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AO-40 search tip



For those of you using FFT "waterfall" programs to look for AO-40,
keep in mind that the beacon is manchester-encoded BPSK. That means
its spectrum is always symmetrical around the (suppressed) carrier,
with little or no energy around the carrier itself. Try to look for
that in your plots. If the signal is too weak to hear you may have to
retune by trial and error to capture both sidebands in your receiver
passband.

Computer noise often resembles BPSK-modulated signals. Try to look for
the characteristic periodicity from AO-40's telemetry frame rate. Each
frame is 10.36 seconds long, but there is idle padding between the
frames that increases the time between frame starts. The spec says
"about 130 bytes" of padding, which would be an extra 2.6 seconds. That
would make the frame rate about once every 13 seconds.

Phil

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