ISS on 437.800 MHz: the cross-band voice repeater

What you hear

FM voice through the most famous repeater in the sky: a queue of amateurs calling through the station, a callsign, a grid square, a quick 73, and the next one, for the ten minutes of a pass. Over Europe and North America the queue is long and several stations often key up together, so the repeater carries whichever is strongest and the rest try again; the regulars keep it to a few seconds each, and a full contact is a callsign heard both ways. The audio is clean when the station is high and noisy near the horizon.

The difficulty is the frequency. The downlink is on 70 centimetres, where the Doppler shift of a station moving at seven and a half kilometres a second is about 10 kHz: the signal arrives on 437.810 MHz as the station rises, passes through 437.800 at the middle of the pass and ends on 437.790 as it sets. A receiver left on 437.800 hears the middle well and the ends distorted or silent, so you follow the signal down in 5 kHz steps, as everyone with a handheld does.

Who transmits and why

ARISS, the amateur radio programme on the station. The repeater is part of the radio system installed in the Columbus module and switched on in September 2020, and it is cross-band on purpose: with the uplink on 2 metres and the downlink on 70 centimetres, an ordinary dual-band handheld can work it, and the two bands do not interfere with each other in the radio. Stations transmit on 145.990 MHz with a 67.0 Hz tone; listening needs no tone and no transmitter at all. The repeater is on most of the time, but the same radio also serves other modes, so during SSTV events, school contacts or maintenance the repeater is off for a while; the ARISS pages announce the changes.

When and where to listen

Almost every pass, four to six times a day from any place between about 60 degrees north and south, each pass up to ten minutes, in groups an hour and a half apart; the footprint is about 2000 kilometres in radius. Evening passes over Europe and daytime passes over North America are the crowded ones; passes over the oceans may carry a single station or none. The receiver must have the 70 centimetre band and be inside the footprint at the time, and the higher the pass, the stronger and longer the signal.

In ShackDeck

Station catalogue → ISS → ISS: voice repeater. When you click, the program asks where the station is at that moment, looks for a public receiver with 70 centimetres inside its footprint, takes the nearest one, tunes it to 437.800 MHz in FM and tells you which receiver the station is over; no receiver, antenna or account of your own is needed. Follow the Doppler with the tuning knob or the mouse wheel, 10 kHz high at the start and down to 10 kHz low at the end. The Satellites tab does the arithmetic for you: it lists the station's transmitters with the downlink corrected for the Doppler at this moment next to a Listen link, shows the track and footprint on the map, the passes of the next 24 hours with rise, maximum elevation and set, and a sky plot, with a bell that reminds you ten minutes before a pass. When the station is out of reach of every receiver, the program says where it is and when the next pass over your own location begins, from the locator in the settings.