[amsat-bb] Receiving PSAT PSK31 FM downlink

Robert Bruninga bruninga at usna.edu
Thu May 21 23:19:30 UTC 2015

Receiving the PSAT (and BRICsat) 435.350 MHz FM downlink is as simple as
placing  your PSK31 laptop microphone next to the speaker on your FM
satellite UHF receiver and just watching the waterfall.

What you see is exactly what everyone else sees (its FM).  There is no
Doppler added to the tones due to your station's position relative to the
satellite.  But you DO have to retune your FM radio at least 3 times
during the pass (+5 KHz, 0, -5 KHz) to stay in the FM passband.

User uplinks, however, will shift in the waterfall according to each
user's position relative to the satellite.  The shift can be as low as 1
Hz per second to as high as 6 Hz per second.  This is because the uplink
is on 10 meters where the Doppler rate is only 1/15th of what it would be
on UHF.

The TELEMETRY channel at 315 Hz (PSAT) or 375 Hz (BRICsat) is FIXED with
no Doppler since it is generated onboard into the FM downlink


1) We will need PSK31 authors to open the PSK31 frequency tracking to
accommodate more than 1 Hz per second Doppler tracking.  Current
implementations can do 1 Hz/s but completely fail at 3 Hz/s.  2 Hz/s might
work a little...

2) Until then, ANY uplink user that is in line with a direct overhead pass
will have minimum Doppler at the start and end of his pass (1 Hz/sec) when
the satellite is going right at him and directly away from him.  (Though
it will be MAX (6Hz/sec) when it passes over his station).

3) Just turn on MULTI CHANNEL window and let the PSK31 decode everyone.
The ones with the least Doppler at any instant may be decoded for a while!

USERS can transmit later when BRNO University says it has completed its
tests.  Brno provided the transponders for use in the PSAT and BRICsat

So start preparing your station to TX PSK31 on 10 meters SSB and to
receive the audio from an FM UHF rig on 435.350 +/- 5 KHz steps of

DOWNLINK Limitations:  The UHF downlink signal is only 300 mW and so a UHF
beam is needed on the downlink.

UPLINK RESTRICTIONS:  *NOTHING MORE THAN* a Vertical 1/4 wave or Dipole is
authorized on the 10m uplink  and no more than 25 Watts (for now).
Remember a 1/4 wave vertical is the ideal antenna because it maximizes the
signal at lower angles and tapers the signal as the satellite gets closer.
This keeps  user uplinks about the same during a pass.  Strong stations
just drive down the AGC and ruin it for everyone.

Use minimum power!!  Remember, this is crossband FULL DUPLEX so  you can
see yourself in the downlink just like everyone else can see you.  Act
accordingly.  And of course DO NOT TRANSMIT if you cannot see the
waterfall  ... Duh!



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