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.
Category: Amateur radio
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.
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.
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.
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.
Telemetry format of 3CAT-2
The team from the NanoSat Lab in Universidad Politècnica de Catalunya have published a telemetry analyser for 3CAT-2. This analyser is designed to connect to a TCP server and get the AX.25 frames in KISS format.
The telemetry format is rather simple, as one can see by looking at the PrintBeacon() function in 3cat2_telemetry.c. As I imagined, the contents of each beacon are just the numerical values of several telemetry channels written in ASCII. For example:
3 7781 0245 07 06 1 0 3.5e-01 2.5e-01 1.6e-01 6.8e-09 1.2e-09 1.8e-08
All the fields are separated by a space, except the 5th and 6th fields, which are separated by a tab. The content of the first 7 fields is as follows:
- Mode. Possible values: 1 survival mode, 2 sun-safe mode, 3 nominal mode, 4 TX communication incoming (data downlink), 5 RX communications (command uplink), 6 and 7 payload mode.
- Battery voltage in mV. In the example 7.781V.
- Current consumption in mA. In the example 245mA.
- EPS temperature (probably in ºC). In the example 7ºC.
- Antenna temperature (probably in ºC). In the example 6ºC.
- Status of the ADCS system. 0 means detumbling enabled. 1 means SS-nominal.
- Control flag of the ADCS routine. Possible values: 0 automatic, 1 manual
The next 3 fields are floating point numbers. If detumbling is enabled, they correspond to magnetomer values in nT for the axes X, Y and Z respectively. If detumbling is not enabled, they correspond to the sun vector, axes X, Y and Z.
The last 3 fields correspond to the control voltages for axes X, Y and Z, regardless of whether detumbling is enabled or not.
Of course, the telemetry format is so easy that it can even be parsed with a “one-line” awk script:
strings sats/3cat2-20160824-pe0sat.kiss | awk '{if ($1==1) printf "Survival"; if ($1==2) printf "Sun-safe"; if ($1==3) printf "Nominal"; if ($1==4) printf "TX"; if ($1==5) printf "RX"; if ($1>=6) printf "Payload"; printf " %.2fV %dmA EPS: %2dºC Ant: %2dºC ", $2*1e-3, $3, $4, $5; if ($7==0) {printf "ADCS auto "} else {printf "ADCS manual "}; if ($6==0) {printf "Detumbling (%f,%f,$f) nT", $8, $9, $10} else {printf "SS-nominal Sun: (%.2f,%.2f,%.2f)", $8, $9, $10}; printf " Control (%.1e,%.1e,%.1e)V\n", $11, $12, $13}'
which shows the following output:
Nominal 8.26V 233mA EPS: 4ºC Ant: 8ºC ADCS auto SS-nominal Sun: (0.49,0.42,1.00) Control (6.9e-09,1.7e-09,1.7e-08)V Nominal 8.28V 221mA EPS: 5ºC Ant: 8ºC ADCS auto SS-nominal Sun: (0.16,0.87,0.57) Control (6.7e-09,1.4e-09,1.7e-08)V Nominal 8.29V 245mA EPS: 5ºC Ant: 8ºC ADCS auto SS-nominal Sun: (0.26,0.96,0.46) Control (6.7e-09,1.4e-09,1.7e-08)V Nominal 8.30V 257mA EPS: 5ºC Ant: 8ºC ADCS auto SS-nominal Sun: (0.62,0.78,0.42) Control (6.7e-09,1.4e-09,1.7e-08)V Nominal 8.30V 257mA EPS: 5ºC Ant: 9ºC ADCS auto SS-nominal Sun: (0.64,0.72,0.49) Control (6.7e-09,1.4e-09,1.7e-08)V Nominal 8.30V 245mA EPS: 5ºC Ant: 9ºC ADCS auto SS-nominal Sun: (0.64,0.66,0.59) Control (6.8e-09,1.5e-09,1.7e-08)V Nominal 8.30V 245mA EPS: 5ºC Ant: 9ºC ADCS auto SS-nominal Sun: (0.60,0.60,0.71) Control (6.8e-09,1.5e-09,1.7e-08)V Nominal 8.30V 245mA EPS: 5ºC Ant: 9ºC ADCS auto SS-nominal Sun: (0.54,0.54,0.86) Control (6.8e-09,1.6e-09,1.7e-08)V Nominal 8.29V 245mA EPS: 5ºC Ant: 10ºC ADCS auto SS-nominal Sun: (0.45,0.49,1.00) Control (6.9e-09,1.7e-09,1.7e-08)V Nominal 8.28V 245mA EPS: 5ºC Ant: 10ºC ADCS auto SS-nominal Sun: (0.32,0.44,1.00) Control (6.9e-09,1.7e-09,1.7e-08)V
The KISS file in question was obtained from the recording on 24/08/2016 at 10:54 by PE0SAT that I mentioned at the end of a previous post.
