Following a discussion on Twitter about how to use satellite signals to check that distributed receivers are properly synchronized, I have decided to write a post about how to use GPS signals to timestamp an SDR recording. The idea is simple: we do a short IQ recording of GPS signals, and then process those signals to find the GPS time corresponding to the start of the recording. This can be applied in many contexts, such as:
Checking if the 1PPS synchronization in an SDR receiver is working correctly.
Timestamping an SDR recording without the need of a GPS receiver or 1PPS input, by first recording GPS signals for some seconds and then moving to the signals of interest (this only works if you’re able to change frequency without stopping the sample stream).
Measuring hardware delays between the 1PPS input and the ADC of an SDR (for this you need to know the hardware delay between the antenna connector and 1PPS output of your GPSDO).
Checking if synchronization is repetitive across restarts or power cycles.
We will do things in a fairly manual way, using a couple of open source tools and a Jupyter notebook. The procedure could certainly be automated more (but if you do so, at some point you might end up building a full fledged GPS receiver!). The post is written with a walk-through approach in mind, and besides the usefulness of timestamping recordings, it is also interesting to see hands-on how GPS works.
One of the things I’ve always wanted to do since Es’hail 2 was launched is to perform two-way ranging by transmitting a signal through the Amateur transponder and measuring the round trip time. Stefan Biereigel DK3SB first did this about a year ago. His ranging implementation uses a waveform with a chip rate of only 10kHz, as it is thought for Amateur transponders having bandwidths of a few tens of kHz. With this relatively slow chiprate, he achieved a ranging resolution of approximately 1km.
The QO-100 WB transponder allows much wider signals that can be used to achieve a ranging resolution of one metre or less. This weekend I have been doing my first experiments about ranging through the QO-100 WB transponder.
On April 28, I got together with a few Spanish radio Amateurs to perform some experiments. One of the things we did was an angle of arrival experiment in the 145MHz Amateur band. The ultimate goal of the experiment was to be able to measure the angle of arrival of meteor reflections of the GRAVES radar at 143.05MHz. However, we also recorded a few other signals, such as the Amateur satellite band at 145.9MHz (intended to perform calibration of the setup) and the APRS terrestrial signals at 144.8MHz.
The Sprites chipsats are tiny satellites built on a 3.5×3.5cm PCB with the bare minimum electronics to do something useful: a CC430 microcontroller with integrated FSK transceiver, an IMU, and solar cells.
The Sprites have been developed as part of the KickSat project, led by Zac Manchester, from Stanford University. The idea is to carry up to 128 Sprites in a cubesat and deploy them in a swarm once the cubesat is in orbit. The first test of this concept was done by the KickSat 3U cubesat in 2014. The test was a failure, since the Sprites couldn’t be deployed before KickSat reentered.
The second test was made this year with the KickSat-2 3U cubesat, a reflight of the KickSat mission carrying 104 Sprites. KickSat-2 was launched to the ISS onboard Cygnus NG-10 in November 2018 and deployed into orbit in February 2019.
All the Sprites transmit in the same frequency using CDMA, so further analysis is required to identify which Sprites were observed by Dwingeloo. Zac said he was working on decoding the recording, however, I haven’t seen any results published yet. Here I show my analysis of the recording made at Dwingeloo. I manage to detect 4 different Sprites.
Today an SSDV transmission session from DSLWP-B was programmed between 7:00 and 9:00 UTC. The main receiving groundstation was the Dwingeloo radiotelescope. Cees Bassa retransmitted the reception progress live on Twitter. Since the start of the recording, it seemed that some of the SSDV packets were being lost. As Dwingeloo gets a very high SNR and essentially no bit errors, any lost packets indicate a problem either with the transmitter at DSLWP-B or with the receiving software at Dwingeloo.
My analysis of last week’s SSDV transmissions spotted some problems in the transmitter. Namely, some packets were being cut short. Therefore, I have been closely watching out the live reports from Cees Bassa and Wei Mingchuan BG2BHC and then spent most of the day analysing in detail the recordings done at Dwingeloo, which have been already published here. This is my report.
