[amsat-bb] Re: Automatic doppler tracking of DSB

John Magliacane kd2bd at yahoo.com
Mon Feb 4 11:34:56 PST 2008

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 :-)
> 1) 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.

> 2) 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
> 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).

> 3) If this is a good idea, does we need a standard tone?

A standard tone would be required.
> 4) 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

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