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AO-40 Experimental Downlink Coding (FEC) |
At the 2002 AMSAT Annual Meeting in Ft Worth, Texas, Phil Karn KA9Q presented his proposal for a forward-error-correcting (FEC) format for AO-40 downlink telemetry. When links are poor due low signal-to-noise ratio, spin modulation or both, FEC promises error free telemetry. The link improvement can be as much as 8 dB (and much more under certain conditions). The software to do the encoding on the AO-40 satellite has now been written and will be exercised from time to time as a proof of concept. It runs on the IHU-2 (the experimental flight computer), so there is no risk to the spacecraft. When downlink FEC is in use, the normal uncoded A-blocks and their FEC encoded versions will alternate. First an A-block is sent and you can collect it in exactly the same way as before. Immediately following will be the same data FEC encoded into 650 bytes, which takes 13 seconds to send. Then there are a few seconds of inter-block idle, and the sequence repeats. The repeat period is about 27 seconds.
The FEC format and a generic 'reference encoder' (in the C language) was designed by Phil Karn KA9Q. His FEC Encoding for AO-40 Telemetry page documents all aspects of his work.
Conversion of the 'reference encoder' to IPS, the spacecraft's "native language" was performed by Jonathon Naylor G4KLX, his first exercise in the genre. Jonathon used IPS running on an IBM-PC as his test bed. IPS information.
Minor edits to the encoder and devising the IHU-2 test framework was done by James Miller G3RUH.
StaRCV 1.0.0 from Paul Willmott, VP9MU for Windows 95/98/NT/2000, is an experimental port of AO40RCV with FEC capture added, with source in Delphi Pascal.
Analogue to Digital Converter Upgrade for G3RUH MkII for P3 data demodulator PCB.
For telemetry decoders and other information about collecting and interpreting AO-40 telemetry, see the AO-40 Telemetry page.
FEC format details: Phil Karn's FEC Encoding for AO-40 Telemetry page.
Reference decoder (40K) for the experimental AO-40 FEC format, from James Miller, G3RUH.
Processing AO-40 Data at the Symbol Level by James Miller, G3RUH. These notes describe methods of processing the 400 symbols/sec generated by an ADC attached to the integrate-and-dump of a G3RUH P3 400 bps PSK Decoder. Symbols could also be generated by a computer's soundcard DSP process, or indeed by simulation. Familiarity with the AMSAT P3 telemetry idiom is assumed. Some sample code is given.
Updated August 1, 2003.
Original text by Stacey Mills, W4SM, James Miller, G3RUH, and Paul Willmott, VP9MU.
Comments to kb5mu@amsat.org