GNUradio decoders for different satellites.

This repository is a collection of GNUradio decoders for the telemetry of several satellites. The decoders don't need a graphical interface to run, so they can be used in an embedded or remote computer. The decoders are designed to run in real time and print the telemetry packets to the terminal as they are received. Optionally, the telemetry packets can be uploaded in real time to the PE0SAT telemetry server or any other telemetry server that implements the SiDS (Simple Downlink Sharing Convention) protocol.

It is also possible to use the decoder with a recording (audio WAV or IQ file), in case that the telemetry wasn't processed in real time. To do this, one has to know the time and date at which the recording was started and the recording has to be played back at normal speed. This allows the decoder to compute the correct timestamps for the packets when uploading them to the telemetry server. It also simplifies Doppler correction of the recording with Gpredict if the recording was not Doppler corrected.

The signal is fed to the decoders using a UDP stream. The format used is the same that gqrx uses. Therefore, you can use gqrx to feed the signal to the decoders. You will have to set the proper frequency, mode and bandpass in gqrx for the satellite you want to receive. This is probably the easiest way to start using the decoders from gr-satellites. Gqrx supports Doppler correction with Gpredict.

It is also possible to use the frontend streamers from gr-frontends. This allow to stream from different SDR hardware without using a GUI SDR program. It is possible to perform Doppler correction with Gpredict. There are also frontend streamers to use a conventional receiver connected via soundcard and recordings (audio WAV and IQ).

Each satellite has its own decoder. You can open the .grc file with gnuradio-companion and edit the parameters (they are on the upper part of the flowgraph). You can also run the .py script and specify the parameters on the command line. Use the -h flag to get help on how to specify the parameters. The decoder will printing each telemetry packet in the terminal as soon as it receives it.

