ISS on 145.825 MHz: the APRS digipeater in orbit

What you hear

In FM, the buzzing bursts of 1200-baud packet radio, the same sound as the APRS frequency in any city, but sparser and coming from much further away. During a pass over Europe a burst arrives every few seconds: a station in Portugal, then one in Finland, then one in Turkey, each packet having gone up four hundred kilometres to the station and back down over the whole footprint. Between them, now and then, the station's own beacon identifies the digipeater. By ear it is all the same noise; the map is where it comes alive.

Decoded, the packets are positions and short texts. Most are from amateurs who send a position with a greeting precisely so that it goes through the station; the path in the packet shows that it did. A few are messages between two stations, bounced off the orbit, and some are from automatic stations that beacon through every pass. The spread is the signature: terrestrial APRS shows what is within a hundred kilometres, the ISS shows the whole continent at once.

Who transmits and why

The radio belongs to ARISS, the amateur radio programme on the station, and the packet system has been part of it, in one form or another, since the early 2000s; the current equipment sits in the Columbus module. The idea is the oldest one in satellite amateur radio: a repeater in orbit that anyone can use with a handheld radio, a packet modem and a few watts, with packets crossing 2000 kilometres and more in one hop. 145.825 MHz is the frequency satellites use for APRS, so other small satellites with digipeaters have been heard there too. The stations sending through the ISS are amateurs on the ground, from clubs to individuals on a balcony.

When and where to listen

Almost every pass, around the clock: the digipeater is on most of the time, and only occasionally switched off when the radio is needed for something else. Passes last up to ten minutes and come four to six times a day for any place between about 60 degrees north and south, in groups an hour and a half apart, with a footprint about 2000 kilometres in radius. Passes over Europe and North America are the busiest, with a packet every few seconds; passes over the oceans carry little. The Doppler shift on 2 metres, around 3.5 kHz, is within what an FM receiver tolerates; nothing to retune. A receiver with 2 metres inside the footprint is all that is needed.

In ShackDeck

Station catalogue → ISS → ISS: APRS digipeater. When you click, the program asks where the station is at that moment, looks for a public receiver with 2 metres inside its footprint, takes the nearest one, tunes it to 145.825 MHz in narrow FM and sets the APRS mode; no receiver, antenna or account of your own is needed. The packets appear in the APRS panel under the waterfall as they come, with the callsign, the kind of station and what it sent, and every station lands on the map with its symbol and callsign; a click on a symbol shows the details, and a click on a row centres the map on that station. By the end of a pass the map has dots from one edge of the continent to the other. 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; the Satellites tab shows the track and footprint, the passes of the next 24 hours and a bell for ten minutes before.