Yesterday, the FM repeater on the Amateur satellite LilacSat-2 was active. I’ve talked about LilacSat-2 before, but so far I hadn’t made any recordings containing subaudio telemetry. While contacting several Spanish stations (EA5TT, EA1JM and EA1IW) throughout the pass, I made an IQ recording to analyse the telemetry later. Here I take a look at the telemetry format and the decoded data.
A scrambler is a function that is applied to a sequence of data before transmitting with the goal of making this data more “random-like”. For instance, scrambling avoids long runs of the bits 0 or 1 only, which may make the receiver loose synchronization or cause spectral concentration of the signal. The receiver applies the inverse function, which is called descrambler, to recover the original data. The documentation for the scrambler blocks in GNUradio is not very good and one may need to take a look at the implementation of these blocks to get their parameters right. Here, I’ll try to explain a bit of the mathematics behind scramblers and the peculiarities of their implementation in GNUradio.
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.
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.
When receiving signals from a satellite, it can be important to correct for the Doppler shift in the signal. Normally, I use Gpredict to track satellites and compute the Doppler shift. Gpredict can control the frequency of a receiver using Hamlib to track the Doppler shift. When using an SDR receiver, there are several possible ways of using Gpredict’s frequency control.
Normally, the SDR software doesn’t support Hamlib control in a way that it’s useful and easy to use for this purpose. This is the case with Linrad, which is the software I use, and probably with many other popular SDR softwares. An easy solution is to let Gpredict completely control the frequency of the SDR receiver through Hamlib and prevent the SDR software from controlling the frequency. With the FUNCube Dongle Pro+, which is the receiver I normally use, this is easy to do. It can be controlled without problem with recent versions of Hamlib, and if you set the dongle in Linrad as an “Undefined” card instead of a FUNCube Dongle, then Linrad will not try to control its frequency.
The problem with this solution is that each time that the frequency gets updated, it does so in a non phase continuous manner, because the PLL of the receiver has to lock on to the new frequency, effectively losing reception for just a tiny amount of time. This supposes no problem for SSB, CW or FM reception, because your ears just don’t notice. However, if you want to receive any digital signal or SSTV, the frequency change usually messes with the decoder software, which loses sync and suffers decoding problems. An alternative solution is to leave the receiver frequency fixed and correct for Doppler shift in software.
- All the decoders and the frontend are run on the same flowgraph. I find this more practical than having to run all the flowgraphs separately. Also, some blocks can be reused in this way.
- It uses gr-gpredict-doppler to compensate for Doppler. As I mentioned in a previous post, I prefer this to the Doppler correcting system included in
- It plots and outputs to a file the signal strength on 437.200MHz and 437.225MHz. This can be used for later analysis.
- It supports recording file input, or live SDR using an ALSA source (for the FUNCube Dongle Pro+, for instance). Another SDR supported by GNUradio can be easily used.
- It supports recording both the raw IQ data and the Doppler corrected IQ data. The raw IQ file can be then played back by simultaneously running Gpredict with the correct settings for the recorded pass. The Doppler corrected IQ file can be played back without running Gpredict.
This receiver can be downloaded from GitHub. The flowgraph is a bit crammed, but that’s what you get for having all the decoders in the same flowgraph. Several of the input/output blocks are disabled, so that you can choose which ones to use.
After having my first QSO through the Harbin Institute of Technology amateur radio satellite LilacSat-2, I decided to give a serious try to the telemetry decoding software. This is available as a GNURadio module. A Linux distribution with all the proper software installed and configured is provided, for an easy use. However, I have used GNURadio in the past, so I wanted to try to setup the GNURadio module directly on my machine.
The web page for LilacSat-2 gives also a description of the different telemetry formats. The satellite has an SDR radio transmitting on 437.200MHz. This radio is used for the FM amateur radio transponder and also to transmit several different telemetry formats. The satellite also transmits telemetry on 437.225MHz, presumably using a different (non-SDR) radio and a different antenna, so that the satellite can keep transmitting telemetry even if the SDR system fails. Typically, when the FM transponder is disabled, the satellite will transmit 9k6 BPSK telemetry on 437.200MHz and 4k8 GFSK telemetry on 437.225MHz. These can be seen in the picture above, which was made using my RF recording and baudline. The packet on the upper right is 4k8 GFSK and the packet on the lower left is 9k6 BPSK. Notice the slight slant due to Doppler.