QO-100 beacon FEC decoder

Since the BPSK beacon on the QO-100 narrowband transponder was first activated, I had thought that it only transmitted messages using the AO-40 uncoded protocol. However, a Twitter conversation a few days ago with Rob Janssen PE1CHL convinced me that FEC messages might be sent in between uncoded messages.

The AO-40 FEC protocol used a concatenated code with a (160, 128) Reed-Solomon code and an r=1/2, k=7 convolutional code, together with scrambling and interleaving to achieve very good performance. The same protocol has then been used in the FUNcube satellites, so I have an AO-40 FEC decoder in gr-satellites since I added support for AO-73.

It is quite easy to notice that the QO-100 beacon transmits both uncoded and FEC messages. Indeed, using my gr-satellites decoder, I see that an uncoded message is transmitted every 23 seconds approximately. Since an uncoded message comprises 514 bytes, it takes 10.28 seconds to transmit it at 400baud, so something else must be sent between uncoded messages.

A FEC message is formed by 5200 symbols (after applying FEC), so it takes 13 seconds to transmit at 400baud. This gives us the total 23.28 seconds that I had observed between uncoded messages. Note that the contents of the uncoded and FEC blocks are different. An uncoded block contains 8 lines of 64 characters plus 2 bytes of CRC. A FEC block only contains 4 lines of 64 characters, and no CRC.

I have added a FEC decoder to the QO-100 decoder in gr-satellites, so that it now decodes both FEC and uncoded messages.

2 Replies to “QO-100 beacon FEC decoder”

  1. Dear Dr. Estevez
    Have you ever tried comparing Doppler and differential Doppler measurements of GEO satellites whose high precision ephemeris are available?. As you may know, exact positional information of some GEO satellites such as Astra-5B, GSAT-8, and etc, which have SBAS payload (http://mgex.igs.org/IGS_MGEX_Status_SBAS.php) are available for free. You can access their ephemeris in this FTP: ftp://ftp.cddis.eosdis.nasa.gov/pub/gnss/data/daily/2019/

    1. Hi John,

      I haven’t tried doing Doppler measurements of GEOs with high precision ephemeris available. It seems an interesting idea. Regarding using the MGEX SP3 ephemeris for SBAS satellites, I think this should work, but I would be a bit careful. For SBAS satellites, the GNSS signal is not generated on-board the satellite, but rather on ground and relayed by a linear transponder on the satellite. The control segment adjusts the transmission so that it appears to originate from the satellite (regarding synchronization). Thus, I am not sure how this process interferes with precise orbit determination for these satellites. The control segment might mask ephemeris errors by adjusting the uplink signal. In any case, SP3 ephemeris for SBAS satellites should still be several orders of magnitude better than TLEs.

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