A fun ham activity is making contacts via satellites. Not only is there the romantic notion of sending messages into outer space, but you have to trace the orbit of the satellite with your antenna while tuning the radio, to compensate for the Doppler effect.
Working the satellites does not require any expensive investments in equipment, and
can be done using a regular 2m/440 HT and a small hand held antenna. It can also be enjoyed by Technician class licenses.
An amateur radio satellite is a little like a repeater in space, a station that relays signal over a broad territory because of the height of the transmitter.
A number of Radio Amateur satellites have been launched, operated, and reached end of life. A number of them are still usable, some even multiple times a day.
There are new ones under development, particularly through AMSAT. Amateur radio satellites will vary by the bands they use/make available, modes, and orbital characteristics.
The International Space Station (ISS) also has a repeater onboard that hams can use, but occasionally, if you’re lucky, the astronauts turn on their radios to make contact directly with hams on the ground.
Modes of operation
Typical configurations include uplink communications (signals TO the satellite) being on separate bands from downlink communications (signals FROM the satellite). This is done to achieve isolation between the signals, as is done on earth by separating a repeater's receive and transmit frequencies.
The modes a satellite operates in will vary by design and in some cases can be changed by control stations. Common modes include FM (voice), single sideband (voice), and digital.
Some digital-capable satellites may operate in a "store and forward" mode instead of immediately reflecting or repeating what they "hear". A store-and-forward configuration offers the (perhaps multiple) retransmission of the data either for a number of times, at a certain time, or when the satellites approximate position reaches a certain area or range.
So far, AMSAT-originated amateur radio satellites have been active in two orbit configurations known as HEO (High Earth Orbit) and LEO (Low Earth Orbit). Generally speaking, HEO's are geo-synchronous or geo-stationary, that is they seem to stay in the same position with respect to an earth-based observer's viewpoint. LEO's will have relatively rapid "passes" that will appear as if they come from different directions and at varying times. This appearance is because both of their orbital paths and earth's rotation. Note that while there's been much interest in placing a HEO(geo-synchronous) amateur satellite, none are currently operational in that orbit.
These are currently working satellites, with uplink and downlink on UHF and/or VHF.
Check the AMSAT Page for up to date information on what satellites are operational and what frequencies to use.