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Possible interference due to malfunction of a satellite

For the past few weeks many 144MHz meteor scatter enthusiasts have been complaining of interference from AFSK packet transmissions. As a 144MHz DX enthusiast, in addition to being VHF manager I took an interest and started monitoring.
The transmissions on 144.390MHz FM AFSK occur at regular intervals, mostly on Northerly headings . The transmissions fill about 15KHz of spectrum on the waterfall display of a SDR receiver.

 

The impact the satellite signals on narrowband MS users can be seen from this video from GM4JJJ

https://www.youtube.com/watch?v=2ZJgMA4i7PY&feature=youtu.be

Because of the inclined orbit Northern European amateurs get a longer pass than those in the US which means that there is a burst every minute for a pass lasting up to about 12 minutes, repeating every 110 minutes orbit time. Whilst that is not continuous QRM one pass is enough to cause a meteor scatter QSO on 144.390 +/- 6KHz to fail if they coincide.

 

Find hereby the text of AMSAT-NA, published in their weekly bulletin:

 

PCSAT Default Beacon Transmissions on 144.390 MHz

 

An FM APRS signal has been received in England causing interference to the MGM frequency and weak-signal Meteor Scatter (MS) operation which is just below 144.390 MHz. A MS DXpedition was disrupted by such activity earlier in the month and interference has been occurring at various times since.

 

Bob Bruninga, WB4APR, explains that the beacon was planned over 15 years ago for operation only when PCSAT is over North America based on published band plans at that time. Now, after 14 years on orbit, the ability to turn that backup beacon off has been lost.

 

PCSAT (now 14 years old) had a backup fail-safe beacon on 144.390 that would activate after any unknown spacecraft reset to give a backup comm link in case the primary 145.825 channel died.  Being on the North American APRS frequency with hundreds of IGates there would always be at least one that would hear this « emergency call home » from PCSAT even though the channel is generally saturated.  It worked.

 

The problem is, that now PCSAT resets on every orbit due to negative power budget and so, on every orbit that beacon comes back on.  Even if we did get a command up to reset it, that setting would last only 15 minutes to the next eclipse.

 

We learned our lesson!  That was our FIRST amateur satellite and we sure learned NOT to use a « connected-packet-command link » that needs ACKS and Retries and logon passwords  just to LOGON before you can even send a SHUTUP command.  All our satellites since, operate without the multiple Send, connect, ACK, retry, ACK, command, ACK overhead…. just to get one command understood.  Now, only the receiver on the spacecraft has to be functional to command it to silence in a single packet.  But too late for PCSAT.


Source: UBA Radioamateurs en Belgique




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