The Doppler is not a constant. Example: For ISS at 240 miles altitude (145.800 mhz), the Maximum doppler is 3.3k. however, most of the time it will not get that high. That is only when the pass is directly over your house (90 degrees).
If you assume a 80 degree pass, then for the first minute the doppler will be 145.803~ for minute 4 the doppler will be 145.800~ and for the last minute the freq will be 145.797~
Leo SSB birds are much harder.
If the pass for your oribt is only 45 degrees, the maximum doppler will be around 2.5k.
The best way to compensat but the most complex is to have a tracking program generate the values in real time for uplink and down link seperatly and then feed thoese into your radio.
I just do it manually.
For FM, you only need to be withing 2khz for a good signal. Just preprogram a few odd splits. For ISS, 3 pre set channels will work. For 2/440 FM birds, 7 preset channels will do the job.
example for IsS voice. a similar pattern will work for 2/440 birds Channel 1 145.802.5 RX 144.487.5 TX Channel 2 145.800.0 RX 144.490.0 TX Channel 3 145.797.5 RX 144.492.5 TX
this link has some tips for using odd splits on ISS FM, a similar theory can be applied to FM birds.
http://www.marexmg.org/fileshtml/roundtableiss.html
--- Scott Wilson s.wilson@yahoo.com wrote:
I have a question regarding software doppler correction - in particular when using linear satellites.
I am working on some control software that is a little more flexible than what I've used. I really like SatPC32 on Windows and Predict/gsat on Linux. I haven't yet fallen in love with any available software, but that's really a different story.
I can imagine that doppler correction for an FM sat is rather easy in that you simply tune the transmitter/receiver to the published frequencies depending on the operating mode.
I'm a little less sure how that would work for a linear satellite. As mentioned before, I do have a chart from AMSAT that lists channelized frequencies for them. If I can tune the transmitter such that the frequency at the satellite is one of the listed frequencies, how sure can I be that the receive is as published?
Can I extend that to the point where I would be able to automatically determine frequencies (at the satellite) of the downlink for a given uplink and then compute doppler from there? It would then be fairly convenient to tune within the satellite's passband and let the computer do the 'thinking'.
Thanks for any feedback.
Scott NW2S
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