# [amsat-bb] Doppler shift question

k6vug at sbcglobal.net k6vug at sbcglobal.net
Wed Jun 5 18:31:18 UTC 2019

Let me take a stab at this...

The obvious thing about doppler is that, as the sat comes at you the frequency sounds higher than  its normal frequency and lower when it goes away from you. Hence we set our receivers at a slightly "higher"  frequency at AOS and step lower until LOS.

The not-so-obvious thing is that a similar effect happens "at the satellite", so if we transmit at the normal frequency, it will appear slightly higher at the satellite, as it comes at you. Since the satellite receiver cannot change, we compensate by transmitting at a slightly lower frequency so it is "normal at the satellite" and we keep stepping it up until LOS.
The amazing/confusing thing about space is that everything is relative, i.e., the sat and the earth station are just moving relative to each other.

Hope that helps !

73!
Umesh
k6vug

On Wednesday, June 5, 2019, 10:45:46 AM PDT, 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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