Many thanks to Juan Fran Muñoz and the rest of the NanoSat Lab team for publishing the telemetry analyser and sharing details about the satellite and the operations.
How hard is it to decode 3CAT-2?
In a previous post, I looked at the telemetry packets transmitted by the satellite 3CAT-2. This satellite transmits 9600bps AX.25 BPSK packets in the Amateur 2m band. As far as I know, it is the only satellite that transmits fast BPSK without any form of forward error correction. LilacSat-2 uses a concatenated code with a (7, 1/2) convolutional inner code and a (255, 223) Reed-Solomon outer code. The remaining BPSK satellites transmit at 1200bps, either using AX.25 without FEC (the QB50p satellites, for instance), or with strong FEC (Funcube, for example). Therefore, I remark that 3CAT-2’s packets will be a bit difficult to decode without errors. But how difficult? Here I look at how to use the theory to calculate this, without resorting to simulations.
RS92-SGP: trying to change the frequency with an external reference
In a previous post, I talked about the possibility of changing the transmit frequency of a Vaisala RS92-SGP radiosonde by modifying the settings on its EEPROM. The lowest frequency you can achieve using this method is 400MHz and the highest probably depends on the particular unit, but it is somewhat between 410MHz and 423MHz. There are also reports of very low output power on the highest frequencies (I’ll explain why below). Clearly, this can’t be used to make the radiosonde transmit in 430MHz, inside the 70cm Amateur band. In fact, from what I’ve read online, the impression is that it’s not possible to modify the radiosonde to make it transmit in 430MHz. However, I wanted to try to feed an external reference to see what happened. Short story: it doesn’t work either. However, I discovered some interesting information about the RF section of the RS92-SGP along the way.
Decoding packets from 3CAT2
On 15th August, a Chinese CZ-2D rocket launched three satellites from Juiuquan (Mongolia). The main payload was the Chinese satellite QSS, designed to do some experiments in quantum communications and entanglement. As anything that has the word quantum on it, this satellite even made it to the mainstream news in Spain. The rocket also launched Lixing 1, another Chinese satellite which will research the upper atmosphere, and 3CAT2, from the Universidad Politècnica de Catalunya (Spain).
3CAT2’s main payload is a GNSS reflectrometer designed to measure the altitude of the Earth and map the oceans. This means that it uses reflections of satellite navigation signals off the surface of the earth and sea to perform mapping. It will mainly use the L1 and L2 signals from GPS, but it can also work with Galileo, GLONASS and BeiDou signals. It also carries a prototype of a magnetometer designed for the eLISA project. This project consists in setting up a laser interferometer in space to observe gravity waves. It is roughly the same as the Earth-based LIGO, that recently confirmed the first detections of gravity waves. However, since eLISA will be in space, its laser arms will much longer than LIGO’s. This permits to study much lower frequencies than it’s possible Earth-based interferometers.
3CAT2 has a downlink in the Amateur 2m band, at 145.970MHz, and transmits 9600bps BPSK. It also has a faster BPSK downlink in the S-band, presumably at 2401MHz (inside the Amateur 13cm band). The days following 3CAT2’s launch I tried to receive its VHF signal, without any luck. I have been in contact with other Amateurs who also listened and didn’t hear anything.
This morning, I’ve received email from Scott K4KDR telling me that he has heard the satellite for the first time and he has managed to do a recording, but he is unable to decode the data.
We where unsure about which encoding that 3CAT2 is using. It could be AX.25, or some custom protocol using FEC. As far as I know, the only other satellite that transmits 9k6 BPSK in the Amateur bands is LilacSat-2, which uses strong FEC. Nevertheless, I’ve taken a good look at Scott’s recording and I’ve been able to decode one packet. This is, as far as I’m aware, the first decoding of 3CAT2 by an Amateur operator.