[amsat-bb] Feedback on BPSK decoding

Phil Karn karn at philkarn.net
Thu Aug 4 14:04:48 PDT 2011

I'm interested in people's comments on the reliability of the BPSK-1000
digital link in the presence of the deep periodic fading that everyone
is reporting. This fading is probably because the spacecraft hasn't
settled down into a stable spin around its preferred spin axis (which
includes the antennas). But I designed the BPSK-1000 format to be
specifically resistant to fading, so I'm really curious to see how well
it's working in practice.

Although the decoders don't explicitly distinguish between lost frames
and idle link time (hard to do given the signal design) you can gauge
the effectiveness of the decoding from the amount of data you do
successfully decode per unit time. If people could estimate how many
bytes per minute of decoded data they're getting, and assuming the
computer doesn't let the link idle very much, that would be a good
indication of how well the system is performing overall.

The channel symbol rate is 1000 Hz. After rate 1/2 Viterbi FEC the
actual link data rate is 500 bps. From this you have to subtract idle
inter-frame time and HDLC frame overhead (flags, 4-byte CRCs, etc) that
varies with the length of the frame, leaving the data. So in 1 minute
you could receive up to 3750 bytes of binary user data. How close are
people coming to this figure?


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