Hi Jim.
Hi John,
What if someone turns on a CTCSS subtone for their FM uplink?
If set high enough, it might make it thru the audio filters, so could be tracked with a PLL at the receive end. (But not so high it's a problem for human ears :-)
- Would this work?
I believe it could be made to work. However, during the absence of any CTCSSed uplink, your PLL would lose lock and your Doppler tracking function would stop.
- Will it cause any problems to AO-16, by raising the duty-cycle
of the DSB downlink? (Clearly this would be a bad idea if it might
cause harm or
unacceptable power-budget issues. Mind you, it can't be too much
worse
than the original BPSK can it?)
The downlink was modulated continuously with BPSK during Pacsat service, but the state of the batteries were almost certainly better then than they are now.
However, in its present configuration, the downlink is modulated by white noise if there is no signal present on the uplink, although the peak-to-average power ratio is probably higher with noise than it was with BPSK (a pure guess on my part).
- If this is a good idea, does we need a standard tone?
A standard tone would be required.
- How would it cope with multiple overlapping uplinks?
That would be a problem, as would any period where there is an absence of an uplink.
If someone was able to automate the downlink Doppler correction, that can then feed right back into the uplink correction too.
Very true.
It would be interesting to them compare the corrections against that calculated from the Keps.
I'd love to hear ideas re above, but most critical is to be sure it's not harmful to the sat in any way.
I have only had the chance to tune into AO-16 once during a time when I happened to catch Jim, WD0E, testing the satellite before it was open for general use, and I thought I heard some residual carrier in the downlink. That "pilot carrier" (if present) could be used for Doppler tracking purposes as well. The problem of finding and keeping track of a very narrowband carrier would be almost as difficult as tracking a CTCSS tone.
(I say "almost" because in the case of using a DSB demodulator, it might be a little easier to find and track a DSB carrier (produced by a CTCSS pilot tone) because of its redundancy than a single pilot tone caused by a residual suppressed carrier. The residual carrier, however, will be farther removed from voice frequencies than any CTCSS tone.)
Considering all of the above, I might be inclined to try a Costas or Squaring Loop along with a DSB demodulator for "closed loop" Doppler correction. With any luck, maybe the loop can acquire and track the white noise that is present when no uplink signals are present.
Another experiment you might try is to see how a receiver with an FM center tuning discriminator meter reacts to AO-16's downlink with and without signals present on the uplink.
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/ . . . .
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