EA4GPZ: First tests of a narrowband data modem for QO-100

  • I take the liberty to link to Daniel's excellent development of a medium speed data modem, which is tailored to the QO100 NB transponder (rather than using a system developed for HF) . Based on some very good reasoning and design choices, he manages to get 6000 bps through 2.7 kHz of bandwidth while sticking to beacon level power and therefore adhering to the rules of operation.


    https://destevez.net/2019/12/f…nd-data-modem-for-qo-100/


    Bear in mind, this is ongoing work and the higher data framing levels are still missing. It does show however what is technically feasible.

  • Interesting. DATV is using a modem and 33ks and 66ks are getting towards narrow band. Maybe we can see some convergence. DVB-S2 is a very efficient coding method close to the Shannon limit. No reason it can't be done in 2.7 kHz with the advantage of efficient FEC, LDPC. It might be possible to use the DVB-S2 modules in gnuradio rather than the standard modulators and demodulators in Daniel's flowgraph.

  • G0MJW

    I agree, DVB-S2X would indeed also be a perfect solution as partly discussed here:   IP over DVB-S2

    Even ultra-low-bitrate would be possible, with only using an manually pointed LNB for reception..

    Kinda SMS or APRS service, even from mobile shall be possible...


    Unfortunately I miss the good old times where we had some international organizations like TAPR defining some common standards like AX.25. Or the AMSAT 400 Bit/s BPSK standard which was developed more than 40 Years :!: ago for the first AMSAT P3 Satellites... and we still use it on QO-100 ^^ Or the famous PACSAT protocol (PB/PG) first used some 30 Years ago...


    I see a lot of different protocols developed or proposed in the ham community, but none of them really optimized for satellite usage and optimized for efficiency in terms of bandwidth and S/N..


    So I hope we will learn from Daniel's developments and get the right people together to add FEC and protocol layers to it,

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG

  • @F4DAV reports some further progress on Twitter:

    Trying variable-MODCOD narrowband DVB-S2 on #qo100. 2400 bit/s of 8PSK 3/4 interleaved with 720 bit/s of QPSK 1/2, all in 2700 Hz. For beacons and other applications that can tolerate the latency of S2.



    I test 13 dB below the PSD of the BPSK beacon. A stronger signal could carry QPSK 1/4, receivable with a bare LNB or a small horn if software demodulation can be made to work at low SNR and low symbol rate. Probably needs more pilots or better clocks.


    ---


    Now we need a protocol layer on top of it, which is optimized for satellite communication, for example PACSAT Broadcast (PB) protocol or similar with some narrow band low speed uplinks as a next generation beacon for QO-100 Uplink from Bochum...

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG

  • Here is an excellent resource with all current documentation about the AMSAT PACSAT Broadcast Protocol and Ground Station Software (Client) from G0KLA: https://www.g0kla.com/pacsat/


    Does anyone know an open source implementation for the PACSAT Server?

    Yes CHRISTOPHER E THOMPSON developed a new PACSAT software, (Pacsat Ground Station Software)

    Christopher also works on FoxTelem - AMSAT Ground Station Software. if somebody know / inform the OM, he may help.!!!

  • I think the (PACSAT software) which had been used by any/many PCASAT satellite, could be use as server .!! its ready and avalable ,for sure need some touches to sute ground station !!

    Hi Nader,


    For the "client" side on the ground exists WISP (also not open source), but the groundstation software PACSAT Suite from G0KLA is written in Java and available on GitHub. It runs on Linux and Windows..


    Unfortunately the "server" side used on the PACSAT satellites never has been Open Source software as far as I know... But that's what we need and what is not part of the groundstation software! Someone needs to implement it...

    73s Peter

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG

  • I (re)wrote a client suite for the PACSAT protocol back around 1994 in C, based on the work by John G0ORX. It did rely on the Linux kernel AX.25 support (which I had a connection to) for the connected mode uplink, downlink would be trivial in comparison. It is available as source code from ftp://ftp.amsat.org/amsat/software/Linux/ with the filename of microsat-0.9-Xaw.tar.gz.

    Jonathan Naylor

    Callsign: G4KLX

    Locator: IO90TT

    QTH: Worthing, UK