Back in 2019, I took advantage of the autumn sun outage season of Es’hail 2 to make some observations as the sun passed in front of the fixed 1.2 metre offset dish I have to receive the QO-100 transponders. Using the data from those observations, I estimated the gain of the dish and the system noise. A few weeks ago, I have repeated this kind of measurements in the spring sun outage season this year. This post is a summary of the results.
This post has been delayed by several months, as some other things (like Chang’e 5) kept getting in the way. As part of the GNU Radio activities in Allen Telescope Array, on 14 November 2020 we tried to detect the X-band signal of Voyager-1, which at that time was at a distance of 151.72 au (22697 millions of km) from Earth. After analysing the recorded IQ data to carefully correct for Doppler and stack up all the signal power, I published in Twitter the news that the signal could clearly be seen in some of the recordings.
Since then, I have been intending to write a post explaining in detail the signal processing and publishing the recorded data. I must add that detecting Voyager-1 with ATA was a significant feat. Since November, we have attempted to detect Voyager-1 again on another occasion, using the same signal processing pipeline, without any luck. Since in the optimal conditions the signal is already very weak, it has to be ensured that all the equipment is working properly. Problems are difficult to debug, because any issue will typically impede successful detection, without giving an indication of what went wrong.
I have published the IQ recordings of this observation in the following datasets in Zenodo:
Back in March, I was helping Julien Nicolas F4HVX to test the S-band image transmitter of AMICal Sat before launch. In my post back then, I explained that AMICal Sat uses a Nordic Semiconductor nRF24L01+ 2.4GHz FSK transceiver chip to transmit Shockburst packets at 1Mbaud. I also explained how the Onyx EV76C664 CMOS image sensor works and how to process raw images.
AMICal Sat was finally launched on 2020-09-03, and since them the satellite team has been busy trying to downlink some images, both using the UHF transmitter (which uses the same protocol as Światowid) and the S-band transmitter. This has proven a bit difficult because the ADCS of the satellite is not working, and the downlink protocols are not very robust.
Julien has been sending me recordings done by their groundstation in Russia with the hope that we could be able to decode some of the data. Before several failed attempts where we were hardly able to decode a few packets, we got a particularly good S-band recording done on 2020-10-05. Using that recording, I have been able to decode a full image.
In a previous post I talked about how the high data rate signal of Tianwen-1 can be used to replay recorded telemetry. I did an analysis of the telemetry transmitted over the high speed data signal on 2020-07-30 and showed how to interpret the ADCS data, but left the detailed description of the modulation and coding for a future post.
Here I will talk about the modulation and coding, and how the signal switches from the ordinary low rate telemetry to the high speed signal. I also give GNU Radio decoder flowgraphs,
tianwen1_hsd.grc, which works with the 8192 bit frames, and
tianwen1_hsd_shortframes.grc, which works with the 2048 bit short frames.
As promised in this post, I will now speak about how to demodulate the Tianwen-1 telemetry signal. This post will deal with demodulation and FEC decoding. The structure of the frames will be explained in the next post. In this post I also give a fully working GNU Radio decoder that can store frames in the format used by the orbit state vector extraction Python script.
The DSCS satellites are a constellation of US military communication satellites. While the constellation is old and it is being replaced by WGS, there are still several active DSCS-III satellites. A few days ago, Scott Tilley VE7TIL tweeted about the DSCS-III-A3 X-band beacon. The satellite DSCS-III-A3, also known as USA-167, is the second most recent DSCS-III satellite, having been launched in 2003. It has an X-band beacon at 7604.6MHz.
Scott’s tweets included a very impressive and interesting find: a Master’s thesis about a DSCS-III beacon decoder made by James Coppola in 1992. The thesis contains a wealth of information about the beacon, as well as the complete C source code for the decoder.
Scott has also been kind enough to share with me some recordings that he made of the beacon, so in this post I’ll be looking at these and how they relate to the information in the thesis.
The launch last Saturday of Crew Dragon Demo-2 undoubtedly was an important event in the history of American space exploration and human spaceflight. This was the first crewed launch from the United States in 9 years and the first crewed launch ever by a commercial provider. Amateur radio operators always follow this kind of events with their hobby, and in the hours and days following the launch, several Amateur operators have posted reception reports of the Crew Dragon C206 “Endeavour” signals.
It seems that the signal received by most people has been the one at 2216 MHz. Among these reports, I can mention the tweets by Scott Tilley VE7TIL (and this one), USA Satcom, Paul Marsh M0EYT. Paul also managed to receive a signal on 2272.5 MHz, which is not in the FCC filing, so this may or may not be from the Crew Dragon.
Scott has also shared with me an IQ recording of one of the passes, and as I showed on Twitter yesterday, I have been able to demodulate the data. This post is my analysis of the signal.
This post is a follow up to my study of the “wiggles” observed in the local oscillator of the QO-100 NB transponder. After writing that post, I have continued measuring the frequency of the BPSK beacon with my station almost without interruptions. Now I have some 44 days of measurements, spanning from April 9 to May 23. This data can be interesting to look at, so I’m doing this short post to share the data and look at it briefly.
The Jupyter notebook with all the data can be found here. The data is also linked in my jupyter_notebooks Github repository, which now uses git-annex to store the data in my home server. See the README for instructions on how to download some or all of the data files in the repository.
The whole time series can be seen in the figure below. We note that the typical Doppler sinusoidal curve varies slowly due to orbit perturbations and sometimes suddenly as a consequence of a station-keeping manoeuvre. I tweeted about one of the manoeuvres a while back.
There are now too many days in order to see things clearly when the frequency curves for each day are overlaid, but hopefully the figure below gives a good idea. We can see that the wiggles still happen approximately between 21:00 and 06:00 UTC, and between 11:00 and 17:00 UTC.
If we add an artificial offset of -15 Hz per day to the curves to prevent them from overlapping, we obtain the figure below. We see that the pattern of the wiggles keeps changing slightly, but also remains quite similar.
In my last post about this topic I said that it seemed that the wiggles repeated with a period of a sidereal day. Now it is clear that it is not the case. The wiggles seem to repeat roughly with a period of a solar day (24 hours). In fact, in 44 days sidereal time “advances” 2.88 hours with respect to solar time. However, it is clear that the wiggles haven’t shifted that much in time.
Some days ago, Hans Hartfuss DL2MDQ sent me an email about some frequency measurements of the QO-100 NB transponder BPSK beacon that he had been doing. The BPSK beacon is uplinked from Bochum (Germany) through the transponder, and as the beacon is generated using a very good Z3081A GPSDO as a reference, the frequency drift observed on the beacon downlink is caused by Doppler and the drift of the local oscillator of the transponder.
In his measurements, Hans observed some small oscillations or “wiggles” that didn’t seem to be caused by Doppler. Decided to investigate this, I started to do some measurements of my own. This post is an account of my measurements and findings so far.
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.