David, I'll step in here with a brief comment which you may already have considered:
Doppler frequency offset changes throughout a pass, so if you transmit on a fixed frequency on the uplink, the downlink frequency will change due both due to Doppler effects on the uplink and downlink frequencies. If this is mode-UV the amount of Doppler will be more on the uplink due to the higher frequency.
Another effect that many do not take into mind is that Doppler changes at a non-linear rate; in other words it changes faster near the point where Doppler goes from positive shift to negative shift in the rest frequency of the satellite. I know it sounds like a lot of math. But as an operator you just need only realize that this will happen resulting in tuning the signal over the duration of the pass (tune faster at mid-point of pass).
When one gets up to modes like LS, it gets more challenging because both Doppler and rate of Doppler change are greater. Probably should consider computer tuning of the radio at this point.
Note that Leo satellites go thru faster frequency change due to the quicker pass overhead. Heo's taking hours for a pass require retuning less often.
Others may provide more definitive help with your question. 73, Ed - KL7UW
At 11:35 AM 10/6/2010, David Palmer wrote:
Hi Guy and Amsat-List!
I'd like to know the answer to this too! I just recently started using twin FT-817ND's and SatPC32 as well, and notice the exact same thing -- audio drifts slightly thorughout the pass, even though I have the dopper correction set pretty close. For me, it only drifts a few hundred Hz at most, but it's still enough that I have to tweak the offset on the uplink. I'm new to SSB sats so not sure as well if there's something I'm doing wrong. I've got the time on the PC clock set within 1 second, and my lat/long within 0.01 degrees.
I'm measuring the drift just roughly based on my own audio coming back over the downlink -- unfortunately as I've called CQ on FO-29 and VO-52 the last few evening's passes (0100-0500 UTC), no one else has been on the sats for a direct QSO! My own audio comes back nice and clear (even out to 5 degrees elevation) so it's not a reception problem, as far as I know.
Any help for those of us new to the satellites would be appreciated!
Thanks,
Dave KB5WIA CM88 Northern California 2xFT817-ND, Elk Log-Periodic, MH-72 Duplexer, Samsung Netbook running SatPC32.
Is anyone else using two Yaesu FT-817 / 857 / 897 rigs with SatPC32? If so, what do you think I am doing wrong and what are your DOPPLER.SQF data lines for AO-7, VO-52, and
FO-29 (as well
as any other critical settings)?
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73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-QRT*, 432-100w, 1296-QRT*, 3400-winter? DUBUS Magazine USA Rep dubususa@hotmail.com ====================================== *temp
Hi Ed,
Thanks for the fast response! Yeah, both the original poster (Guy) and myself are using computer tuning of both radios. I know for sure my SatPC32 is set up for adjusting both the uplink and the downlink, and I see the readout on the radio changing accordingly, so it seems to be working ... what puzzles me is why the computer tuning seems to be so poor, since even though it's complex math, it's just math and the computer should be able to keep up!
Of course, since there are many on the list here using SatPC32 and twin radios successfully, I'm almost certain it's operator error that's causing this slight drift (or lack of complete correction), but I just can't figure out what it is yet. :-)
One thing I *did* notice with the Twin FT-817's, is that those radios (817,857, 897) do not update the TX frequency by CAT while they're transmitting ... so as I call CQ, I definitely hear my voice drop in pitch because the SatPC32 is busy updating the downlink frequency but the radio is busy so is not updating the uplink frequency. I accept that as normal (and just unkey to let the computer reset the radio for proper shift) but ...
... what's confusing me is that the amount of correction seems to change from pass to pass and even during the pass ... at first (out of the box) SatPC32 was about 2400 Hz low on the uplink based on the values in the doppler.sqf file. I just figured it could be due to radio calibration, etc, so I applied the calibration factor, and now it's much closer ... but still I seem to have to adjust +/- 100 to 200 Hz at the start of the pass, and periodically during the pass, to keep my return pitch constant.
Thanks everyone!
Dave KB5WIA CM88 Northern California.
When one gets up to modes like LS, it gets more challenging because both Doppler and rate of Doppler change are greater. Probably should consider computer tuning of the radio at this point.
participants (2)
-
David Palmer -
Edward R. Cole