SO-50: the FM repeater satellite on 436.795 MHz

What you hear

FM voice from orbit for ten to twelve minutes, the length of a pass. The satellite has one channel, and on a pass over Europe or North America a queue of stations works through it: a callsign, a grid square, a quick report, 73, and the next one. Several people often key up at once and the repeater carries whichever is strongest, so there are collisions and repeats; the regulars keep it short. Near the horizon the signal is noisy; overhead it is as clean as a local repeater.

What is unusual is the frequency rather than the sound. The satellite moves at about seven and a half kilometres a second, and on 70 centimetres the Doppler shift is about 10 kHz: the downlink arrives on 436.805 MHz as the satellite rises, passes through 436.795 at the middle of the pass and ends on 436.785 as it sets. A receiver left on the nominal frequency hears the middle of the pass well and the ends distorted or silent; on a tunable receiver you follow the signal down on the waterfall.

Who transmits and why

SaudiSat 1C was built by the Saudi space research institute and launched on 20 December 2002 with a group of small satellites. Its amateur payload got the OSCAR number 50, hence SO-50: an FM repeater listening on 145.850 MHz and transmitting on 436.795 MHz with a fraction of a watt. The uplink needs a 67.0 Hz tone, and a 74.4 Hz tone sent for a couple of seconds switches the repeater on for ten minutes when it has gone quiet, which is why a pass sometimes starts with silence and then comes alive. It has outlived everything launched with it and is the longest-serving FM satellite in amateur use, the one most people make their first satellite contact through, since a handheld and a small beam are enough. Listening needs no tone and no transmitter at all.

When and where to listen

From a low orbit a few hundred kilometres up, SO-50 passes over any given place four to six times a day, in clusters a few hours apart, each pass up to twelve minutes; the high passes are long and clear, the low ones short and noisy. The footprint is a circle a couple of thousand kilometres across, so a receiver must be inside it at that moment, and it must have the 70 centimetre band. Passes over Europe in the evening and over North America in the daytime are the crowded ones; passes over the oceans carry a few stations or none.

In ShackDeck

Station catalogue → Fax and digital signals → SO-50: the satellite FM repeater. When you click, the program asks where the satellite is at that moment, looks for a public receiver with 70 centimetres inside its footprint, takes the nearest one, tunes it to 436.795 MHz in FM and tells you which receiver the satellite is over; no receiver, antenna or account of your own is needed. If the satellite is over an ocean, with no receiver under it, the program says where it is and when the next pass over your own location begins, using the locator from the settings. Follow the Doppler with the tuning knob or the mouse wheel: 10 kHz high at the start, down to 10 kHz low at the end. The Satellites tab shows the same satellite on the map with its track and footprint, the passes of the next 24 hours with rise, maximum elevation and set, a sky plot, and its transmitters with the Doppler-corrected downlink next to a Listen link; a bell there reminds you ten minutes before the pass.