[amsat-bb] Fwd: Re: PCSAT2 PSK31 tests

["Robert Bruninga" <bruninga@xxxxxxxx>]

Attached is Peter Martinez' summary of the changes needed
to PSK-31 software to meet theDoppler challenges offered
by PCSAT2.  He is willing to work with any PSK-31 authors
to add second order DOppler tracking so that many more
people can persue this exciting capability...

de WB4APR, Bob
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From: "Peter Martinez" <peter.martinez>
To: "Robert Bruninga" <bruninga@usna.edu>
References: <s2fdecd7.068@FSGWHUB.usna.edu>
Subject: Re: PCSAT2 PSK31 tests
Date: Sun, 14 Aug 2005 20:32:12 -0000
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Bob:

Quite a bit more activity today, but as far as I know, no two-way QSO. Most 
stations still seem to be mesmerised by hearing their own signals drifting 
far faster than they can handle, and have not yet moved on to the problems 
of simultaneously receiving another station.

I predict that there will very soon be a demand for Doppler tracking aids 
that can handle the PSK31 uplink Doppler correction requirement. There are 
several ways this could be done.  If we just think of using existing Doppler 
correction software, then it needs to handle a step size of the order of 
1Hz, and a Doppler rate of abour 14Hz/sec maximum (for ISS). I suspect that 
this may be beyond the capabilities of some of the presently available 
programs, and may be beyond the ability of some presently available 
transceivers. The stepsize and the update rate are the problem areas. At 
least one station has tried with a 10Hz stepsize system and it's not 
workable. If a 1Hz stepsize radio existed, it would need to have it's 
frequency updated at least 14 times per second.

Another solution is to extend the capabilities of the PSK31 programs. One 
way to do this would be to incorporate a link between such a PSK31 program 
and an existing Doppler tracking program, so that the PSK31 program can 
slide the transmitter audio tone frequency up in the required manner. The 
tricky part here is that we still need the 1Hz stepsize and the 14Hz/sec 
rate, and, as I said above, the current Doppler tracking programs may not 
have this resolution. One way round this might be for the PSK31 program 
receiving the Doppler data, at for example 10 second intervals, to 
interpolate between the frequencies provided in order to 'smooth out' the 
frequency of the transmit audio tone. This might require that the Doppler 
program calculate frequency values ahead of time, for example 5 secs ahead 
if the PSK31 program was going to linearly interpolate over 10 secs. This 
solution would be limited to uplinks where the total Doppler swing was less 
than about 2.4kHz, in practice to uplinks below 30MHz.

Both of the above solutions are 'open loop', and could be applied to 
automated uplinks.

Another way to solve the PSK31 uplink Doppler correction requirement is the 
way I am doing it, where the transmitting station feeds-back the Doppler 
correction from reception of his own downlink signal. This solution is 
unique to the linear-up/FM-down transponder scheme and can be implemented 
entirely in software for uplinks below 30MHz. The advantage is that it 
doesn't need keplers but the disadvantage is that is does require the sender 
to transmit continuously, and QRM of the sender's downlink can disrupt the 
tracking, so it's not a solution that can be applied to an automated uplink, 
only to a manually-operated one.  I have verified that a second-order AFC 
loop is quite capable of handling any Doppler drift likely to be 
experienced. This kind of loop is not stressed by Doppler drift rate itself 
but there are limitations on the double-rate-of-change of Doppler that such 
a system can handle. The maximum that PCSAT2/ISS presents for this parameter 
is +/-0.23 Hz/sec/sec on an overhead pass. There are two critical points 
about 25-30 seconds each side of TNA. My scheme would appear to handle this 
well on a simulated signal, but since I never get an ovehead pass from the 
ISS here, I can't verify that on the real ISS. The worst I experience is 
0.14 Hz/sec/sec.

I believe we should actively solicit designers/programmers to tackle this 
project urgently, drawing attention to the problem and providing the details 
needed to make sure that these guys can come up with usable solutions within 
the lifetime of PCSAT2.  If we don't tackle this soon, I think the users 
will migrate away from PSK31 on PCSAT2 and swap to other wider-bandwidth 
modes where the Doppler shift problem does not need to be solved. The result 
will be a far less efficient use of the transponder bandwidth.

I do not have the capability or desire to do this kind of mass-market 
software myself. I am just an experimenter. I am very willing to pass on the 
results of any of my experiments to others. This is how I worked with the 
original PSK31 systems. I just demonstrated that it could be done (in 
software on a soundcard/PC), and the end-user programs were written by 
others far more skilled in this task than me.

Copy this to whoever you wish.

73
Peter G3PLX
----
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