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PSK31 on AO-10, test results (long)



Today I did some experiments with PSK31 on OSCAR 10.  Essentially, I worked
myself through the satellite.  I ran a text file through the PSK31 program
off the air, and tape recorded the sound card output.  Then I transmitted
the tape recording while receiving the downlink.

The good news is that PSK31 performed beautifully.  I was able to receive
my text with very good copy.  And many of the errors occurred because of my
tuning errors during QSB, rather than the QSB itself.  The transponder
frequency varies slightly.  When the frequency shifts and "spin cycle" QSB
did not coincide, PSK31 could copy right throught the QSB with no errors,
or one or two missed characters, even at apogee.  When frequency shifts
occurred during a fade, I lost a bit more, as I couldn't see where the
signal was moving.

*Very* careful on-the-fly tuning was required.  I had to keep my eyes glued
on the phase circle, and "ride" the receive tone frequency with the left
and right arrow keys. It was like trying to keep a small airplane level in
a variable crosswind.  

The signal could vary back and forth 25 Hz or more during an approximately
2-minute transmission.  I found that if I watched the phase circle
carefully, and hit the appropriate arrow key 3-5 times (Hz) when the phase
indicator began to shift, I could keep the signal tuned.  This variation
was independent of the doppler shift.  

Whenever I observed the beacon, it appeared to move in only one direction,
and as the doppler figures for the pass would predict.  I don't know what
is shifting the transponder--perhaps a combination of signal load in the
passband and voltage variations caused by satellite spin.  I have a feeling
there may be a cyclic component to the transponder frequency changes, but
it was not in complete "synch" with the QSB "spin cycle," and I didn't
analyze it.

So the bad news is that while the transponder is stable enough for CW or
SSB, it's not quite stable enough for the tuning requirements of PSK31.
You have to really ride the receive tone frequency, and if you stop to read
what you're copying, you'll miss something. A couple of determined
operators will be able to pull off a QSO on BPSK, but they will have to
work for it.  That said, if anybody wants to schedule a QSO, please email me.

Conclusions:  PSK31 shows promise for satellite work.   When tuned in
properly, BPSK works beautifully on AO-10.  As on HF, PSK31 shows an
amazing ability to pull excellent conversational copy out of noisy and weak
signals, even through AO-10's notorious "spin cycle" QSB.  However, even on
BPSK, the PSK31 program's automatic frequency control (AFC) can't cope with
AO-10's transponder instability without much manual intervention.  QPSK
seems out of the question, and Doppler shifts on LEO satellites would be
problematic at best.  

But remember, these tests are of a program designed and optimized for HF
conditions.  If someone were to take the public domain G3PLX code and add
some really robust frequency control,  we might have something!  Just sound
card tone control would suffice for AO-10.  A combination of radio control
(coarse tune) and sound card (fine tune) would be needed for LEO work.  The
mode could be optimized for satellite work by adjusting the data rate,
narrowness of the bandwidth and tolerance for off-tuning.

*****
The details:
*****

For those interested, I have included several examples of actual received
text.  My station is a Yaesu FT-736R with 100 watt TE amps for both bands.
I use the preamp in the 2m amp, behind a DCI helical filter--this works
better than using a Landwehr preamp at the antenna.  About 40 feet of 9913
and 10 feet of regular RG-213 coax to the antennas.  My antennas are
M-Squared circular beams, 14 elements RHCP on 2m and 30 elements RHCP on
70cm. They are the short, 10-foot long booms rather than the larger,
20-foot KLMs that some folks have.  I also use a Timewave 599zx DSP filter.

*****
First, here is the test file I sent:
*****

This is KD7MW, Peter in Seattle, Washington, testing PSK31 on OSCAR 10.
This text comes from the PSK31 Help file, "PSK31 Overview."

The basic idea of PSK is that keying is done by phase-shifting the
carrier rather than frequency-shifting it. The bandwidth is equal to the
baudrate rather than to the shift plus the baudrate. With the chosen
baudrate of 31.25, that brings the bandwidth down from the 300-500Hz of
other modes down to 31Hz. By using an alphabet similar to morse with
short codes for common letters, the text speed of PSK31 comes out at
about 50 wpm.

By using the narrowest possible filters in the transmitter and receiver,
the performance of PSK31, even without error-correction, is certainly
better than most, and has the added advantage for live QSOs that the
performance degrades very gracefully as the signal drops.

