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.

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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.

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Receiving the Vaisala RS92-SGP radiosonde launched from Madrid-Barajas

Each day, at 01:00UTC and 11:00UTC a Vaisala RS92-SGP radiosonde is launched from Madrid-Barajas airport. This is a small electronics package tied to a helium balloon that ascends up to between 24 and 28km high before bursting and descending on parachute. It is designed to measure atmospheric parameters on its way up. It includes temperature, pressure and humidity sensors, as well as a GPS receiver. The launch on Wednesdays at 11:00UTC also includes a plug-in ozone sensor (which is a much larger and more expensive package). The data is transmitted at 403MHz using Manchester-encoded 4800bps GMSK and protected using Reed-Solomon. You can find more information about the RS92-SGP model in its technical datasheet and about the launches at Madrid-Barajas and other launches in Spain in the Spanish AIP Section 5.3 (other activities of a dangerous nature). Also, there is somebody who feeds the radiosonde data into the APRS network using SM2APRS, so you can track the launches by following OKER-11 on aprs.fi.

Usually, the Sondemonitor software is used to receive and plot the parameters measured by the radiosonde and track the GPS data. Of course, this program is very nice and complete, but it is shareware, costs 25€ and runs only in Windows. I wanted to try if it is possible to track the GPS data in Linux using free software.

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Update on phase noise of 27MHz references

This is a follow up to a previous post where I investigated the phase noise of 27MHz references to be used for a 10GHz receiver. Dieter DF9NP has being kind enough to send me a 10MHz 0.25ppm TCXO to do some more tests.

I’ve connected the 10MHz TCXO to the DF9NP 27MHz PLL and used it to receive the beacon of BADR-5, as I did in the previous post. The phase noise of the 10MHz TCXO + 27MHz PLL can be seen in the following figure.

10MHz 0.25ppm TCXO and 27MHz PLL
10MHz 0.25ppm TCXO and 27MHz PLL

For comparison, see below the phase noise with the DF9NP 10MHz GPSDO and 27MHz PLL. There is not much difference between both. This seems to indicate that the culprit of the phase noise is the 27MHz PLL, as the 10MHz TCXO should be quite clean.

10MHz GPSDO and 27MHz PLL
10MHz GPSDO and 27MHz PLL

Phase noise of a Baofeng UV-5R 10GHz signal

Several of the Baofeng chinese handheld radios generate a weak 10GHz signal while in receive mode. Thus, they are a popular cheap and quick 10GHz signal source for tests. To generate a 10GHz signal, you have to tune the Baofeng to the 70cm band (for instance, 432MHz). The radio will generate a weak 24th harmonic while in receive mode. If you want a steady carrier, you have to set the squelch to zero. Otherwise you will just get beeps as the radio wakes up periodically to check for a signal. Lately, I’ve being investigating phase noise and reciprocal mixing of 10GHz receiver systems. A natural question is how good is the phase noise of a Baofeng used as a 10GHz signal source and whether it can be used to check if the phase noise performance of a receiver is acceptable. It turns out that it is not so noisy as one may first think.

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Ship reflection on ED5YAE 10GHz beacon

In my previous post, I mentioned the possibility of receiving 10GHz beacons reflecting off ships in the Mediterranean sea through the Paella Team WebSDR, in Alicante. Luis EA5DOM tells me that these reflections happen often. However, I didn’t get any in the time I was doing the recordings for the previous post.

After making much longer recordings, I have seen a couple of reflections. I would say that a dozen or so happen every day. However, they last for quite long. Here you can see a reflection lasting for almost 20 minutes. The Doppler shift ranges between -300Hz and -200Hz. At its strongest moment, the reflection is only 10dB weaker than the beacon.

ED5YAE: direct signal and reflection
ED5YAE: direct signal and reflection

Phase noise of 27MHz references for a Ku-band LNBF

Today, I’ve being measuring the phase noise of the different 27MHz references that I have for my Ku-band LNBF. The LNBF is an Avenger PLL321S-2. I’ve modified it, removing the 27MHz crystal and including a connector for an external 27MHz reference signal. In my lab, I have the following equipment to generate a 27MHz signal:

  • OCXO/Si5351A kit. This kit includes a 27MHz OCXO and a Si5351A frequency synthesizer. The Si5351A can act as a buffer and output the OCXO signal directly or generate a 27MHz clock.
  • A DF9NP 27MHz PLL and a DF9NP GPSDO. The GPSDO generates a 10MHz signal which is locked to GPS. The PLL generates a 27MHz from the 10MHz signal.

I’ve used linrad to receive the beacon of BADR-5 at 11966.2MHz using different references for the 27MHz signal. The AFC in linrad tries to compensate for any drift in the reference or the satellite beacon. By averaging, one can get good plots of the sideband noise of the beacon. This is far from a proper lab test, but it gives a good idea of the performance of the references.

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Trying to decode data from ÑuSat

Last Monday, a Chinese CZ-4B rocket launched the Chinese Earth observation satellite ZY-3 and the Argentinian satellites ÑuSat-1 and 2. These two satellites are the first of the Aleph-1 constellation of Earth observation satellites. ÑuSat-1 carries LUSEX, an Amateur payload which consists of a U/V linear transponder. Also, the two ÑuSat satellites transmit backup telemetry in the 70cm Amateur band, as one can see in the IARU frequency coordination application. In fact, the latest news is that ÑuSat-1 transmits telemetry on 436.445MHz and ÑuSat-2 uses 437.445MHz. According to the public announcements, the telemetry was supposed to be 9200 baud or 19200 baud. However, some people have noticed that, on the contrary, it is 40 kbaud. Although the modulation and coding specifications are not public, I’ve taken a look at an IQ recording of ÑuSat-2 by Mike DK3WN to see if I can decode anything. Here are my findings.

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GOMX-3 data download

This weekend, Mike DK3WN caught GOMX-3 downloading a good amount of data. See his post here. This data consists mainly of the satellite retransmitting a lot of beacons that were generated during the last 16 hours or so.

GomSpace has recently released a complete parser for GOMX-3 beacons of type 1 0 (these are the beacons that contain ADS-B data). I have already incorporated this code into my gr-ax100 fork.

The binary data in KISS format (almost 250KB) and the parsed beacon data received during this data download is in gist. Probably the most interesting thing is the ADS-B data. Below you can see all the aircraft on the map. Clicking on any of them will show the details for that aircraft.

Since the orbit of GOMX-3 has an inclination of 51.6º, the satellite doesn’t usually detect aircraft above 55ºN or below 55ºS. GomSpace has an image which shows lots of flights received with GOMX-3. There, the major air routes and hubs are apparent.