Decoding packets from 3CAT2

On 15th August, a Chinese CZ-2D rocket launched three satellites from Juiuquan (Mongolia). The main payload was the Chinese satellite QSS, designed to do some experiments in quantum communications and entanglement. As anything that has the word quantum on it, this satellite even made it to the mainstream news in Spain. The rocket also launched Lixing 1, another Chinese satellite which will research the upper atmosphere, and 3CAT2, from the Universidad Politècnica de Catalunya (Spain).

3CAT2's main payload is a GNSS reflectrometer designed to measure the altitude of the Earth and map the oceans. This means that it uses reflections of satellite navigation signals off the surface of the earth and sea to perform mapping. It will mainly use the L1 and L2 signals from GPS, but it can also work with Galileo, GLONASS and BeiDou signals. It also carries a prototype of a magnetometer designed for the eLISA project. This project consists in setting up a laser interferometer in space to observe gravity waves. It is roughly the same as the Earth-based LIGO, that recently confirmed the first detections of gravity waves. However, since eLISA will be in space, its laser arms will much longer than LIGO's. This permits to study much lower frequencies than it's possible Earth-based interferometers.

3CAT2 has a downlink in the Amateur 2m band, at 145.970MHz, and transmits 9600bps BPSK. It also has a faster BPSK downlink in the S-band, presumably at 2401MHz (inside the Amateur 13cm band). The days following 3CAT2's launch I tried to receive its VHF signal, without any luck. I have been in contact with other Amateurs who also listened and didn't hear anything.

This morning, I've received email from Scott K4KDR telling me that he has heard the satellite for the first time and he has managed to do a recording, but he is unable to decode the data.

We where unsure about which encoding that 3CAT2 is using. It could be AX.25, or some custom protocol using FEC. As far as I know, the only other satellite that transmits 9k6 BPSK in the Amateur bands is LilacSat-2, which uses strong FEC. Nevertheless, I've taken a good look at Scott's recording and I've been able to decode one packet. This is, as far as I'm aware, the first decoding of 3CAT2 by an Amateur operator.

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Trying to decode data from ÑuSat

Last Monday, a Chinese CZ-4B rocket launched the Chinese Earth observation satellite ZY-3 and the Argentinian satellites ÑuSat-1 and 2. These two satellites are the first of the Aleph-1 constellation of Earth observation satellites. ÑuSat-1 carries LUSEX, an Amateur payload which consists of a U/V linear transponder. Also, the two ÑuSat satellites transmit backup telemetry in the 70cm Amateur band, as one can see in the IARU frequency coordination application. In fact, the latest news is that ÑuSat-1 transmits telemetry on 436.445MHz and ÑuSat-2 uses 437.445MHz. According to the public announcements, the telemetry was supposed to be 9200 baud or 19200 baud. However, some people have noticed that, on the contrary, it is 40 kbaud. Although the modulation and coding specifications are not public, I've taken a look at an IQ recording of ÑuSat-2 by Mike DK3WN to see if I can decode anything. Here are my findings.

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GOMX-3 data download

This weekend, Mike DK3WN caught GOMX-3 downloading a good amount of data. See his post here. This data consists mainly of the satellite retransmitting a lot of beacons that were generated during the last 16 hours or so.

GomSpace has recently released a complete parser for GOMX-3 beacons of type 1 0 (these are the beacons that contain ADS-B data). I have already incorporated this code into my gr-ax100 fork.

The binary data in KISS format (almost 250KB) and the parsed beacon data received during this data download is in gist. Probably the most interesting thing is the ADS-B data. Below you can see all the aircraft on the map. Clicking on any of them will show the details for that aircraft.

Since the orbit of GOMX-3 has an inclination of 51.6º, the satellite doesn't usually detect aircraft above 55ºN or below 55ºS. GomSpace has an image which shows lots of flights received with GOMX-3. There, the major air routes and hubs are apparent.

Decoding packets from GOMX-3: modulation and coding

Recently, Mike DK3WN pointed me to some decoder software for the satellite GOMX-3. This satellite is a 3U cubesat from GomSpace and transmits in the 70cm Amateur band. It has an ADS-B receiver on board, as well as an L-band SDR. As far as I know, no Amateur has decoded packets from this satellite previously, and Mike had some problems running the decoder software. I have taken a look at the software and tried my best to decode some packets from GOMX-3. So far, I have been able to do Reed-Solomon decoding and get CSP packets. However, I don't have the precise details for the beacon format yet. Here, I describe all of my findings.

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A bunch of data from AAUSAT-4

Recently, I have published a Gnuradio AAUSAT-4 decoder in Github. It is based on software from the university team, but several parts have been rewritten completely.

The current features of this decoder are as follows:

  • FEC decoding of both long frames and short frames using the code from bbctl (this code is included in the Gnuradio decoder)
  • CSP header parsing according to the specifications in Wikipedia
  • Parsing of the COM and EPS fields in telemetry beacons, using the code from the university team

In the future, I would like to be able to parse more data from the satellite, but I don't have the format specifications. I'm trying to get the university team to send me some information.

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Receiving Ku-band geostationary satellite beacons

After sorting out some problems with several connectors which caused huge phase noise in the external 27MHz reference, I have my 10GHz receiver up and running as it should. This station will be used to receive Es'hail-2 in the future. The station is composed of a 95cm offset dish, an Avenger PLL321S-2 Ku-band LNBF modified to use an external 27MHz reference, an OCXO/Si5351A kit used as the 27MHz reference, an RTL-SDR, and a cheap DVB-S2 receiver as a power supply (this allows me to change polarizations and LO frequency easily).

The dish is pointing to the 26ºE or 25.5ºE orbital position, where Es'hail-2 will be. Actually, I have pointed the dish to peak the beacon from BADR-5 the best I can. To test the performance of the station, I have tried to receive the beacons from several Ku-band satellites. Here are the results.

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First signals from AAUSAT-4

Today I woke up early to receive the signals from AAUSAT-4 as it passed over Spain for the first time. This satellite was launched from Kourou yesterday at 21:02UTC into a Sun-synchronous orbit. The main payload for the launch was Sentinel-1B, a 5GHz Synthetic Aperture Radar satellite from the Copernicus project of the ESA. The remaining satellites that were launched by the Soyuz rocket were Microscope, from the French CNES, designed to test Einstein's equivalence principle and the three cubesats in the Fly You Satellite! program: OUFTI-1, from the University of Liège, which carries a D-STAR amateur radio transponder, e-st@r-II, from the University of Torino, and AAUSAT-4, from the University of Aalborg, which carries an AIS receiver. Since the launch was into a polar orbit, the first pass of the Fly Your Satellite! cubesats over Spain was at 05:42UTC today.

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Scanning Ku band satellites with the FUNCube Dongle

I've recently installed my satellite dish and modified LNBF in my garden. This equipment will be used to receive Es'hail 2, the first geostationary satellite carrying an amateur radio transponder. Here I'll look at the hardware I'm using, how I did the alignment to the 25.5ºE geostationary orbital position where Es'hail 2 will be located, and how to have some fun scanning the direct broadcast satellites in the Ku band with a FUNCube Dongle Pro+.

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