[Ground-station] Need advice on DVB-S2 modem

[Mike Parker]

A team headed by the University of Arizona is working on a 6-U satellite that will fly at 500 km altitude.  The satellite, named CatSat after the UofA wildcat mascot, was described in a paper presented at the Amsat Annual Meeting.  We are shooting for a launch that could be as early as one year from now.  The plan is to have a downlink using DVB-S2 modulation generated by an FPGA on the AstroSDR card provided by Rincon Research.  This modulation was chosen so that hams that have been working phase4 ground DVB-S2X might receive and demodulate the link.  We are planning to transmit a limited number of ModCod’s.  Modcod 7, QPSK 3/4, and modcod 17, 8-PSK 9/10 are likely choices.

Two experiments plan to use DVB-S2 modulation on the downlink.  One involves transmitting high quality video, and the other will capture and retransmit narrowband pieces of the HF ham bands containing WSPR and FT8 signals.  Closing the link to our 6.1 meter diameter ground station is no problem, and we are planning on a modulation bandwidth up to 20 MHz and bit rates over 50 Mbps when using the satellite’s inflatable directive antenna provided by FreeFall.  But closing the link to a ham 0.6 meter dish when operating with an onmi-directional transmit antenna poses a very different problem. We will likely require small downlink bandwidths on the order of 200 kHz.

We are looking for a demodulator to assist in testing the satellite, and also a demodulator for use in the ground station.  So we are reaching out to you for suggestions.  We hope that an available commercially for an affordable price, or perhaps something designed by a member of this group.  We have neither the desire or time to reinvent the wheel.
Several things concern us we would appreciate advice on.
1) We read with interest a paper by Downey, Evans, and Tollis, “DVB-S2 Experiment over NASA’s Space Network”.  It said “ typical commercial DVB-S2 receivers are not designed for symbol rates below 300 kbaud”.  That is consistent with our observation that many commercial demodulators do not seem to have a lowest bandwidth specification.  Anyone know of one that goes lower in bandwidth while having a high bandwidth capability?
2)  We need to have a demodulator that will output raw DVB-S2 frames, bypassing any transport layer protocols which are normally used with DVB-s2 such as Multi-protocol Encapsulation(MPE) or Geeric Stream Encapsulation (GSE).  Downey, Evans, and Tollis used a Newtec MDM6000 modem.   Is there a better or cheaper solution? (I haven’t priced one yet).
3) Doppler shifts are also a concern, especially at a low data rate, but we have a plan to solve that if necessary using a local oscillator in the ground station that is swept according to ephemeris predictions to de-Doppler the signal before demodulation.

Oh yes, some good news.  The first of our 6.1 meter dishes has been reassembled in Rincon’s parking lot in Centennial, CO.  A picture with assembly in progress is attached.  I’m flying up tomorrow with a 10 GHz feed horn and LNA to see if we can hear signals!
Mike Parker, KT7D
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