state vector is valid. See time & reference frames.
GMST: Greenwich Mean Sidereal Time — the rotation angle from the inertial frame to Earth-fixed. See time & reference frames.
Ground track: The path of a satellite's subsatellite point traced over Earth's surface. See ground tracks.
Hohmann transfer: A two-burn transfer between coplanar circular orbits. See maneuvers.
Inclination (i): The tilt of the orbital plane relative to Earth's equator. See classical orbital elements.
J2: The dominant Earth-oblateness gravity term; also the app's secular-drift propagator. See propagation models.
LOS (loss of signal): The moment a pass ends as a satellite drops below the elevation mask. See ground sites & access.
Maneuver: An impulsive burn that changes an orbit, defined by time, frame, and ΔV. See maneuvers.
Mean anomaly (M): A fictitious angle that advances uniformly in time, convenient for propagation. See classical orbital elements.
Mean elements: Averaged elements meaningful only inside a model such as SGP4; contrast osculating. See how to read a TLE.
Miss distance: The minimum range between two objects at closest approach. See conjunction screening.
Nodal regression: The J2-driven secular drift of the ascending node over time. See propagation models.
Osculating elements: The instantaneous ellipse matching a state's position and velocity right now. See how to read a TLE.
Periapsis (perigee): The closest point of an orbit to its central body — called perigee for Earth orbits.
Propagator: The physics model that advances an object along its orbit: two-body, J2, or SGP4. See choosing a propagator.
RAAN (Ω): Right ascension of the ascending node — the swivel of the orbital plane about Earth's axis. See classical orbital elements.
RIC: The local frame built from a state (r, v): radial (R̂ = r/|r|), cross-track (Ĉ = (r×v)/|r×v|), and in-track (Î = Ĉ×R̂). See time & reference frames.
SDP4: The deep-space branch of SGP4 for orbital periods of about 225 minutes or more. See propagation models.
Semi-major axis (a): Half the ellipse's long axis; it sets the orbit's size and period. See classical orbital elements.
SGP4: The operational analytic propagation model that TLEs are fitted for. See propagation models.
Stale TLE: A TLE whose epoch is more than 7 days old, flagged in the catalog. See satellite catalog.
State vector: An orbit described as position and velocity (r, v) at an epoch. See state vectors.
Subsatellite point: The point on Earth directly beneath a satellite; it traces the ground track. See ground tracks.
TCA (time of closest approach): The instant two objects are nearest during a conjunction. See conjunction screening.
TEME: True Equator, Mean Equinox — SGP4's native output frame. See time & reference frames.
TLE: Two-line element set, the standard mean-element format for satellites. See how to read a TLE.
True anomaly (ν): The actual angle from periapsis to the body's current position. See classical orbital elements.
Two-body: The idealized single-attractor Keplerian model with no perturbations. See propagation models.