K2SAT is a cubesat developed by the Aeroespace Systems and Control Lab in KAIST, a university in Daejeon, South Korea. It will be launched later this year, between September and October. The main mission of the satellite is to test the transmission of images taken with its onboard camera using an S-band QPSK transmitter that supports up to 2Mbps data rate. This will use the 2.4GHz Amateur satellite band, and the satellite has already coordinated a downlink frequency of 2404MHz. The K2SAT team at KAIST is the same one that built the QB50 KR01 (LINK) cubesat.
Since February, I have been collaborating with Pilwon Kwak and the rest of the K2SAT team to produce a GNU Radio receiver for the S-band image downlink and add it to my gr-satellites project. This receiver has now been publicly released. Here I explain the main details of the transmitter and protocol used by K2SAT and the implementation of the receiver.
Continuing with my research on using WSJT-X modes through linear transponder satellites in low Earth orbit (see part I and part II), a few days ago I transmitted and recorded an FT8 signal through the V/U linear transponder on FO-29 during a complete pass. The recording started at 2017/10/23 20:26:00 UTC and ended at 20:42:30 UTC. It was made with a FUNcube Dongle Pro+ set to a centre frequency of 435.850MHz and connected to a handheld Arrow satellite yagi through a duplexer. Here the duplexer was used to avoid desense on transmit.
An FT8 signal was transmitted on every even period during the recording, at a fixed frequency of 145.990MHz, using a Yaesu FT-817ND and the Arrow antenna. The signal was transmitted using lower sideband (i.e., inverted in the frequency domain) to get a correct FT8 signal through the inverting transponder. The transmit power was adjusted often to get a reasonable signal through the transponder and avoid using excessive power. There have been reports and complaints of people using too much power with digital modes through linear satellites. In this post, a study of the power is included to show that it is possible to use digital modes effectively without putting any pressure on the satellite’s transponder.
Out of the 33 even periods, a total of 24 can be decoded by WSJT-X using the best TLEs from Space-Track. No measures were taken to correct for the time offset \(\delta\) that has been studied in the previous posts, as the TLEs already provided a good Doppler correction. Regarding the choice of TLEs, there are still some remarks to make. First, the epoch of the TLEs used was 2017/10/23 21:39:16 UTC, so these TLEs were actually taken after the pass. The previous TLEs were taken a few hours before the pass, and it is likely that they also provided a good correction, perhaps by using a time offset \(\delta\) if necessary. However, I do not know if these previous TLEs were also available from CelesTrak before the start of the pass, as it seems that TLEs take a while to propagate from Space-Track to Celestrack. To explain why the TLEs with no time offset correction are enough, it will be interesting to study the rate of change of TLE parameters for FO-29. This will be done in a future post.
The results of this test look very promising. Even though this wasn’t an overhead pass (the maximum elevation was 40º), the maximum rate of change of the Doppler was over 20Hz/s for the self-Doppler seen on the FT8 signal and 35Hz/s for the downlink Doppler seen on the CW beacon. Most of the periods which couldn’t be decoded were near the start or end of the pass. This is the only test that I know of that has decoded FT8 signals in the presence of high rates of change of Doppler. The previous tests by other people were made at low elevations, where the rate of change of Doppler is small. This test has shown that it is possible to get many decodes with high rates of change of Doppler, even using no corrections to the TLEs. Here I continue with a detailed analysis of the recording.
I have also been thinking about how to study the polarization of signals in a dual polarization recording (two coherent channels with orthogonal polarizations). My main goal for this is to study the polarization of the signals of Amateur satellites in low Earth orbit. It seems that there are many myths regarding polarization and the rotation of cubesats, and these myths eventually pop up whenever anyone tries to discuss whether linearly polarized or circularly polarized Yagis are any good for receiving cubesats.
Through the Breakthrough Listen paper I’ve learned of the Stokes parameters. These are a set of parameters to describe polarization which are very popular in optics, since they are easy to measure physically. I have immediately noticed that they are also easy to compute from a dual polarization recording. In comparison with Jones vectors, Stokes parameters disregard all the information about phase, but instead they are computed from the averaged power in different polarizations. This makes their computation less affected by noise and other factors.
As I also wanted to get my hands on the Breakthrough Listen raw recordings, I have been computing the Stokes parameters of the Voyager 1 signal in their recording. Since the Voyager 1 signal is left hand circularly polarized, the results are not particularly interesting. It would be better to use a signal with changing polarization or some form of elliptical polarization.
