[amsat-bb] Re: Question about doppler correction software
ka1rrw at yahoo.com
Fri Aug 31 11:07:19 PDT 2007
The Doppler is not a constant.
For ISS at 240 miles altitude (145.800 mhz), the
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
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.
--- Scott Wilson <s.wilson at 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.
> Sent via AMSAT-BB at amsat.org. Opinions expressed are
> those of the author.
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