El 30/10/17 a las 23:23, Robert Bruninga escribió:
Im not following this closely, but the transponder I think is inverting. Thus the uplink dopper is subtracted from the downlink dopper whereas the beacon on the satellite is impacted by the full dopper of the speed of the spacecraft. So they will not be the same. Do the math and see if that comes out to the 1.26 you observed.
Hi Bob,
I'm not looking for a Doppler effect here. I've already corrected for that using the TLEs. It's the local oscillator drift in FO-29 that I'm trying to study.
However, I've done some math that matches my observed 1.26 within 5% error. The transponder is indeed inverting, with uplink on 145.95MHz and downlink on 435.85MHz. Assuming that the IF frequency f is below 145.95MHz (a 10.7MHz or 70MHz IF would be typical), there are two possibilities:
a) The first LO runs at 145.95 - f and the second LO runs at 435.85 + f b) The first LO runs at 145.95 + f and the second LO runs at 435.85 - f
In both cases, the total frequency shift through the linear transponder is 145.95 + 435.85. Assuming that both LOs are derived from the same oscillator (or that otherwise drift similarly), then the drift in the transponder would be proportional to 145.95 + 435.85, whereas the drift in the beacon would be proportional to 435.85. Now the quotient
(145.95 + 435.85)/435.85 = 1.33
matches the 1.26 I've measured within 5% error.
So for me this explains completely the effect I'm seeing, but it would be good to confirm it with the block diagram.
73,
Dani.