• sat_3cat2 3CAT-2, which transmits 9k6 AX.25 BPSK telemetry in the 2m band. You must use wide SSB mode to receive this satellite.
• aausat_4 AAUSAT-4, which transmits 2k4 or 9k6 GFSK telemetry in the 70cm band. It uses the CSP protocol and FEC with an r=1/2, k=7 convolutional code and a (255,223) Reed-Solomon code. You must use FM mode to receive this satellite.
• ao73 AO-73 (FUNcube), which transmits 1k2 BPSK telemetry in the 2m band. It uses the AO-40 FEC protocol, which includes block interleaving, an r=1/2, k=7 convolutional code, CCSDS scrambling and two interleaved (160,128) Reed-Solomon codes. You must use SSB mode to receive this satellite.
• aisat AISAT, which transmits 4k8 AF GMSK telemetry in the 70cm band. It uses the CSP protocol and FEC with a (255,223) Reed-Solomon code. It also uses a CCSDS scrambler. There is no telemetry parser yet, as the beacon format is unknown. This satellite has an AIS receiver on board. You must use FM mode to receive this satellite.
• athenoxat-1 ATHENOXAT-1, which transmits 4k8 AF GMSK telemetry in the 70cm band. It uses the CSP protocol and FEC with a (255,223) Reed-Solomon code. It also uses a CCSDS scrambler. There is no telemetry parser yet, as the beacon format is unknown. This satellite is on a low inclination orbit, so it can only be received near the equator. You must use FM mode to receive this satellite.
• beesat BESAT-1,-2 and -4, which transmit 4k8 FSK telemetry in the 70cm band. They use the Mobitex-NX protocol, which includes FEC with a (12,8,3) linear code and CRC-16CCITT for error detection. You must use FM mode to receive these satellites.
• by701 BY70-1, which transmits 9k6 BPSK telemetry in the 70cm band. It uses FEC with an r=1/2, k=7 convolutional code and a (255,223) Reed-Solomon code (the same as the LilacSat-2 9k6 BPSK telemetry). You must use wide SSB mode to receive this satellite. It has an optical camera on board and it transmits JPEG images together with the telemetry. by701 includes a complete telemetry decoder and image receive software. This satellite launched on 28 December 2016 into a 520x220km orbit. The perigee is too low because of a problem in the launch. The orbit will only last a couple months.
• galassia GALASSIA, which transmits 4k8 AF GMSK telemetry in the 70cm band. It uses the CSP protocol and FEC with a (255,223) Reed-Solomon code. It also uses a CCSDS scrambler. There is no telemetry parser yet, as the beacon format is unknown. This satellite is on a low inclination orbit, so it can only be received near the equator. You must use FM mode to receive this satellite.
• gomx_1 GOMX-1, which transmits 4k8 AF GMSK telemetry in the 70cm band. It uses the CSP protocol and FEC with a (255,223) Reed-Solomon code. It also uses a CCSDS scrambler. The beacons include information from ADS-B beacons transmitted by terrestrial aircraft. You must use FM mode to receive this satellite.
• gomx_3 GOMX-3, which transmits 19k2 GFSK telemetry in the 70cm band. It uses the CSP protocol and FEC with a (255,223) Reed-Solomon code. It also uses a G3RUH scrambler. The beacons include information from ADS-B beacons transmitted by terrestrial aircraft. Note that GOMX-3 will decay during October 2016. You must use FM mode to receive this satellite.
• ks_1q KS-1Q, which transmits 20k FSK telemetry in the 70cm band. It uses KISS framed CSP packets and FEC with an r=1/2, k=7 convolutional code and a (255,223) Reed-Solomon code (the protocol is very similar to LilacSat-2). It also uses a CCSDS scrambler. You must use FM mode to receive this satellite.
• lilacsat2 LilacSat-2, which transmits 9k6 BPSK, 4k8 GFSK and FM subaudio telemetry in the 70cm band. It uses FEC with an r=1/2, k=7 convolutional code and a (255,223) Reed-Solomon code. The decoders for this satellite are organized a bit different from the decoders for other satellites, because LilacSat-2 transmits in several different frequencies using several different modes. You can use lilacsat2 as a usual single-frequency single-mode decoder. You can use gqrx or one of the frontends from gr-frontends to feed an UDP audio stream to lilacsat2. However, you can decode only one frequency and mode using this method. You should tune to 437.200MHz in wide SSB mode to receive 9k6 BPSK telemetry, to 437.200MHz in FM mode to receive FM subaudio telemetry and to 437.225MHz in FM mode to receive 4k8 GFSK telemetry. lilacsat2 will recognise the telemetry format automatically. To receive all the frequencies and modes at the same time, you need to use an SDR receiver. The receivers lilacsat_fcdpp and lilacsat_rtlsdr can be used with a FUNcube Dongle Pro+ and an RTL-SDR respectively. These are complete receivers and decoders. They submit telemetry to the PE0SAT server and can use Doppler correction with Gpredict, in the same way as the frontends from gr-frontends. When using Doppler correction with Gpredict, you have to set 437.200MHz as the downlink frequency in Gpredict.
• nayif1 Nayif-1, which transmits 1k2 BPSK telemetry in the 2m band. It uses the AO-40 FEC protocol, which includes block interleaving, an r=1/2, k=7 convolutional code, CCSDS scrambling and two interleaved (160,128) Reed-Solomon codes. You must use SSB mode to receive this satellite.
• tw_1a, tw_1b, tw_1c TW-1A, TW-1B, TW-1C, which transmit 4k8 GFSK telemetry in the 70cm band. They use the CSP protocol and FEC with a (255,223) Reed-Solomon code. They also use a G3RUH scrambler. The transceiver is the GomSpace NanoCom AX100, the same transceiver used in GOMX-3. There is no beacon parser yet, as the beacon format is unknown. The only difference between the 3 receivers is that the NORAD ID is set for the correct satellite when doing telemetry submission. You must use FM mode to receive these satellites.
• ukube1 UKube-1 (FUNcube-2), which transmits 1k2 BPSK telemetry in the 2m band. It uses the AO-40 FEC protocol, which includes block interleaving, an r=1/2, k=7 convolutional code, CCSDS scrambling and two interleaved (160,128) Reed-Solomon codes. You must use SSB mode to receive this satellite.

The following GNUradio out-of-tree modules are required in several of the decoders. You should probably install all of them.

• gr-kiss Tools for AX.25 and KISS
• gr-synctags Tools for dealing with GNUradio synctags easily
• gr-csp Tools for CSP protocol
• gr-sids Telemetry submission using the SiDS protocol
• gr-libfec FEC decoders using Phil Karn's libfec.

You also need to install Phil Karn's KA9Q libfec for some of the satellites that use Reed-Solomon or convolutional codes (other include their own decoder). A fork that builds in modern linux systems can be found here.

The following GNUradio out-of-tree modules are only required for the decoder of one particular satellite. You may install only the ones you're interested in.

• gr-3cat2 3CAT-2 telemetry parser
• gr-aausat AAUSAT-4 decoder and telemetry parser
• gr-ao40 AO-40 FEC decoder, used in the FUNcube satellites
• gr-ax100 Decoders and telemetry parsers for satellites using GomSpace radios: AISAT, ATHENOXAT-1, GOMX-1, GOMX-3, TW-1A, TW-1B, TW1-C
• beesat-sdr BEESAT decoder and TNC
• gr-ks1q KS-1Q decoder
• gr-lilacsat LilacSat-2 decoder

Some of the decoders use hierarchichal flowgraphs. These are GNU Radio flowgraphs that can be used as a block in another flowgraph. To use these hierarchical flowgraph blocks, you must open each hierarchical flowgraph with gnuradio-companion and press the "Generate" button (next to the "Play" button). The Python code and XML description of the block will then be generated and saved within your GNU Radio installation. Consider this step as part of installing gr-satellites.

