Third alpha for gr-satellites 3

gr-satellites v3 is a large refactor of the gr-satellites codebase that I introduced in September. Since then, I have been working and releasing alphas to showcase the new features and get feedback from the community. Today I have released the third alpha in the series: v3-alpha2.

Each of the alphas has focused on a different topic or feature, and v3-alpha2 focuses on extending the number of satellites supported and bringing back most of the satellites supported in gr-satellites v2. Whereas previous alphas supported only a few different satellites, this alpha supports a large number. Therefore, I think that this is the first gr-satellites v3 release that is really useful. I expect that interested people will be able to use v3-alpha2 as a replacement of gr-satellites v2 in their usual activities.

In this post, I explain the main features that this alpha brings. For the basic usage of gr-satellites v3, please refer to the post about the second alpha.

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First tests of a narrowband data modem for QO-100

Since a while ago, I have had the idea to design a data modem for the NB transponder of QO-100 (Es’hail 2). The main design criteria of this modem is that it should fit in a bandwidth of 2.7kHz and be able to work at a signal power equal to that of the transponder BPSK beacon, since these are the bandwidth and power constraints when using the NB transponder.

Currently, the following modes are used for medium speed data (understood as a few kbps) on the NB transponder. First, there are the FreeDV modes, whose use has been covered in this Lime microsystems community post. Most of these modes use OFDM or multi-carrier modems and are designed having HF fading channels in mind. These don’t give good performance over the QO-100 transponder, since the frequency instabilities of the transmitters and receivers give problems with OFDM modems. A single carrier modem is much better. David Rowe VK5DGR has made some modifications to the FreeDV 2020 modem to improve performance over QO-100, and it certainly works quite well, but better results can be obtained with a single carrier modem.

There are some people using DRM for DSSTV. This is also an OFDM modem intended for HF, and the symbol time is quite long, so the frequency instabilities can give problems. Finally, there is KG-STV, which was relatively unpopular before QO-100 but it is seeing a lot of use due to its good performance. It uses a single carrier MSK modem. This is probably the most popular medium speed mode on the NB transponder, but it is only 1200bps.

One important characteristic of the NB transponder is that there is a lot of SNR available. The rule is that no signal should be stronger than the beacons, but the BPSK beacon has a CN0 of around 54dB as received in my station. It is also not difficult (in terms of uplink EIRP) to achieve the same power as the beacon. Therefore, it is a reasonable assumption that stations interested in using a medium speed data modem will adjust their uplink power to be as strong as the BPSK beacon. I already hinted at what is possible with such a strong signal in this post.

I have decided to do some preliminary tests to check the performance of a 2kbaud 8PSK signal over the NB transponder. This post summarizes my results. The material for the post can be found in the qo100-modem Github repository.

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Measuring the Allan deviation of a GPSDO with an SDR

A few days ago I tried to measure the QO-100 NB transponder LO stability using my DF9NP 10MHz GPSDO. It turned out that my GPSDO was less stable than the LO, so my measurements showed nothing about the QO-100 LO. Carlos Cabezas EB4FBZ has been kind enough to lend me a Vectron MD-011 GPSDO, which is much better than my DF9NP GPSDO and should allow me to measure the QO-100 LO.

Before starting the measurements with QO-100, I have taken the time to use the Vectron GPSDO to measure the Allan deviation of my DF9NP GPSDO over several days. This post is an account of the methods and results.

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Second alpha for gr-satellites 3

Following the first alpha, I have released today v3-alpha1, the second alpha for gr-satellites 3. As I introduced in September, gr-satellites 3 will be a large refactor of gr-satellites, bringing many UI and under-the-hood changes. I am releasing a series of alphas during the development to get feedback from users. Each of the alphas focuses on a different aspect.

The second alpha is focused on input and output formats. New functionality has been implemented to allow the user to choose the input and output in a flexible way. This post describes the main features added.

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First alpha for gr-satellites 3

In my last post, I introduced my plans to do a large refactor of gr-satellites, which when ready will originate a version 3.0.0 running on GNU Radio 3.8. During the development of this refactor, I intend to release alpha versions showing important new concepts or functionalities. The main goal of this is both to test if my ideas work well in practice and that interested people can start testing the new software and give feedback.

I have now published the first alpha release, which is called v3-alpha0. In this post I describe the functionality implemented in this alpha and how to use the software.

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gr-satellites roadmap

In my talk at GRCon19 last week I presented the roadmap I have planned for gr-satellites for the next months and some longer term developments. The relevant slide can be seen below.

gr-satellites roadmap in my talk in GRCon19

Here I will describe the roadmap in more detail, including how certain things will be done (or how to find your way among the different releases and branches in the Github repository), in order to get feedback from the community.

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Światowid image decoder

Yesterday I spoke about the Światowid image downlink protocol. Today, Piotr Kuligowski SQ4NOW has published an image that he has been able to decode from Światowid. The image was taken at around 3:29 UTC and downlinked at 6:38 UTC over Warsaw.

