Yesterday I looked at decoding some data transmitted by LES-5. Today I have analysed a longer recording made by Scott Tilley VE7TIL to perform an eclipse timing on 2020-03-25. The study has been done in this Jupyter notebook, which looks at the sequences of symbols extracted before and after the eclipse (they are kept as two separate sequences because the transmit frequency changed slightly after the eclipse, so decoding required two separate passes).
Category: Experiments
Measurements, transmissions, receptions, etc. with the objective of obtaining and analysing some data.
Decoding LES-9
After decoding a recording of the LES-5 236.7MHz telemetry beacon made by Scott Tilley VE7TIL, I have decoded an older recording made by Scott of the S-band beacon of LES-9. This satellite was launched in 1976 and it has a 100 baud BPSK beacon at 2250MHz. Scott twitted about it in April 2019, and in January 2020 he reported that the modulation had stopped and the beacon was now a CW carrier.
I have used this recording made by Scott in 2020-01-13. The GNU Radio demodulator, which is very similar to the one for LES-5, is here and the Jupyter notebook with the results is here. Below, I make a brief summary of the results.
Decoding LES-5
LES-5 is a satellite launched in 1967. It was built by the MIT Lincoln Laboratory and its main payload was an experimental transponder for the military 230MHz band. It was placed in a subsynchrounous orbit with an altitude of around 33400km (GEO altitude is 35786km). Its operations ceased in 1971.
A couple days ago, Scott Tilley VE7TIL discovered that LES-5 was still transmitting, and was able to receive its beacon at 236.749MHz. Scott reports that LES-5 is the oldest GEO-belt object that he knows to be still transmitting.
The beacon is modulated, rather than being a CW carrier, so Scott sent me a short recording for analysis. This post is a summary of my study.
Coherence and QO-100
My tweet about the AMSAT-BR QO-100 FT8 QRPp experiment has spawned a very interesting discussion with Phil Karn KA9Q, Marcus Müller and others about weak signal modes specifically designed for the QO-100 communications channel, which is AWGN albeit with some frequency drift (mainly due to the imperfect reference clocks used in the typical groundstations).
Roughly speaking, the conversation shifted from noting that FT8 is not so efficient in terms of EbN0 to the idea of using something like coherent BPSK with \(r=1/6\) CCSDS Turbo code, then to observing that maybe there was not enough SNR for a Costas loop to work, so a residual carrier should be used, and eventually to asking whether a residual carrier would work at all.
There are several different problems that can be framed in this context. For me, the most interesting and difficult one is how to transmit some data with the least CN0 possible. In an ideal world, you can always manage to transmit a weaker signal just by transmitting slower (thus maintaining the Eb/N0 constant). In the real world, however, there are some time-varying physical parameters of the signal that the receiver needs to track (be it phase, frequency, clock synchronization, etc.). In order to detect and track these parameters, some minimum signal power is needed at the receiver.
This means that, in practice, depending on the physical channel in question, there is a lower CN0 limit at which communication on that channel can be achieved. In many situations, designing a system that tries to approach to that limit is a hard and interesting question.
Another problem that can be posed is how to transmit some data with the least Eb/N0 possible, thus approaching the Shannon capacity of the channel. However, the people doing DVB-S2 over the wideband transponder are not doing it so bad at all in this respect. Indeed, by transmitting faster (and increasing power, to keep the Eb/N0 reasonable), the frequency drift problems become completely manageable.
In any case, if we’re going to discuss about these questions, it is important to characterize the typical frequency drift of signals through the QO-100 transponder. This post contains some brief experiments about this.
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.
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.
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.
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.
QO-100 BPSK beacon frequency measured at Bochum
The experiments about measuring the frequency stability of the local oscillator of the QO-100 NB transponder with a Vectron MD-011 GPSDO I made a few days ago indicated that the Allan deviation of the local oscillator was probably better than \(10^{-11}\) for \(\tau\) between 1 and 100 seconds. The next step in trying to characterize the stability of the local oscillator is to use a reference clock which is more stable than the Vectron.
I contacted Achim Vollhardt DH2VA asking him if it was possible to record the downlink of the BPSK beacon at Bochum, so as to have a recording referenced to the Z3801A GPSDO in Bochum, which is much more stable than the Vectron. He and Mario Lorenz DL5MLO have been very kind and they have taken the effort to make a recording for me. This post is an analysis of this recording made at Bochum.
More frequency measurements of the QO-100 NB transponder
This post is a follow up to my experiments about measuring the stability of the QO-100 NB transponder local oscillator. I am now using the Vectron MD-011 GPSDO that Carlos Cabezas EB4FBZ has lent me to reference all my QO-100 groundstation (see more information about the Vectron GPSDO in this post).
The Vectron MD-011 has an Allan deviation of \(10^{-11}\) at \(\tau = 1\,\mathrm{s}\) and \(2\cdot10^{-11}\) at \(\tau = 10\,\mathrm{s}\) according to the datasheet, so it is an improvement of an order of magnitude compared to my DF9NP TCXO-based GPSDO. I have made more measurements with the Vectron MD-011 as in my previous experiments, measuring the phase of the BPSK beacon transmitted from Bochum and a CW tone transmitted with my station. This post summarizes my results and conclusions.