Simulating JT modes: how low can they get?

In this post I’ll show how one can use the signal generation tools in WSJT-X to do decoding simulations. This is nothing new, since the performance of the modes that WSJT-X offers has being thoroughly studied both with simulations and real off-air signals. However, these tools seem not very widely known amongst WSJT-X operators. Here I’ll give some examples of simulations for several JT modes. These can give the operators a hands-on experience of what the different modes can and cannot achieve.

Please note that when doing any sort of experiments, you should be careful before jumping to conclusions hastily. You should make sure that the tools you’re using are working as they should and also as you intend to (did you enter correctly all the parameters and settings?). Also, you should check that your results are reproducible and agree with the theory and other experiments.

Another warning: some of the software that I’ll be showing here, in particular the Franke-Taylor soft decoder for JT65 and the QRA64 mode, is still under development. The results that I show here may not reflect the optimal performance that the WSJT-X team aims to achieve in the final release version.

After all these warnings, let’s jump to study the modes. We’ll be considering the following modes: WSPR, JT9A, JT65A, JT65B and QRA64B. To give our tests some purpose, we want to find the decoding threshold for these different modes. This is the signal to noise ratio (SNR) below which the probability of a successful decode is too small to be useful (say, lower than 20%). For each mode, we will generate 100 test files containing a single signal with a fixed SNR. We will then see how many files can be successfully decoded for each SNR.

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LilacSat-1 Codec 2 downlink

LilacSat-1 is one of the satellites that will form part of the QB50 constellation, a network of 50 cubesats built by different universities around the world that will conduct studies of the thermosphere. LilacSat-1 is Harbin Institute of Technology’s satellite in the QB50 constellation, and is expected to launch late this year. Incidentally, his “brother” LilacSat-2 launched in September 2015, and it has become a popular satellite because of its Amateur Radio FM repeater.

Apparently, LilacSat-1 will feature a very novel transponder configuration: FM uplink and Codec2 digital voice downlink. I have discovered this yesterday while browsing the last updates to the Harbin Institute of Technology gr-lilacsat github repository. In fact, there is no mention of digital voice in the IARU coordination page for LilacSat-1. According to the coordination, the transponder will be mode V/U (uplink in the 144MHz band and downlink in the 435MHz band). However, it seems that only downlink frequencies have been coordinated with IARU. Hopefully the uplink frequency will lie in the satellite subband this time. LilacSat-2 is infamous because of its uplink at 144.350MHz, which lies in the SSB subband in the Region 1.

Codec2 is the open source digital voice codec that is used in FreeDV. This makes LilacSat-1 very exciting, because Codec2 is the only codec for digital voice radio that is not riddled with patents. Moreover, it performs much better than its main competitor: the AMBE/IMBE family of codecs, which are used in D-STAR, DMR and Yaesu System Fusion. Codec2 can achieve the same voice quality as AMBE using roughly half the bitrate.

Harbin Institute of Technology has recently published a GNUradio decoder for the Codec2 downlink and an IQ recording to test the decoder. Here I take a quick look at this code and I talk a bit about the possibilities of using Codec2/FreeDV in satellites.

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Concurso QSL V-UHF

Today I’ve hiked to Cerro de San Pedro, SOTA summit EA4/MD-020 (1425m), to work in the last national V-UHF contest of the year: concurso QSL. This contest is a bit particular, because it coincides with the IARU-R1 UHF & up contest, so the contacts in the UHF & up bands count for both contests. As always, I’m participating in the 6 hours category with my QRP station: a FT-817ND with 5W and a 3 element yagi on 144MHz and 7 element yagi on 432MHz (the Arrow satellite yagi).

I arrived at the summit at 8:00UTC and worked until the end of the contest, at 14:00UTC, so I could enjoy almost 6 full hours of operation. As expected, after 12:00UTC there where few people left in the contest, as almost everybody had gone for lunch. The map of stations worked is below. Stations in green where worked both on 144MHz and 432, stations in blue where worked only on 144MHz and my operating position is marked in red.

Participation has been perhaps a bit low and propagation was not so good at times, but overall I’m happy with my results, which compare well with other contests this year. I missed some usual stations from the zones EA3 and EA5. I think that propagation to these zones was only open briefly during the contest.


It turns out that the satellites AISAT and ATHENOXAT-1 use the NanoCom U482C transceiver from GomSpace. This is the same transceiver that GOMX-1 uses, so the same decoder can be used.

I’ve added example flowgraphs and wav recordings to gr-ax100 and complete decoders to gr-satellites. Note that there is no telemetry parser yet, because I don’t have the telemetry format used by these satellites. Thanks to Jan PE0SAT for sending me an AISAT recording and to Roland PY4ZBZ for sending an ATHENOXAT-1 recording (note that this satellite is on a low inclination orbit, so it can only be received near the equator).

