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-------- Original message -------- From: Robert Bruninga [email protected] Date: 05/21/2015 16:19 (GMT-07:00) To: [email protected] Subject: [amsat-bb] Receiving PSAT PSK31 FM downlink
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
WHAT TO DO:
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 satellites.
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 Doppler.
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!
Enjoy!
Bob, WB4APR _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb