Glossary

Every technical term used on this site, explained in plain language. If a guide or prediction uses a word you do not recognise, you will find it here.

AOS — Acquisition of Signal

The moment a satellite rises above the observer's horizon and becomes trackable or visible. The start of a pass.

Apogee

The highest point in an orbit — the moment the satellite is farthest from Earth. For the near-circular ISS orbit, apogee and perigee are almost equal.

Azimuth

Compass direction measured in degrees from true north (0°) clockwise — east is 90°, south 180°, west 270°. Used to tell you where on the horizon a pass starts or ends.

BSTAR — B* drag term

A number in a TLE that encodes atmospheric drag. Higher values mean the satellite is losing altitude faster. The ISS has a relatively high B* because it flies low enough to feel the upper atmosphere.

CBM — Common Berthing Mechanism

The standard docking interface used to attach modules on the US segment of the ISS. Two halves are bolted together by the Station's robotic arm.

Culmination

The highest point a satellite reaches during a pass, as seen from your location. Also called 'maximum elevation'. The moment the Station is closest to you and typically brightest.

Drag

The tiny but persistent force of the upper atmosphere slowing the Station down. Without periodic reboosts, drag would pull the ISS into a lower and lower orbit until re-entry.

ECI — Earth-Centred Inertial

A coordinate frame fixed to the stars, not the rotating Earth. SGP4 outputs positions in ECI; converters then rotate them into latitude and longitude.

ECEF — Earth-Centred Earth-Fixed

A coordinate frame that rotates with the Earth. Once an ECI position is converted to ECEF, you can plot it on a map.

Eccentricity

How elongated an orbit is. 0 is a perfect circle; close to 1 is a long ellipse. The ISS eccentricity (~0.0001) means its orbit is almost perfectly circular.

Elevation

The angle above the horizon, in degrees. 0° is the horizon; 90° is straight overhead (the zenith). Higher elevation passes are brighter and easier to see.

EMU — Extravehicular Mobility Unit

The American spacesuit used for spacewalks. A self-contained one-person spacecraft with oxygen, cooling, power, and communications.

Epoch

The exact date and time a TLE describes. All orbital elements in the set are a snapshot at this moment. Propagators like SGP4 wind the clock forward from here.

EVA — Extravehicular Activity

A spacewalk — any activity performed by a crew member outside the pressurised modules of the Station.

Inclination

The tilt of an orbit relative to the equator, in degrees. The ISS orbits at 51.6°, meaning it passes over everywhere between 51.6°N and 51.6°S.

ISS — International Space Station

A multinational laboratory in low Earth orbit, continuously crewed since November 2000. Operated by NASA, Roscosmos, ESA, JAXA, and CSA.

LEO — Low Earth Orbit

Orbits between roughly 200 km and 2 000 km altitude. The ISS flies at about 410 km — well within LEO.

LOS — Loss of Signal

The moment a satellite drops below the observer's horizon. The end of a pass.

Magnitude

A measure of brightness on an inverted logarithmic scale — lower (more negative) is brighter. The ISS at its best reaches about −4, rivalling Venus.

Mean anomaly

Where the satellite is along its orbit at the epoch, expressed as an angle. Combined with the other elements, it places the object at any future instant.

Mean motion

How many orbits a satellite completes per day. The ISS's mean motion of ~15.49 gives an orbital period of about 92.9 minutes.

NORAD ID

A unique catalogue number assigned to every tracked object in orbit. The ISS is object 25544.

OMM — Orbit Mean-elements Message

A modern, structured format for orbital data (XML, JSON, CSV) that carries the same information as a TLE but is easier for machines to parse.

Orlan

The Russian spacesuit used for EVAs from the ISS. Features a rear-entry hatch — the cosmonaut climbs in through the back of the suit.

Perigee

The lowest point in an orbit — the moment the satellite is closest to Earth.

Phase angle

The angle Sun–satellite–observer. A small phase angle means the satellite is nearly fully lit (face-on sunlight); a large one means you see it edge-on and dimmer.

RAAN — Right Ascension of the Ascending Node

Where the orbit crosses the equator heading north, measured against the stars. It precesses slowly due to Earth's equatorial bulge — the main reason pass times shift week to week.

Reboost

A short engine burn by a docked spacecraft (usually Progress) to raise the Station's orbit and counteract atmospheric drag.

SGP4 — Simplified General Perturbations 4

The standard mathematical model for propagating TLEs into positions. It accounts for Earth's shape, atmospheric drag, and other perturbations. Used by almost every satellite tracker, including this one.

Terminator

The boundary between the sunlit and shadowed parts of the Earth. The ISS becomes invisible when it crosses into Earth's shadow — this is often seen as a sudden fade mid-pass.

TLE — Two-Line Element set

Two lines of numbers that describe a satellite's orbit at a specific moment. Published several times a day for the ISS by sources such as CelesTrak. The foundation of all pass predictions.

Zenith

The point in the sky directly overhead — 90° elevation. A pass that reaches the zenith crosses right above you and is at its absolute brightest.