Ham Radio Satellite Glossary
AOS, TCA, LOS, TLE, and every other term you'll encounter
Satellite operating comes with its own vocabulary. Some terms come from aerospace, some from ham radio convention, and some are specific to the satellite operating community. This glossary covers everything you'll encounter when using Ham Sat Tracker or reading about satellite operations.
Pass Timing Terms
AOS — Acquisition of Signal
The moment a satellite rises above your horizon and its signal becomes receivable. AOS marks the start of a usable pass. Ham Sat Tracker shows the time and azimuth (compass direction) of AOS for each pass.
TCA — Time of Closest Approach
The point in the pass when the satellite is closest to you — which corresponds to its maximum elevation angle. At TCA the Doppler shift is near zero and signals are typically strongest. Also called MEL (Maximum ELevation) point.
LOS — Loss of Signal
The moment the satellite drops below your horizon and the signal is lost. LOS marks the end of the usable pass. Ham Sat Tracker shows the time and azimuth of LOS for each pass.
Pass Duration
The total time from AOS to LOS. Typical LEO satellite passes last 5–15 minutes depending on the maximum elevation. Higher-elevation passes are longer; low scraper passes may be only 3–4 minutes.
Orbital Terms
TLE — Two-Line Element Set
A standardised data format describing a satellite's orbital parameters — its altitude, inclination, eccentricity, and other values needed to calculate its position at any given time. TLEs are published by CelesTrak and updated regularly. Ham Sat Tracker fetches current TLEs automatically and caches them for offline use.
LEO — Low Earth Orbit
An orbit at roughly 160–2,000 km altitude. All amateur radio FM and SSB satellites operate in LEO. The relatively low altitude means passes are brief (minutes rather than hours) but signals are strong and equipment requirements are modest.
Orbital Inclination
The angle between a satellite's orbital plane and the equator. A 0° inclination is equatorial (always over the equator). Most amateur satellites have inclinations of 40–100°, meaning they pass over a wide range of latitudes. Near-polar orbits (inclination ~90°) pass over nearly every point on Earth.
Footprint
The circular area on Earth's surface from which a satellite is visible above the horizon at a given moment. For a satellite at 400 km altitude the footprint diameter is roughly 4,500 km — meaning operators spread across a huge area can all use the satellite simultaneously during a pass.
Eclipse
When a satellite passes through Earth's shadow and receives no sunlight. Solar-powered satellites like AO-91 go offline during eclipse. Battery-backed satellites like SO-50 continue operating. Eclipse periods vary by season and orbital geometry.
Antenna and Pointing Terms
Azimuth
The compass bearing to the satellite, measured in degrees clockwise from North. 0° = North, 90° = East, 180° = South, 270° = West. Ham Sat Tracker shows azimuth at AOS, TCA, and LOS for each pass.
Elevation
The angle above the horizon to the satellite, in degrees. 0° is the horizon, 90° is directly overhead (zenith). Higher elevation passes produce stronger signals and are more likely to clear local obstructions. Passes below 10° are often difficult to work reliably.
Maximum Elevation (Max El)
The highest elevation angle the satellite reaches during a pass, occurring at TCA. A pass peaking at 70° is near-overhead and excellent. A pass peaking at 10° barely clears the horizon. Ham Sat Tracker shows maximum elevation for every pass.
Rotator
A motorised mount that automatically points a directional antenna toward a satellite as it moves across the sky. Used for fixed station work. Portable operators track manually using a handheld yagi and a pass predictor like Ham Sat Tracker's AntTrack feature.
Polar Plot / Sky View
A circular diagram showing a satellite's track across the sky during a pass, viewed from above. The outer edge represents the horizon, the centre represents the zenith. Ham Sat Tracker displays a sky view polar plot for every pass.
Frequency and Signal Terms
Uplink
The frequency you transmit on to reach the satellite. For SO-50 the uplink is 145.850 MHz. The satellite receives your uplink signal and retransmits it on the downlink frequency.
Downlink
The frequency the satellite transmits on, which you receive. For SO-50 the downlink is 436.795 MHz. The downlink is what you tune your receiver to during a pass.
Doppler Shift
The change in received frequency caused by the relative motion between the satellite and your location. As the satellite approaches, the downlink appears higher than nominal. As it recedes, it appears lower. The total shift from AOS to LOS can be 6–9 kHz on 70 cm. Ham Sat Tracker calculates Doppler-corrected frequencies automatically.
Transponder
The equipment on a satellite that receives signals on one frequency band and retransmits them on another. FM transponders work like repeaters — one signal in, one signal out. Linear transponders handle a full passband of frequencies simultaneously, allowing multiple QSOs at once.
Linear Transponder
A satellite transponder that receives and retransmits a range of frequencies simultaneously. Multiple stations can use the satellite at the same time on different frequencies within the passband. FO-29, RS-44, and AO-7 use linear transponders. Also called a bent-pipe transponder.
Inverting Transponder
A linear transponder that reverses the position of signals within the passband — an uplink signal at the low end of the uplink band appears at the high end of the downlink band. Most amateur SSB satellites use inverting transponders, which means LSB on uplink produces USB on downlink.
CTCSS — Continuous Tone-Coded Squelch System
A subaudible tone transmitted alongside your audio to open the squelch of an FM repeater. SO-50 requires a 67.0 Hz CTCSS tone on the uplink, and the ISS repeater requires 67.0 Hz. Without the correct tone the satellite won't respond to your transmission. Also called PL tone or subtone.
Full Duplex
The ability to transmit and receive simultaneously on different frequencies. Full duplex operation lets you hear your own signal through the satellite's downlink while transmitting — useful for SSB satellites to monitor your signal position in the transponder passband. The Icom IC-9700 and IC-705 support full duplex. The Yaesu FT-818 does not.
Half Duplex
Transmitting and receiving on different frequencies but not simultaneously — you either transmit or receive, not both at once. Most handheld radios operate half duplex. Works fine for FM satellites; makes SSB satellite work slightly more challenging.
Satellite Community Terms
AMSAT — Radio Amateur Satellite Corporation
The non-profit organisation that designs, builds, launches, and operates amateur radio satellites. Founded in 1969, AMSAT has been responsible for most of the OSCAR satellites. They also maintain the satellite status page at amsat.org/status which reports current operating conditions for each bird.
OSCAR — Orbiting Satellite Carrying Amateur Radio
The naming convention for amateur radio satellites. OSCAR-1 was launched in 1961. Current satellites carry both their own names (SO-50, FO-29, RS-44) and OSCAR numbers (AO-7 = OSCAR 7). The OSCAR designation confirms the satellite carries amateur radio equipment.
Bird
Informal term for a satellite, widely used in the amateur radio satellite community. "Working the birds" means making contacts through amateur satellites.
Pass
One complete transit of a satellite across your sky — from AOS to LOS. Each orbit of the Earth produces multiple passes visible from different locations. A satellite in a typical LEO orbit may pass over a given location 4–6 times per day.
Grid Square
A Maidenhead locator — a short alphanumeric code (e.g. FN25) identifying your location on Earth. Used by satellite pass predictors, VHF contests, and for exchanging location information on the air. Ham Sat Tracker accepts grid squares as location input.
73 de VE3AKK