[amsat-bb] Doppler shift question
ke4al at yahoo.com
Wed Jun 5 18:08:57 UTC 2019
SO-50 is a VHF uplink and UHF downlink (V/u) satellite. AO-91 and AO-92 are UHF uplink and VHF downlink (U/v) satellites.
While there is some doppler effect on VHF, the amount is minimal (2-3 KHz). We can pretty much ignore that, with the exception of AO-92, which can require you to make an adjustment near the end of the pass. (You will notice your received audio getting scratch.
So, let's focus on UHF doppler effect, which can change as much as +/- 10 KHz during a pass
- Remember the train example. The sound of a train at a distance and approaching will sound higher pitched, continually decreasing in pitch until it arrives (on frequency) at your location (closest), and continues decreasing in pitch as it passes and pulls a way from you.
- On SO-50, you have to adjust your UHF rx frequency to the higher pitched frequency at the start of the pass (AOS), on frequency at mid-pass (TCA), and lower pitched frequency at end of the pass (LOS). Thus, you will need to be 10 KHz higher at AOS, 5 Khz half way between AOS and TCA, on frequency at TCA, -5 KHz halfway between TCA and LOS, and -10 Khz at LOS. Just remember you are adjusting your RX frequency to compensate for the effects of Doppler
- On AO-91 and AO-92, it is the opposite. You adjust your UHF uplink frequency so the satellite hears your signal at the designed uplink frequency. At AOS, doppler effect will cause your signal to sound higher to the satellite, so you need to start at a lower pitched frequency (-10 KHz), Halfway between AOS and TCA, you adjust to -5 KHz, and so on, and so on.
I hope this helps. If not, don't hesitate to ask.
Robert Bankston, KE4ALAMSAT-NA VP of User Services
Twitter: @KE4ALabamaWebsite: KE4AL.wordpress.com
On Wednesday, June 5, 2019, 12:46:07 PM CDT, 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
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