Plotting spectrum measurements by SMOG-P

The SMOG-P 1P PocketQube that was launched recently has an interesting payload: a UHF spectrum monitor that records power spectral density measurements. Lately, I have been adding support in gr-satellites to decode the telemetry frames transmitted by SMOG-P and ATL-1 (which also carries a similar spectrum monitor), using the code published here as a reference.

As a result of this work, now it is possible to save and plot the spectrum data transmitted by SMOG-P and ATL-1 using gr-satellites. This post explains how.

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SMOG-P short codes

In my previous post I talked about the FEC used by the SMOG-P and ATL-1. In there, I reverse-engineered the long frames transmitted by SMOG-P and found that they use the AO-40 FEC protocol.

After publishing that post I started reverse-engineering the short frames. Meanwhile, Peter Horvath pointed me to a Github repository containing an implementation of the FEC used for short frames and long frames. I hadn’t seen that repository before (it’s not easy to search for SMOG-P or ATL-1 in Google, as many unrelated results come up). Indeed this repository contains the source of a FEC decoder for the short frames, so there is no need to reverse-engineer it.

Timur Kristóf, the author of that repository, says that the team plans to release the source for the decoder, but that they are currently very busy with the early operations of the satellites. This is very good news.

I have studied the code in the Github repository and included a decoder for the short FEC frames in gr-satellites.

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Decoding SMOG-P and ATL-1

Last Friday, an Electron rocket from Rocket Lab was launched from Mahia Launch Complex, New Zealand, carrying the ALE-2 microsatellite and 6 PocketQubes into a 400km polar orbit. Two of these PocketQubes are SMOG-P and ATL-1 from Budapest University of Technology and Economics.

They transmit in the 70cm Amateur satellite band, and although they have beeen successfully coordinated with IARU (see here and here), documentation about the protocols they use has not been published. There is some groundstation software available here, but the interesting part is implemented in the atlgnd_x86_64 and smogpgnd_x86_64 binary executables, for which source code is not available. As far as I know both satellites transmit using the same (or very similar) protocols.

In this post I describe my first attempts at reverse-engineering the transmissions of SMOG-P, with successful results. Preliminary support for decoding SMOG-P and ATL-1 has been added to gr-satellites in the maint-3.8 branch.

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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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Lucky-7 decoded

Lucky-7 is a Czech Amateur 1U cubesat that was launched on July 5 together with the meteorological Russian satellite Meteor M2-2 and several other small satellites in a Soyuz-2-1b Fregat-M rocket from Vostochny. The payload it carriers is rather interesting: a low power GPS receiver, a gamma ray spectrometer and dosimeter, and a photo camera.

It transmits telemetry in the 70cm Amateur satellite band using 4k8 FSK with an Si4463 transceiver chip. Additional details about the signal can be found here.

After some work trying to figure out the scrambler used by the Si4463, I have added a decoder for Lucky-7 to gr-satellites. This post shows some of the technical aspects of the decoder.

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