This is KD7MW, Peter in Seattle, Washington, testing PSK31 on OSCAR 10.
This text came from the PSK31 Help file, "PSK31 Overview."

End of test.

*****
Here is text received at 2331 UTC on 23 April 1999.  Power output was about
30 watts.  The Timewave DSP was on voice bandwidth, basically doing
nothing.  Satellite range was about 26,800 km.  The errors at the very
beginning of the file occurred while I was tuning the signal in.  
*****

R 10.
This .Ft come
 j om –ePS1 Help file, "PSK31Overview."

The basic idea of P is that keying is doie bykha
E   tl the
carrier rather than frequency-shifting it. The bandwidth is equal to the
baudrate rather than to the shift plus the baudrate. With tme chosen
baudrate of 31.25, that brings the bandw th dow   ftop ) aiq0Hz of
other modes down to 31Hz. By using an alphabet similar to morse with
short codes for common letters, the text speed of PSK3f comes oÁ at
about 50 wpm.

By using the narrowest possible filters in the transmitter aad receiver,
the performance of PSK31, even without error-correction, is certainly
better than most, and has the added advantage for live QSOs that the
performance degrades very gracefully as the signal drops.

This is KD7MW, Peter in Seattle, Washington, testing PSK31 on OSCAR 10.
This text/ame;rom the PSK31 Help fi|, "PSK31 Overview."
En t of test.

*****
Here is another test 20 minutes later, 2351 UTC 23 April 1999.  In this
test, I used the Timewave 599zx DSP filter at 300 Hz bandwidth and 600 Hz
center frequency, with noise reduction at Aggressiveness of 2.  This didn't
really help the PSK31 program copy, in fact it may have degraded copy a
little.  But it allowed me to "see" the signal on the phase circle further
into the noise, and tune more comfortably during fades.
*****

This is KD7M., P,z ivSeattle, Washington, testing PSK31 on OSCAR 10.
This text comes from the PSK31 Help file, "PSK31 Overview."

The basic idea of PSK is that keying is done by phase-shifting the
carrier rathet  han frequency-shifting it. The bandwidth is equal to the
baudrate rather than to the shift plus the baudrate. With the chosen
baudrate of 31.25,tòt brgs the bandwidth fown nrom the 300-500Hz of
other modes down to 31Hz. By using an alphabet similar to morse with
short codes for coo mon letters, the text speed of PSK31 comes out no
about 50 wpm. 
By using the narrowe
  possible filters in the transeeitter and receiver,
the performance of PSK3C, even without error-correction, is
certainlysbetter than most, and has the added advantage for live QSOs that the
erformance degradesery a racefully as the signalqrops. 
This is KD7, Peter in Seattle, Washington, testing PSK31 	 BSCAR .
This text oame ftom the P nK3k e f e7b Bverview."

,nd o test.

*****
Now, here's a test near apogee, at 0349 Z on 24 April 1999.  Power output
was about 65 watts.  Satellite range was about 36,000 km.  DSP:  No noise
reduction, DSP CW filter at 600 Hz center frequency, 600 Hz bandwidth.
Fades were quite a bit deeper and longer at this part of the orbit.
*****

  e2his #Ea ð*oe te_ol=d n shington, testing PSK31 on OSCAR 10.
This text comes from the PSK31 Help fL  o  e io   o& n n TK

tt =  ldidea of PSK is that keying is done by phase-shift tt eorcattier
rather than freq o  y-shifting it. The bandwidth is equal-o the
baudrate rather than to the shift plus the baudratege r
eseÚyaudrate on 31.25, that brings the bandwidth down from the 300-500Hz of
other modes down to 31Hz. By using an alphabet similar to morse with
short codes for common letters, the text speei ѤSK31 comes out at
about 50 wpm.

By using the narrowest possible;ilters in the transmitter and receiver,
the performance of PSK31, even without error-correction, is cer t  neco
ieÝoEeF  - ngB|n_ dn5ge for live QSOs that the
performance degrades Iery gracefully as the signal drops.

This is KD7MW, Peter in Seattle, Washington, testing PSK31 on OSCAR¢0.
Tmº text cOlúih the PSK31 Help file, "PSK31 Overview."

End of test.

*****
I hope this is useful. 
73 from KD7MW,

--- Peter
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/ b b ,| ,| ,| ,|  ,| ,|  '  ,|   | ,|   |__|__|__|   |__|   |  |   |  |
                                ~'                            ========= 

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