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Doppler tuning



Domenico I8CVS wrote:

For satellite work it is a good practice to keep the receiver frequency
constant and compensate for Doppler using the VFO of  our transmitter.
Using this procedure each participant in a QSO will stay in the same frequency all the time for the benefit of the others instead to ride up and down tuning the band.

Wayne replies:

This is only true if all the stations in the satellite footprint have the same Doppler effects.  When AO40 is 60,000km away, it is indeed a reasonable approximation that everybody has the same Doppler.

Widely spaced ground stations will have different Doppler shifts when the satellite is closer to earth (LEO satellites or AO40 near perigee).  Under this circumstance the best solution is "full Doppler tuning", where every station stays on the same frequency at the satellite's transponder.  This requires automated tuning of both the uplink and downlink frequency.  KB5MU's classic 1994 article "The One True Rule for Doppler Tuning" is fascinating reading:

http://www.amsat.org/amsat/features/one_true_rule.html <http://www.amsat.org/amsat/features/one_true_rule.html> 

Several satellite tracking programs now support full Doppler tuning: The Station Program, InstantTrack/InstantTune, SatPC32, WiSP, and others.

Wayne Estes W9AE
Mundelein, IL, USA
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