[amsat-bb] Doppler shift question

[Dave Taylor]

Look at it from the satellite’s point of view.  The satellite is receiving your Doppler-shifted signal.  If you continue transmitting on a fixed frequency, the satellite sees the frequency dropping.  To keep the frequency seen by the satellite steady, you need to offset the drop by increasing your transmit frequency at the same rate.

Dave, W8AAS

> On Jun 5, 2019, at 1:41 PM, Philip Jenkins via AMSAT-BB <amsat-bb at amsat.org> wrote:
> 
> This came up at AMSAT Academy at Hamvention, and I still can't wrap my head
> around it (something simple I'm not getting, I'm sure). I know the
> xmit/receive frequencies aren't shifted, stay the same at the satellite.
> 
> SO-50 has a 435 Mhz downlink; as the satellite approaches me from AOS I
> lower my receive frequency (and continue lowering it as the bird approaches
> LOS). So far so good.
> 
> AO 91/92 have a 435 Mhz uplink,; as the satellite approaches me from AOS, I
> go up in my transmit frequency.
> 
> Here is where I get lost: Why do I* lower* the frequency on 435 Mhz when
> receiving a satellite, but *raise* the 435 Mhz frequency when transmitting
> to a satelllite?
> 
> So, my question boils down to - why should transmit doppler shift go in the
> opposite direction from receive on the same band? In both cases, the
> satellites are approaching me (from AOS).
> 
> Basically, why the difference when I'm transmitting  and when I'm
> receiving?
> 
> 73
> 
> Philip N4HF
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