I’m on the lookout for any other satellites using the NanoCom U482C transceiver or the NanoCom AX100 transceiver (this is the transceiver that GOMX-3 uses), as it should be possible to decode them with gr-ax100.

Decoding GOMX-1 telemetry

GOMX-1 is a 2U cubesat from GomSpace that was launched in November 2013 into a sun-synchronous orbit. As far as I know, it was the first satellite with an ADS-B receiver payload. It transmits telemetry on the 70cm Amateur band, including some data from the ADS-B receiver, as GOMX-3 does. Some Amateurs, including me, had tried to decode its telemetry on several occasions, without success. GOMX-3 will decay in about 4 weeks, as it was launched from the ISS on October 2015. Therefore, it now becomes more interesting to decode GOMX-1, which is in a longer term orbit. After one more serious try, I’ve been able to decode the telemetry. This is the first time that an Amateur decodes telemetry from GOMX-1 completely. The decoder code can be found in gr-satellites and gr-ax100, including an example wav file in gr-ax100/examples/gomx-1.wav.

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Some notes on BEESAT and Mobitex-NX

The family of BEESAT satellites from the TU Berlin transmit telemetry on the Amateur bands using the Mobitex-NX protocol. Some of the BEESAT satellites also include a digipeater using this same protocol. There is a GNUradio implementation from TU Berlin of a software TNC for these satellites. This software has some shortcomings (for instance, FEC decoding wasn’t working properly). I’ve made my own fork where I’ve fixed some of the problems. Here I’ll talk about various aspects of the Mobitex-NX protocol and the GNUradio implementation.

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A brief try at decoding HORYU-4 1k2 AFSK telemetry

In the previous post I’ve talked about HORYU-4 CW telemetry. Here I report my findings when trying to decode 1200baud AFSK telemetry. Since the satellite transmits digital telemetry only over Japan, the recordings I’ve analysed have being kindly provided by Tetsurou JA0CAW. There is a telemetry format document from Kyutech, but as it is the case with the CW document, it is rather incomplete and lacks several important details.

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HORYU-4 CW telemetry format

HORYU-4 is a small satellite from Kyushu Institute of Technology (Japan) designed to test a high voltage solar array in space and observe the effects produced by the charge on the spacecraft due to the high voltage. It transmits telemetry on the 70cm and 13cm amateur bands. It has a CW beacon at 437.375MHz, a 1200baud AFSK telemetry downlink at 437.375MHz and a 100kbaud BPSK telemetry downlink at 2400.3MHz. The digital telemetry downlinks are only active over Japan and use a custom packet format. Here we take a brief look at the format of the CW telemetry.

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IARU R1 145MHz contest

Today I hiked with all the family to La Najarra, SOTA summit EA4/MD-013 (2122m), to participate in the IARU Region 1 145MHz contest. Unfortunately, for some weird reason very few stations in Spain participate in this contest. My plan was to make a combined contest activity and SOTA activation, making QSOs with whoever was working SSB in the contest, but spending most of the time calling in FM. This gives me the opportunity to contact many more stations, because not many hams have a VHF yagi and SSB radio, but many have a VHF vertical and FM radio. It also gives these local hams the possibility to work a SOTA summit (most SOTA activity is in HF here) and work some DX with a quite basic FM station (100km or more are easy to achieve).

I worked from around 9:30UTC to 11:30UTC. The station was, as usual, an FT-817ND with 5W and an Arrow satellite yagi (3 elements).

I’ve put in the contest log all stations that were able to give me their locator (many hams that work only FM have no clue about what locators are). This is OK with the contest rules. The other stations went only to the SOTA log. Below, you can find the map of contest contacts. I made a total of 17 contacts, but, of course, I can’t put on the map the stations that didn’t know their locator.

Introducing gr-satellites

This post is to present my gr-satellites project. The goal of this project is to make a collection of GNUradio decoders for the telemetry of different satellites. The decoders support submitting telemetry in real time to the PE0SAT telemetry server. Another goal is that the decoders are as easy to use as possible, to try to make more people interested in receiving digital telemetry from satellites and collaborating in online telemetry submission.

The decoders can be used with the Gqrx SDR software, using its UDP audio streaming capabilities. This is the easiest way to use the decoders. It is also possible to use the GNUradio frontends in the companion project gr-frontends. These support several different SDR hardware, WAV and IQ recordings, and conventional receivers connected through a soundcard. They are design to be flexible and to allow its use in headless and automated receiving configurations.

The long-term goal of this project is to provide an alternative software chain to the UZ7HO soundmodem, AGW packet engine and DK3WN telemetry forwarder. The use of GNUradio makes these decoders more configurable and flexible and eases programming decoders for non-AX.25 satellites, which usually employ strong forward error correction.

Currently, the satellites supported by gr-satellites are 3CAT-2, AAUSAT-4 and GOMX-3. I plan to continue adding support for more satellites in the future.