Last week I presented a JT4G detection algorithm intended to detect very weak signals from DSLWP-B, down to -25dB SNR in 2500Hz. I have now processed the recordings of yesterday’s transmissions with this algorithm and here I look at the results. I have also made a Python script with the algorithm so that people can process their recordings easily. Instructions are included in this post.
In March this year I spoke about the Amateur VLBI with LilacSat-2 experiment. This experiment consisted of a GPS-synchronized recording of LilacSat-2 at groundstations in Harbin and Chongqing, China, which are 2500km apart. The experiment was a preparation for the Amateur VLBI project with the DSLWP lunar orbiting satellites, and I contributed with some signal processing techniques for VLBI.
As you may know, the DSLWP-B satellite is now orbiting the Moon since May 25 and the first Amateur VLBI session was performed last Sunday. The groundstations at Shahe in Beijing, China, and Dwingeloo in the Netherlands performed a GPS-synchronized recording of the 70cm signals from DSLWP-B from 04:20 to 5:40 UTC on 2018-06-10. I have adapted my VLBI correlation algorithms and processed these recordings. Here are my first results.
Now that DSLWP-B has already been for 17 days in lunar orbit, there have been several tests of the 70cm Amateur Radio payload, using 250bps GMSK with an r=1/2 turbo code. Several stations have received and decoded these transmissions successfully, ranging from the 25m radiotelescope at PI9CAM in Dwingeloo, the Netherlands (see recordings here) and the old 12m Inmarsat C-band dish in Shahe, Beijing, to much more modest stations such as DK3WN‘s, with a 15.4dBic 20-element crossed yagi in RHCP. The notices for future tests are published in Wei Mingchuan BG2BHC’s twitter account.
As far as I know, there have been no tests using JT4G yet. According to the documentation of WSJT-X 1.9.0, JT4G can be decoded down to -17dB SNR measured in 2.5kHz bandwidth. However, if we don’t insist on decoding the data, but only detecting the signal, much weaker signals can be detected. The algorithm presented here achieves reliable detections down to about -25dB SNR, or 9dB C/N0.
This possibility is very interesting, because it enables very modest stations to detect signals from DSLWP-B. In comparison, the r=1/2 turbo code can achieve decodes down to 1dB Eb/N0, or 25dB C/N0. In theory, this makes detection of JT4G signals 16dB easier than decoding the GMSK telemetry. Thus, very small stations should be able to detect JT4G signals from DSLWP-B.
Last Sunday, I used Scott Tilley VE7TIL’s Doppler measurements of the DSLWP-B S-band beacon to perform orbit determination using GMAT. Yesterday Scott sent me the Doppler data he has been collecting during this week. I have re-run my orbit determination process to include this new data.
Below I show the Keplerian state that was determined on 2018-06-03, in comparison with the new state determined on 2018-06-10 (both are referenced to the same epoch of 2018-05-26 00:00:00 UTC).
The Vaisala RS41 radiosonde is a weather radiosonde that is currently being launched in Madrid Barajas and other sounding sites in Spain, Europe and Australia. I have already spoken about how to decode it. One of the most interesting aspects of this model is that the RS41 contains a STM32F1 ARM Cortex-M3 microcontroller, a SiLabs FSK transmitter, and a uBlox GPS receiver, whereas the older RS92 contained custom ASICs to perform these functions. Thus, it is easy to reflash this radiosonde and write custom firmware for it, giving a lot of possibilities for experimentation.
In STARcon 2018, Julián Santamaría from AEMET (the Spanish meteorological office) gave me an RS41. While I have some long-term ideas about how to use it as a propagation sounder, I have just started playing with it. In this brief note, I explain how to flash the radiosonde with custom firmware.
A week ago, Mike Rupprecht DK3WN told me that he had discovered JPEG images in the packets transmitted by 1KUNS-PF. I’ve had some time now to take a look at the information he sent me (including KISS dumps of the packets) and add an image decoder to gr-satellites.
JPEG images are not so difficult to spot amongst binary data, since they contain JFIF in ASCII in their header (4a 46 49 46 in hex). Another telltale sign of JPEG is the ff d8 ff that marks the start of the file. Presumably this is how Mike first noticed the images.
It seems that all the image packets are 138 bytes long. The first four bytes are the CSP header. Then we have 16bit big-endian field which counts the number of chunk, starting by 00. The last four bytes of the packet are most likely a checksum, and the remaining bytes are the corresponding chunk of the JPEG file.
I don’t know how to detect the end of one image other than by taking note of the beginning of the next image. In fact, this is the last packet of this image:
The ff d9 that occurs mid-packet is the end-of-file of the JPEG file. I don’t know what to make of the rest of the data following it. Since not all of it is zero, it doesn’t look as deliberate padding. It might be the case that the satellite is sending information left over in a buffer.
The image below is the whole JPEG file contained in the packets received by Mike. It was sent using 72 chunks, for a total of 9216 bytes (at 128 bytes per chunk) and its resolution is 640×480. Most of the other images sent by 1KUNS-PF are smaller.
This is a follow-up on the series about DSLWP-B’s orbital dynamics (see part I and part II). In part I we looked at the tracking files published by Wei Mingchuan BG2BHC, which list the position and velocity of the satellite in ECEF coordinates, and presented basic orbit propagation with GMAT. In part II we explored GMAT’s capabilities to plan and perform manoeuvres, making a tentative simulation of DSLWP-B’s mid-course correction and lunar orbit injection. Now we turn to the study of DSLWP-B’s elliptical lunar orbit.
In this post we will examine the Keplerian elements of the orbits described by each of the tracking files published so far. We will also use Scott Tilley VE7TIL’s Doppler measurements of the S-band beacon of DSLWP-B to validate and determine the orbit.