Looking at SatNOGS recordings of this event, I have noticed that the image data is sent with sequence numbers, contrary to what I stated in the description of the protocol in my previous post. This is something that SatRevolution must have added down the road, since it wasn’t present when I worked with them in June.

The protocol is as I described, but the first two bytes of each Reed-Solomon block are used as a little-endian block counter. The remaining 46 bytes are used to send the JPEG file data. The block counter is reset to zero at the start of a new file, and is increased for each Reed-Solomon block.

This block counter allows for automatic detection of lost blocks and start of new images, so I have added an image decoder to the Światowid decoder in gr-satellites. The decoder is based on the 1KUNS-PF image decoder. If there are missing blocks, gaps full of zeros are inserted in the JPEG file in their position. This allows easily merging files decoded from different groundstations just by ORing the files.

As an example showing the image decoder, I have processed this SatNOGS recording, which was made by the station of Cees Bassa in the Netherlands. To process a SatNOGS recording with the gr-satellites decoder, the OGG audio must be converted to WAV (using oggdec, for example), and the gain of the “Multiply Const” block in swiatowid.grc must be changed from 10 to 1, since SatNOGS recordings usually have too much gain.

The recording only contains the beginning of the transmission. The pass was west to east and the transmission was done when the satellite was in view of Warsaw, so by the middle of the transmission the satellite is already below the horizon in the Netherlands. Still, 1128 blocks could be decoded correctly. This amounts to 51888 bytes. The complete file is 204796 bytes long.

The partial image decoded from the SatNOGS recording is shown below.

Image transmitted by Światowid on 2019-08-31 06:38 (partial)

This image matches the one that Piotr has shown on Twitter. I find it interesting that the SatRevolution logo is already added on-board the satellite to the top left corner of the image.

Decoding Światowid

Światowid is an Earth observation 2U cubesat built by the Polish company SatRevolution. It carriers a camera with a resolution of 4 metres per pixel and an Amateur radio U/V FM transponder that was never activated due to power budget constraints. The cubesat was launched to the ISS on April 17 this year and released in orbit on July 3. It transmits on the 70cm Amateur satellite band, using 1k2 AFSK AX.25 APRS plaintext for telemetry and 9k6 FSK with a custom protocol for downlinking the camera images. According to the IARU frequency coordination sheet and SatRevolution, it can also transmit images in the 13cm Amateur satellite band at 500kbps.

During June, I worked under a contract with SatRevolution to adapt gr-satellites for their use with Światowid and KRAKsat. Since I am well aware of the problem of private companies using the Amateur satellite bands as “free spectrum” for their satellites, when I was first contacted by SatRevolution regarding this project I did a small background check and saw that Światowid and KRAKsat had obtained an IARU frequency coordination successfully.

I also showed my IARU R1 proposal to SatRevolution and told them that, even though I was signing an NDA for the project, according to ITU regulations they had to publish all the details for the protocols they used on Amateur bands. Formally, these details were not covered by the NDA, and we also agreed that the modified version of gr-satellites would be publicly released under the GPLv3. The decoder was released here on July 4, and this was also announced by SatRevolution on Twitter.

Some Amateurs were not at all happy with the news that the FM transponder was not going to be activated, and accused SatRevolution of adding only the FM transponder to get through the IARU coordination, without having any real intention to activate it, of possibly causing interference to SO-50 and of not giving back anything to the community. However, all of this happened by the time I was already finishing my project with SatRevolution.

After finishing this project, I didn’t merge back to the main version of gr-satellites any of the modifications I did for SatRevolution, and I am not aware of SatRevolution having published any technical information about the 9k6 custom protocol used by Światowid. I didn’t see any reports of people receiving the 9k6 signal (only the APRS telemetry beacon was often seen), so I didn’t consider sorting this out as a priority, since I wasn’t even sure if the 9k6 protocol was actually being used (maybe they were only using S-band to download the images).

A couple days ago, I saw that Piotr Kuligowski SQ4NOW, Maciej Nowak and Tomek SP9TMQ, from the PW-Sat2 team managed to decode one of the images transmitted by Światowid using the 9k6 custom protocol. Talking with Piotr, I learned that they had used my modified gr-satellites version, but as it didn’t provide a complete solution to decode images (below I explain what was missing), they had to do some reverse engineering of the custom protocol.

Now that I’ve learned about the effort of Piotr, Maciej and Tomek, I have decided to add a complete decoder solution for the Światowid 9k6 custom protocol to the main gr-satellites version and to write this post to document completely the protocol.

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Software for my QO-100 groundstation

You may have heard about my groundstation for QO-100 (Es’hail-2), which is based on a BeagleBone black and a LimeSDR Mini. A description of this station appeared in a LimeSDR field report. However, I haven’t spoken in detail about the software yet, since I was testing various things and using a makeshift setup until I had some time to put together a solution that I really liked.

Now I am quite happy with the result and indeed I have decided to start up a Github repository with the software I am using in case anyone finds it useful. This post is a description of the software I am using for the narrowband transponder.

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