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.

I used a FUNcube Dongle Pro+ and a handheld Arrow Satellite antenna (a 7 element yagi on 70cm) to try to record the signals from AAUSAT-4 and e-st@r-II. The software used was Linrad, and my locator IN80DO. I have several buildings around my garden, but signals from AAUSAT-4 where quite strong (reaching 20dB over the noise floor) when the satellite was in clear view. e-st@r-II was not heard at all (and I haven't seen any reports of someone else receiving it, either).

The beacon format for AAUSAT-4 is well described on their webpage. The satellite beacons every 30 seconds, and one out of ten beacons is in 30wpm CW. The remaining beacons are in 2k4 FSK FEC coded CSP (the image below the title of this post shows a CSP packet in Linrad's waterfall). I managed to receive one CW beacon and several strong CSP beacons.

The CW beacon is the first thing I decoded after sending the reception report and IQ recording to the AAUSAT-4 team. Below you can see a spectrogram (done with Audacity). The beacon content was "OZ4CUB B8.0 T16+", indicating a battery voltage of 8.0V and a temperature of 16ºC.

CW beacon
CW beacon

One interesting thing I've noticed is that the frequency jumps a bit sometimes. Look carefully at the letters C, 8 and 0 in the image above. This can also be observed in CSP transmissions, but it is only noticeable during a long transmission.

Then I asked the satellite team if they had some software decoder for the CSP beacons. It turns out that they have some GNURadio custom blocks that may take some effort to get working but that definitely can be used to decode the beacons.

I did my own clock recovery and bit slicing routines in GNURadio, since I was already doing the signal filtering and FM demodulation in Linrad. You can see below my flowgraph. It's very similar to what is done in the decoder from the university. A small remark is that it pays off to set a relatively high value for the threshold in the Correlate Access Code block, because FEC can recover packets having many bit errors. The syncword used is OZ4CUB in ASCII. The signal is first multiplied by a constant to make it range between -1 and +1, which is what the clock recovery block expects to work optimally. The constant is negative because otherwise the bits are inverted for some reason.

GNURadio CSP beacon receiver
GNURadio CSP beacon receiver

The results from the decoder are in gist. A total of 5 beacons could be decoded.

The first CSP packet I recorded was a long transmission lasting several seconds. Probably the satellite responding to some command from its ground station in Aalborg. The beacon decoder shows that this packet includes the contents for a normal beacon, but I haven't decoded the remaining data. You can see that the "packet sent" count went up by 10, so this long transmission was probably composed of 10 different CSP packets.

The IQ file for the recording can be downloaded from Google Drive. It is a wav file with 192kHz sampling rate. The centre frequency is 437.450MHz and the start of the recording is 2016-04-26 05:42:34UTC.

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