The QB50 project consists in a constellation of cubesats with the goal of studying the thermosphere. The cubesats are built by different universities around the world and each of them carries one of three different scientific instruments. A total of 36 cubesats have been built for the QB50 project. All of them transmit on the 70cm Amateur satellite band. A total of 28 were launched to the ISS on April 18th on the Cygnus CRS-7 resupply ship. Over the last two weeks, they have been released from the ISS. The complete launch schedule and radio information can be found here (note that the launches on May 23rd were delayed due to an unforeseen EVA). Several other non-QB50 cubesats, some of them transmitting in the Amateur bands, have also been released together with the QB50 satellites. This is probably the time that more Amateur satellite have been released at the same time. The satellites have not separated much yet, giving a great opportunity to record a single pass and analyse the telemetry of all the satellites.
A few days after the release of all the 28 QB50 cubesats, on May 29th at 18:25:29 UTC, I made an SDR recording of the complete pass of all the cubesats. The recording spans the 3MHz of the 70cm Amateur satellite band (435-438MHz) and lasts 23 minutes and 08 seconds. It was made from locator IN80do using a 7 element handheld yagi (the Arrow satellite yagi) held in the vertical polarization and a LimeSDR. The gain of the LimeSDR was set to maximum, but no external LNA was used. Here I look at the recording, list the satellites heard, and decode their telemetry.
These days I have been experimenting with my LimeSDR board. This is an SDR board based on the LMS7002M transceiver chip. The drivers for the LimeSDR are called LimeSuite. This bundle contains a SoapySDR driver called SoapyLMS7, which makes the LimeSDR accessible through SoapySDR and also in GNU Radio through gr-osmosdr; some lower level drivers for the LMS7002M chip; and a GUI called LimeSuiteGUI that allows one to play with all the settings and parameters of the LMS7002M by hand.
In my tests I have come across a couple of driver-related bugs which I find quite annoying. This is not surprising, as the LMS7002M is a very complex piece of hardware and the LimeSDR drivers must control a huge number of settings and parameters and provide different interfaces to access the SDR hardware. I have reported them in the GitHub issues page of LimeSuite, but there are many other bugs open and LimeSuite is still seeing heavy development, so it doesn’t look likely that they will be fixed very soon.
To overcome this bugs, I have done some workarounds. Rather than trying to find the root cause of the problem, these disable the part of the software that is not working as it should. These workarounds are in the dirtyfixes branch of my LimeSuite fork.
The first bug I found was related to the baseband filter. This filter has a selectable bandwidth. Some bandwidths didn’t work properly, because the passband was far from flat or the cut-off frequency was way off. Moreover, just changing the bandwidth slightly sometimes produced a very different filter shape. I have been tweeting some pictures showing this effect (see also my replies to this tweet). I’ve found that the reason for this is that the parameters to tune the filter are usually cached by the drivers in order to save computations, but this cache system doesn’t work properly. My workaround is to disable the cache and always compute the filter parameters.
The second bug is related to DC spur hardware removal. We are used to the fact that many IQ SDR hardware have a (sometimes huge) DC spur in the centre of the passband, due to several hardware imperfections. This is also the case for the LimeSDR, but the LMS7002M has a hardware system (called RX DC correction) which is quite effective at removing the DC spur. I noted that DC spur removal was much better in LimeSuiteGUI than in GNU Radio or SoapySDR based applications. You can see the difference here. At first, I thought that this was an issue with the IQ calibration. However, it turns out that the RX DC correction was always being disabled by the SoapyLMS7 driver, even though it was supposed to be enabled by default. The reason for this is that a lower-level function from the LMS7002M seems to lie and says that the RX DC correction is disabled, even though it is not. I have bypassed this lower-level function in my workaround. You can see the effects of RX DC correction here.
Update: The second bug has just been fixed. It seems that the DC_BY_RXTSP bit that controls the RX DC correction was being overwritten somewhere else in the TX setup code because of a typo. I have reverted the workaround in my “dirtyfixes” branch and merged the proper fix. This branch still contains the workaround for the lowpass filter calibration.