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.

Orbit characteristics

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 orbit, very stretched and tilted near 63.4°, the tilt at which Earth's bulge does not turn its high point. Named after the Soviet Molniya communications satellites.
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.

Identifiers and data

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.
Orbital elements: The numbers that fix an orbit at one moment, from which software such as SGP4 works out where a satellite is; azmth calls them a satellite's full orbit. The older two-line format (TLE) runs out at catalogue number 99,999, which the catalogue passed on 11 July 2026.
SGP4: Simplified General Perturbations model 4. The mathematical model used to predict where a satellite will be at any given time, based on its orbital elements.

Orbital elements

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.

Watching satellites

Pass: One crossing of a satellite over your sky, from rising above the horizon to setting. A typical low-orbit pass lasts a few minutes. Whether you can SEE it depends on the satellite being sunlit while your sky is dark.
Azimuth: The compass direction to look toward, measured in degrees clockwise from north: 0° is north, 90° east, 180° south, 270° west. azmth takes its name from this word.
Elevation (Altitude): How high above the horizon something appears, in degrees. The horizon is 0°, straight overhead (the zenith) is 90°. A closed fist held at arm's length spans about 10°.
Zenith: The point directly overhead, at 90° elevation. A satellite passing near the zenith is at its closest to you and usually at its brightest.
Magnitude (Brightness): How bright an object appears; the SMALLER the number, the brighter. The ISS can reach magnitude -4 (as bright as Venus); most satellites are +3 to +6, near the limit of naked-eye visibility.
Earth's Shadow (Satellite Eclipse): Satellites shine by reflecting sunlight. When one flies into the shadow Earth casts into space, it abruptly fades out mid-sky, a signature satellite behavior often mistaken for an object "disappearing". This is why most sightings happen in the hours after sunset or before sunrise.

Military satellites

Counts, sources and the one rule for every country: Military satellites, counted and explained →

Approximate orbit: An orbit azmth can describe without placing the satellite on it. On Earth, the height and tilt the public catalogue lists for a satellite whose full orbit is not published, drawn to scale. At the Moon and Mars, a measured orbit whose current direction is unpublished or long out of date. Never a position.
No public orbit: A satellite whose orbital elements the US satellite catalogue does not publish. The catalogue's code is NEA, 'No Elements Available', and CelesTrak's page for these is called Restricted Objects. azmth may still show its listed height and tilt.
No recent orbit: A satellite whose public orbit has not been updated in 30 days. CelesTrak lists these on its Lost Objects page; azmth says no recent orbit, because the orbit was public before.
GCAT: The General Catalog of Artificial Space Objects, kept by astrophysicist Jonathan McDowell and free to reuse with credit (CC BY 4.0). azmth uses it to tell which satellites are military, what they do and whether they still work.
Military class (GCAT): GCAT's class D, 'Defense (govt. military, intelligence)', usually set from the owner or operator organisation, with exceptions. BD and CD mean commercial or civil use alongside military use.
Military* (azmth): azmth's label for a satellite GCAT does not class as military, when its own government or operator says officially that its armed forces or defence ministry own it, run it or have taken it over. Both voices are shown, each dated, and these satellites are counted beside GCAT's figure.
Signals intelligence: Collecting radio and radar signals from space. azmth uses the term where GCAT assigns the category; operators rarely describe their satellites this way.
Missile early warning (Missile warning): Watching from orbit for the heat of missile launches (GCAT's category: 'Missile early warning, including ballistic missile defence tracking'). azmth's mission chip calls it Missile warning.
Space surveillance: Watching and identifying other objects in orbit.
Rendezvous (proximity operations): Manoeuvring close to another satellite on purpose, to inspect or service it. azmth tags it only where the operator describes it.
Geodesy (Measuring Earth): Measuring Earth's shape and gravity from orbit. azmth's mission chip calls it Measuring Earth.
Radar imaging (SAR): Making images with synthetic aperture radar, which works through cloud and at night.
UN Register of Objects Launched into Outer Space: The register the UN Secretary-General keeps under the 1975 Registration Convention, from what each launching state files: each object's orbit as the state registered it and its general function, open to everyone. azmth prints these values as the registered orbit.
Satellite warning: Timing activity on the ground around when a reconnaissance satellite passes. azmth builds no tools for it.

Other

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.

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