During the last few days I've been experimenting with feeding signals from GNU Radio into Linrad using Linrad's network protocol. Linrad has several network protocols designed to share data between different instances of Linrad, but generally these protocols are only supported by Linrad itself. The only other example I know of is MAP65, which can receive noise-blanked data from Linrad using the timf2 format.

Another possible use of gr-linrad is as an instrumentation for any kind of GNU Radio flowgraph. It is very easy to stream data into Linrad, so it can be used as a very nice waterfall display or to do any sort of signal processing, such as noise blanking or adaptive polarization. Here I describe how to get the test flowgraph in gr-linrad working and some aspects of the network protocol.

During the last few days, I have been talking with Edson PY2SDR about using GNU Radio to decode digital telemetry from AO-73 (FUNcube-1) and other FUNcube satellites. I hear that in Virginia Tech Groundstation they have a working GNU Radio decoder, but it seems they never published it.

The modulation that the FUNcube satellites use is DBPSK at 1200baud. The coding is based on a CCSDS concatenated code with a convolutional code and Reed-Solomon, but it makes extensive use of interleaving to combat the fading caused by the spin of the spacecraft. This system was originally designed by Phil Karn KA9Q for AO-40. Phil has a description of the AO-40 FEC system in his web and there is another nice description by James Miller G3RUH.

I took a glance at this documents and noted that it would be a nice and easy exercise to implement a decoder in GNU Radio, as I have most of the building blocks that are needed already working as part of gr-satellites. Today, I have implemented an out-of-tree module with a decoder for the AO-40 FEC in gr-ao40. There is another gr-ao40 project out there, but it seems incomplete. For instance, it doesn't have any code to search for the syncword. I have also added decoders for AO-73 and UKube-1 to gr-satellites.

The signal processing in gr-ao40 is as described in the following diagram taken from G3RUH's paper.

First, the distributed syncword is searched using a C++ custom block. It is possible to set a threshold in this block to account for several bit errors in the syncword. De-interleaving is done using another C++ custom block. For Viterbi decoding, I have used the "FEC Async Decoder" block from GNU Radio, since I like to use stock blocks when possible. Then, CCSDS descrambling is done with a hierarchical block from gr-satellites. Finally, the interleaved Reed-Solomon decoders are implemented in a C++ custom blocks that uses Phil Karn's libfec.

The complete FEC decoder is implemented as a hierarchical block as show in the figure below.

Coding for HIT satellites (and other CCSDS satellites)

The Harbin Institute of Technology satellites LilacSat-2, BY70-1 and the upcoming LilacSat-1 all use a concatenated code with an $r=1/2, k=7$ convolutional code and a (255,223) Reed-Solomon code according to the CCSDS TM Synchronization and Channel Coding blue book specifications. The GNU Radio decoder gr-lilacsat by Wei BG2BHC includes a custom implementation of the relevant part of the CCSDS stack, probably ported into GNU Radio from some other software.

Recently, I have been working on decoding KS-1Q and I've seen that it uses the same CCSDS coding as the HIT satellites. This has made me realise that most of this CCSDS coding can be processed using stock GNU Radio blocks, without the need for custom blocks. The only exception is Reed-Solomon decoding. This can be done easily with gr-libfec, which provides an easy interface from GNU Radio to Phil Karn's libfec. Here I look at the details of the CCSDS coding and how to process it with GNU Radio. I've updated the decoders in gr-satellites to use this kind of processing. I'll also talk about the small advantages of doing it in this way versus using the custom implementation in gr-lilacsat.

KS-1Q decoded

In a previous post, I talked about my attempts to decode KS-1Q. Lately, WarMonkey, who is part of the satellite team, has been giving me some extra information and finally I have been able to decode the packets from the satellite. The decoder is in gr-ks1q, together with a sample recording contributed by Scott K4KDR. I've also added support for KS-1Q in gr-satellites. Here I look at the coding of the packets in more detail.

