Satellite and orbit glossary

Every term you will meet while tracking satellites, in plain English. These same explanations appear as inline tooltips throughout the live tracker.

Orbit regimes

LEO (Low Earth Orbit): Below 2,000 km altitude. Most satellites live here, including the ISS (~420 km) and Starlink (~550 km). Objects in LEO circle Earth every ~90 minutes.
MEO (Medium Earth Orbit): Between 2,000 and 35,786 km. Home to navigation satellites like GPS (~20,200 km) and Galileo. Orbital period is 2-24 hours.
GEO (Geostationary Orbit): Exactly 35,786 km altitude. Satellites here orbit at the same speed as Earth's rotation, appearing stationary in the sky. Used for TV, weather, and communications.
HEO (Highly Elliptical Orbit): An elongated orbit that swings close to Earth at perigee and far away at apogee. Includes Molniya and Tundra orbits used for high-latitude coverage.

SSO (Sun-Synchronous Orbit): A near-polar LEO that crosses every point on Earth at the same local solar time, ideal for consistent imaging conditions. Inclination is around 96-103°.
Polar Orbit: Inclination near 90°, so the satellite passes over both poles each revolution. Used for global Earth observation. SSO is a special case of polar orbit.
Equatorial Orbit: Inclination near zero. The satellite stays close to the equator. Most GEO satellites qualify, plus a few low-inclination LEOs.
Retrograde Orbit: Inclination greater than 90°, meaning the satellite orbits opposite to Earth's rotation. Rare and usually intentional, often for radar imaging.
Molniya Orbit: A 12-hour HEO with ~63.4° inclination. Used by Russia to give long dwell times over high northern latitudes that GEO cannot reach.
Tundra Orbit: A 24-hour HEO with ~63.4° inclination. Like Molniya but with a longer dwell over a single hemisphere. Sirius XM used these for satellite radio.
GTO (Geostationary Transfer Orbit): A temporary high-eccentricity path used to lift a satellite from LEO up to GEO. Most GTO objects end up in GEO once they fire their apogee motor.
Graveyard Orbit: About 300 km above GEO. Retired geostationary satellites are pushed up here to free space in the GEO ring.

TLE (Two-Line Element): A standardized data format that describes a satellite's orbit using six parameters (Keplerian elements). Updated regularly by the US Space Force.
NORAD ID: A unique number assigned to every object tracked in Earth orbit by the North American Aerospace Defense Command.
COSPAR ID: International designator for a space object. Format: YYYY-NNNP where YYYY is launch year, NNN is launch number that year, and P is the piece (A = payload, B/C = rocket bodies).
Epoch: The date and time when the orbital data was last measured. Older data becomes less accurate over time.
SGP4: Simplified General Perturbations model 4. The mathematical model used to predict where a satellite will be at any given time, based on its TLE data.

Inclination: The tilt of the orbit relative to Earth's equator, in degrees. 0° = equatorial, 90° = polar, above 90° = retrograde.
Apogee / Perigee: Apogee is the highest point of an orbit (farthest from Earth). Perigee is the lowest point (closest to Earth). A circular orbit has roughly equal apogee and perigee.
Eccentricity: How 'oval' an orbit is. 0 = perfect circle, closer to 1 = very elongated ellipse. Most LEO satellites have near-zero eccentricity (circular orbits).
Mean Motion: How many times a satellite completes a full orbit per day. ISS has ~15.5 rev/day (one orbit every ~93 minutes). GEO satellites have ~1 rev/day.
RAAN: Right Ascension of the Ascending Node. The angle that defines where the orbit crosses the equator going northward, measured from a fixed reference direction in space.
B* Drag: A measure of atmospheric drag on the satellite. Higher values mean the orbit is decaying faster. Satellites with very high B* may reenter Earth's atmosphere soon.

Starlink Train: A visible line of Starlink satellites shortly after launch, before they spread out to their operational orbits. Often visible to the naked eye as a 'string of pearls' in the night sky.

azmth is a free, real-time satellite tracker showing 15,000+ satellites on an interactive 3D globe. No signup, no ads.