modes

Modes

  • V 2m
  • U 70 cm

FM Satellites

Name Code Uplink Mode Downlink Mode PL Notes Status
Saudi-OSCAR 50 SO-50 145.850 FM 436.795 FM 67.0 (74.4 to activate) Active
Hope Oscar 68 HO-68 145.825 FM 435.675 FM 67.0 Active Beacon only
ISS3 ARISS 437.800 FM 145.800 FM N/A Active
European-OSCAR 806 EO-80 435.080 FM 145.840 FM 210.7 Active Beacon only

Doppler shift[edit] 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 and therefore, the receiver frequency at the ground station must be adjusted higher in order 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 in order to be received by the satellite. After the satellite passes overhead and begins to move away, this process reverses itself. 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:

f{d} = doppler corrected downlink frequency

f{U} = 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 (3×10^8 m/s).

Change in frequency Downlink Correction Uplink Correction

  • Delta F = f x (v/c)

Downlink Correction

  • Fd = f(1+(v/c))

Uplink Correction

  • Fu = f(1-(v/c))

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.