An amateur radio satellite is an artificial satellite built and used by amateur radio operators. It forms part of the Amateur-satellite service. These satellites use amateur radio frequency allocations to facilitate communication between amateur radio stations.
Many amateur satellites receive an OSCAR designation, which is an acronym for Orbiting Satellite Carrying Amateur Radio. The designation is assigned by AMSAT, an organization which promotes the development and launch of amateur radio satellites. Because of the prevalence of this designation, amateur radio satellites are often referred to as OSCARs.
These satellites can be used free of charge by licensed amateur radio operators for voice (FM, SSB) and data (AX.25, packet radio, APRS) communications. Currently, over 18 fully operational amateur radio satellites are in orbit. They may be designed to act as repeaters, as linear transponders, and as store and forward digital relays.
Amateur radio satellites have helped advance the science of satellite communications. Contributions include the launch of the first satellite voice transponder (OSCAR 3) and the development of highly advanced digital "store-and-forward" messaging transponder techniques.
The Amateur Radio Satellite community is very active in building satellites and in finding launch opportunities. Lists of functioning satellites need updating regularly, as new satellites are launched and older ones fail. Current information is published by AMSAT. AMSAT has not been actively involved in the launch and operation of most amateur satellites in the last two decades beyond allocating an OSCAR number.
Satellite communications
Currently, amateur satellites support many different types of operation, including FM voice and SSB voice, as well as digital communications of AX.25 FSK (Packet radio) and PSK-31.
Mode designators
Uplink and downlink designations use sets of paired letters following the structure X/Y where X is the uplink band and Y is the downlink band. Occasionally, the downlink letter is rendered in lower case (i.e., X/y). With a few exceptions, the letters correspond to IEEE's standard for radar frequency letter bands...
| Designator |
H |
A |
V |
U |
L |
S |
S2 |
C |
X |
K |
R |
| Band | 15 m | 10 m | 2 m | 70 cm | 23 cm | 13 cm | 9 cm | 5 cm | 3 cm | 1.2 cm | 6 mm |
| Frequency (General) |
21 MHz | 29 MHz | 145 MHz | 435 MHz | 1.2 GHz | 2.4 GHz | 3.4 GHz | 5 GHz | 10 GHz | 24 GHz | 47 GHz |
Prior to the launch of OSCAR 40, operating modes were designated using single letters to indicate both uplink and downlink bands. While deprecated, these older mode designations are still widely used in casual conversation.
- Mode A: 2 m uplink / 10 m downlink
- Mode B: 70 cm uplink / 2 m downlink
- Mode J: 2 m uplink / 70 cm downlink
Doppler shift
Due to the high orbital speed of the amateur satellites, the uplink and downlink frequencies will vary during the course of a satellite pass. This phenomenon is known as the Doppler effect. While the satellite is moving towards the ground station, the downlink frequency will appear to be higher than normal. Hence, the receiver frequency at the ground station must be adjusted higher to continue receiving the satellite. The satellite in turn, will be receiving the uplink signal at a higher frequency than normal so the ground station's transmitted uplink frequency must be lower to be received by the satellite. After the satellite passes overhead and begins to move away, this process is reversed. The downlink frequency will appear lower and the uplink frequency will need to be adjusted higher. The following mathematical formulas relate the Doppler shift to the velocity of the satellite.
| Where: | ||
|---|---|---|
| fd | = | doppler corrected downlink frequency |
| fu | = | doppler corrected uplink frequency |
| f | = | original frequency |
| v | = | velocity of the satellite relative to ground station in m/s. Positive when moving towards, negative when moving away. |
| c | = | the speed of light in a vacuum (3×108 m/s). |
| Change in frequency | Downlink Correction | Uplink Correction |
|---|---|---|
|
Δf=f×vc |
fd=f(1+vc) |
fu=f(1−vc) |
Due to the complexity of finding the relative velocity of the satellite and the speed with which these corrections must be made, these calculations are normally accomplished using satellite tracking software. Many modern transceivers include a computer interface that allows for automatic doppler effect correction. Manual frequency-shift correction is possible, but it is difficult to remain precisely near the frequency. Frequency modulation is more tolerant of doppler shifts than single-sideband, and therefore FM is much easier to tune manually.
FM satellites
A number of low Earth orbit (LEO) OSCAR satellites use frequency modulation (FM).[6] These are also commonly referred to as "FM LEOs" or the "FM Birds". Such satellites act as FM amateur radio repeaters that can be communicated through using commonly available amateur radio equipment. Communication can be achieved with handheld transceivers using manual doppler correction.[7] Satellite passes are typically less than 15 minutes long.