BY70-1 is a Chinese Amateur satellite that will launch on Monday 26 December. It has a V/U FM repeater, a camera and a 9k6 BPSK downlink on 70cm that transmits telemetry and the JPEG images taken by the camera. The BPSK downlink uses the same modulation and coding as LilacSat-2, of which I have spoken several times. Recently, Wei MingChuan BG2BHC has added support for the image downlink of BY70-1 to gr-lilacsat and a bit stream recording to test the image receiver.

Unfortunately, the image decoder is closed-source, as it contains some certification methods used to avoid fake packets over the internet. However, Wei gave me a brief description of how the image downlink protocol works. After seeing the closed-source decoder running, I had enough to figure out how the protocol works. I have implemented an open-source image decoder as a python GNU Radio block. It is in my gr-lilacsat fork, and it will soon be included in the upstream gr-lilacsat repository. Here I look at the protocol used for the image downlink.

This is a follow up post to my experiments studying OTH radar. I have found that the signal processing I did there to obtain the cross-correlation was far from optimal. This produced the strange side-bands below the main reflection. The keen reader might have noticed that I was doing the cross-correlation with a template pulse that lasted the whole pulse repetition cycle. However, the pulses from the radar are shorter. Therefore, it is a better idea to use a shorter template pulse. Ideally, the template pulse should have the same length as the transmitted pulse. However, I don't know this length precisely, because multipath propagation makes the received pulses a bit longer. However, I think that 6.5ms is a good estimate.

I have also changed the window for the pulse. I'm now using a Dolph-Chebyshev window. I use scipy to compute this window, because it is not included in GNU Radio. This window has the property that the side-bands have constant attenuation. Indeed, it minimizes the $L^\infty$ norm of the side-bands. There is a parameter that tunes the side-bands attenuation. For higher attenuations, you have a wider main lobe, while if you want a narrower main love you get less side-band attenuation. These properties make this window useful in radar applications.

Below I'm doing the cross-correlation in GNU Radio with a shorter template pulse shaped with a Dolph-Chebyshev window set for 55dB attenuation.

The good thing about the settable attenuation of the Dolph-Chebyshev window is that it can be used to trade-off performance between different features. First, we use an attenuation of 100dB. The side-bands are below the noise floor in this case, so we have no "false responses" in our cross-correlation. The drawback is that the main lobe is wide so the resolution of the features of the ionosphere in the image below is not very good.

Next we try with 55dB attenuation. This narrows the main lobe, improving the resolution and crispness of the features of the ionosphere in the image below. However, side-bands start being visible above the noise floor, producing "false responses". However, comparing with the image above, we now know where the false responses are.

I have updated the gist with the GNU Radio flowgraph and python script used to produce the images.

Looking at an HF OTH radar

Most amateur operators are familiar with over-the-horizon radars in the HF bands. They sometimes pop up in the Amateur bands, rendering several tens of kilohertzs unusable. Inspired by Balint Seeber's talk in GRCon16, I've decided to learn more about radars. Here I look at a typical OTH radar, presumably of Russian origin. It was recorded at my station around 20:00UTC on 8 December at a frequency around 6860kHz. This radar sometimes appears inside the 40m Amateur band as well.

Testing Opera sensitivity with GNU Radio

Some fellow Spanish Amateur Operators were talking about the use of the Opera mode as a weak signal mode for the VHF and higher bands. I have little experience with this mode, but I asked them what is the advantage of this mode and how it compares in sensitivity with the JT modes available in WSJT-X. I haven't found many serious tests of what is the sensitivity of Opera over AWGN, so I've done some tests using GNU Radio to generate signals with a known SNR. Here I'll talk about how to use GNU Radio for this purpose and the results I've obtained with Opera. Probably the most interesting part of the post is how to use GNU Radio, because it turns out that Opera is much less sensitive than comparable JT modes.

Reverse-engineering Outernet in GNU Radio blog

I have published a post in the GNU Radio blog about my reverse engineered GNU Radio Outernet receiver gr-outernet. I cover more or less the same information as in a previous post in this blog, but I include lots of screenshots. Many thanks to Ben Hilburn and Johnathan Corgan for contacting me to write this post in the GNU Radio blog and for their useful suggestions.