This is the list of hierarchical flowgraphs in gr-satellites:

• ccsds_descrambler.grc CCSDS additive descrambler (using unpacked PDUs)
• ccsds_viterbi.grc Viterbi decoder with CCSS/NASA-GSFC convention (POLYB, ~POLYA). Output is unpacked bits.
• sync_to_pdu.grc Find a syncword and extract a PDU of fixed length containing unpacked bits
• sync_to_pdu_packed.grc Find a syncword and extract a PDU of fixed length containing packed bytes

This is the usual procedure to build and install an OOT module:

mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig

To sumbit telemetry to the PE0SAT telemetry server (or another SiDS telemetry server), you have to specify your callsign and coordinates. The callsign is specified using the --callsign parameter and the latitude and longitude are specified using the --latitude and --longitude parameters if you are using the .py script. If you are using the .grc file with gnuradio-companion, you can set these parameters by editing the boxes on the upper part of the flowgraph.

The format for the latitude and longitude is of the form 00.00000 or -00.00000. The - means South (for latitude) or West (for longitude).

If you want to submit telemetry from a recording, you have to specify the UTC date and time when the recording was started. This allows the decoder to compute the proper timestamp for the packets. The format is YYYY-MM-DD HH:MM:SS and it is specified using --recstart if using the .py script or with the parameter box on the upper part of the flowgrah if using the .grc file with gnuradio-companion.

It is also very important that the decoder and the recording streamer are started simultaneously. This can be achieved by something like

gr-frontends/wav_48kHz.py -f recording.wav & \
gr-satellites/sat_3cat2.py --recstart="2016-01-01 00:00" --callsign=N0CALL --latitude=0.000 --longitude=0.000

There are many satellites that use standard packet radio AX.25 and can be received with any software TNC such as Direwolf. gr-satellites includes kiss_submitter to perform telemetry submission when using a software TNC.

kiss_submitter connects to the software TNC as a KISS TCP client. The frames received by the software TNC will be submitted by kiss_submitter. To use kiss_submitter, you must specify your callsign and coordinates as when submitting telemetry using any of the decoders. You also need to specify the NORAD ID of the satellite you are receiving. This can be done by setting using --norad if using the .py script or with the parameter if using the .grc file. It is very important that you set the NORAD ID correctly. You can search the NORAD ID in celestrak.

You must start the software TNC first and the run the .py script or the .grc file for kiss_submitter.

It is also possible to use the flowgraphs in gr-satellites to submit telemetry to the Harbin Institute of Technology servers using proxy_publish.py in gr-lilacsat/examples/proxy_publish. To enable this, you must open the flowgraphs in gnuradio-companion and enable the "Socket PDU" block (usually on the lower right corner of the flowgraph). This block is disabled by default because when it is enabled the flowgraph won't run unless proxy_publish.py is running. Also see this information about how to set the proper ports in proxy_publish.py.

Some modes (9k6 BPSK, for instance) need to be received using SSB mode, but the bandwidth of the signal is larger than the usual 3kHz bandwidth of a conventional SSB receiver. Therefore, an SDR receiver or a heavily modified conventional SSB receiver is needed (a 9k6 BPKS signal is about 15kHz wide).

The decoders for satellites using these kind of wide SSB signals expect the signal to be centred at an audio frequency of 12kHz. This means that you have to dial in USB mode to a frequency 12kHz lower than the nominal frequency of the satellite (+/- Doppler). If your SDR program allows this (gqrx does), the best idea is to set an SSB audio filter from 0Hz to 24kHz and then tune the signal in the middle of the passband. Alternatively, you can use the --bfo parameter if using the .py file or edit the corresponding parameter in the .grc file to use a frequency different from 12kHz.

If you are using the wide SSB receivers from gr-frontends you don't need to do anything special, as these receivers already dial in USB mode to a frequency 12kHz than the nominal and use a 24kHz wide audio filter.

We are all used to the two SSB modes: USB (which is sideband-preserving) and LSB (which is sideband-inverting). When receiving FM (or FSK), there is the same concept. An FM receiver can be sideband-preserving or sideband-inverting. This makes no difference when receiving analog FM (both sound the same) or AX.25 (which uses a differential protocol).

However, some satellites which use FSK (AAUSAT-4 and GOMX-3, for instance) need a sideband-preserving FM receiver. If your receiver is sideband-inverting, you can use set --invert=-1 while running the .py file or edit the corresponding parameter in the .grc file to invert the signal again in the decoder and recover the original signal with the correct sidebands.

To run the decoder and save the output to a file, it is possible to do something like

python2 -u aausat_4.py | tee /tmp/aausat4.log

This will both print the beacons in real time and also save all the output to the text file /tmp/aausat4.log.