by WB4SON » Tue Nov 26, 2013 2:43 am
Just completed first CW QSO with NM3B, Wayne, in PA (I'm in RI). Prior failures were due to failing to apply a large enough UL correction factor to get my signal to appear in the downlink.
I'm using SatPC32, which adjusts for both UL and DL Doppler. I can set it to show me what frequencies are heard by the satellite or by my rig. The published DL/UL of 145.960/435.140 is from the satellite perspective, of course. Both of those signals are subject to Doppler, which makes them different at the ground -- but that correction is taken care of by SatPC32, as long as the PC time and the TLE is accurate.
So in my case, my DL was close to 145.955 (at the satellite, not at my rig). I would have expected my UL to be 435.145 (at the satellite, not at my rig). Instead my UL had to be adjusted to be 435.155 (at the satellite) -- I had to shift my UL up by 10KHz in order to be heard on the DL.
That +10KHz adjustment varied over the course of the pass from about 8.5KHz to 10.5KHz -- so it is an indication that the Doppler correction is a tad bit off. Usually there is some sort of correction that must be applied to the published relationship (like 145.960/435.140) and the one you need to use when the bird is in space. I'm used to seeing values like +/-250Hz to +/-2500Hz (like on VO52/AO7/FO29). So this one just caught me by surprise since it was more like 10000 Hz -- so large that I never even looked that far away prior to this last pass.
And I was talking about a CW contact, which means I had the UL shifted by about 700 Hz in order to produce my desired tone. So the UL correction factor for SSB would be about 700 Hz lower.
Anyway, mystery solved, but, as I said, it was enough of a shift from what I used to seeing that it was keeping me from making a contact until now.
73, Bob, WB4SON