Simulating the TED gain for a polyphase matched filter

Trying to improve the performance of the demodulators in gr-satellites, I am switching to the Symbol Sync GNU Radio block, which was introduced by Andy Walls in GRCon17. This block covers the functionality of all the other clock synchronization blocks, such as Polyphase Clock Sync and Clock Recovery MM, while fixing many bugs.

One of the new features of the Symbol Sync block is the ability to specify the gain of the timing error detector (TED) used in the clock recovery feedback loop. All the other blocks assumed unity gain, which simply causes the loop filter taps to be wrong. However, the TED gain needs to be calculated beforehand either by analysis or simulation, as it depends on the choice of TED, samples per symbol, pulse shaping, SNR and other.

While Andy shows how to use the Symbol Sync block as a direct replacement for the Polyphase Clock Sync block in his slides, he leaves the TED gain as one, since that is what the Polyphase Clock Sync block uses. In replacing the Polyphase Clock Sync block by Symbol Sync in gr-satellites, I wanted to use the correct TED gain, but I didn’t found anyone having computed it before. This post shows my approach at simulating the TED gain for polyphase matched filter with maximum likelyhood detector.

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Decoding ESA Solar Orbiter

Solar Orbiter is an ESA Sun observation satellite that was launched on February 10 from Cape Canaveral, USA. It will perform detailed measurements of the heliosphere from close distances reaching down to around 60 solar radii.

As usual, Amateur observers have been interested in tracking this mission since launch, but apparently ESA refused to publish state vectors to aid them locate the spacecraft. However, 18 hours after launch, Solar Orbiter was found by Amateurs, first visually, and then by radio. Since then, it has been actively tracked by several Amateur DSN stations, which are publishing reception reports on Twitter and other media.

On February 13, the spacecraft deployed its high gain antenna. Since it is not so far from Earth yet, even stations with relatively small dishes are able to receive the data modulation on the X band downlink signal. Spectrum plots showing the sidelobes of this signal have been published in Twitter by Paul Marsh M0EYT, Ferruccio IW1DTU, and others.

I have used an IQ recording made by Paul on 2020-02-13 16:43:25 UTC at 8427.070MHz to decode the data transmitted by Solar Orbiter. In this post, I show the details.

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Extracting AX.25 satellites from SatNOGS DB

In my last post about gr-satellites 3, I announced that gr-satellites would start to support all the AX.25 satellites transmitting in Amateur bands. Historically, gr-satellites didn’t support packet radio (AFSK and FSK AX.25) satellites since there were too many of them and there were already other good decoders such as Direwolf. At one point Rocco Valenzano W2RTV convinced me to add “generic” packet radio decoders to gr-satellites and since then these have been seeing quite some use.

In gr-satellites 3 it is very easy to add new satellites, since this is done with a SatYAML file, which is a brief YAML file describing basic information about the satellite and its transmitters. Therefore, I decided to make a script to get this data from SatNOGS DB and write the SatYAMLs automatically for all the AFSK and FSK AX.25 satellites.

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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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DSLWP-B whole mission telemetry

Recently, together with people from Harbin Institute of Technology and CAMRAS, we have published in Zenodo the data collected during the DSLWP-B mission. This data release includes all the raw telemetry frames uploaded to the DSLWP telemetry server.

I have made a Jupyter notebook that loads up and parses the telemetry, with the idea of providing a simple way to study the data.

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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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Decoding FloripaSat-1

FloripaSat-1 is a 1U cubesat developed by Universidade Federal Santa Catarina, Brazil. It was launched two days ago from Taiyuan, China, together with the Chinese-Brazilian Earth observation satellite CBERS 4A and other small satellites. FloripaSat-1 transmits in the 145MHz and 436MHz Amateur satellite bands using the NGHam protocol developed by Jon Petter Skagmo LA3JPA in 2014. NGHam is intended as a modern replacement for AX.25 in packet radio. It uses a 32 bit syncword, a CCSDS scrambler, and Reed-Solomon FEC. Additionally, FloripaSat-1 transmits also using AX.25, for retrocompatibility and performance comparison.

The information published online about the protocols used by FloripaSat-1 makes a great impression at first. There are descriptions of the coding and telemetry format and an open source decoder. However, a deeper look reveals many flaws. The documentation doesn’t describe accurately or in enough detail what the satellite is transmitting, and several functions are not implemented in the open source decoder. To my best knowledge, the NGHam Reed-Solomon and the whole AX.25 implementations in the satellite seem to be broken, and these are not implemented in the decoder.

Nevertheless, I have added a decoder for FloripaSat-1 to gr-satellites maint-3.8 branch, covering the functions that do work. This post gives a closer look at some of the technical details.

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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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