Acquisition and wipeoff for JT9A

Lately, I have been playing around with the concept of doing acquisition and wipeoff of JT9A signals, using a locally generated replica when the transmitted message is known. These are concepts and terminologies that come from GNSS signal processing, but they can applied to many other cases.

In GNSS, most of the systems transmit a known spreading sequence using BPSK. When the signal arrives to the receiver, the frequency offset (given by Doppler and clock error) and delay are unknown. The receiver runs a search correlating against a locally generated replica signal which uses the same spreading sequence. The correlation will peak for the correct values of frequency offset and delay. The receiver then mixes the incoming signal with the replica to remove the DSSS modulation, so that only the data bits that carry the navigation message remain. This process can be understood as a matched filter that removes a lot of noise bandwidth. The procedure is called code wipeoff.

The same ideas can be applied to almost any kind of signal. A JT9A signal is a 9-FSK signal, so when trying to do an FFT to visually detect the signal in a spectrum display, the energy of the signal spreads over several bins and we lose SNR. We can generate a replica JT9A signal carrying the same message and at the same temporal delay than the signal we want to detect. Then we mix the signal with the complex conjugate of the replica. The result is a CW tone at the difference of frequencies of both signals, which we call wiped signal. This is much easier to detect in an FFT, because all the energy is concentrated in a single bin. Here I look at the procedure in detail and show an application with real world signals. Recordings and a Python script are included.

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D-SAT image downlink

In a previous post, I spoke about the cubesat D-SAT. The thing that first caught my attention about this satellite is its image downlink and the quality of some of the images that Mike DK3WN has managed to receive. Yesterday, Mike sent me an IQ recording of D-SAT downlinking a couple of images. After using the Groundstation software by the D-SAT team to verify that the images in the recording can be decoded, I have reverse engineered the protocol used to transmit images and added an image decoder to the D-SAT decoder in gr-satellites.

The image decoder can be tested with the dsat-image.wav recording in satellite-recordings. This WAV file contains the image below, which shows the Southwestern part of Spain and Portugal. The image was taken by D-SAT on 2017-08-17 10:09:54 UTC and received by Mike during the 19:10 UTC pass that evening.

Image of Spain and Portugal taken by D-SAT

According to the TLEs, at the time this image was taken, D-SAT was just above Rincón de la Victoria, in Málaga, passing on a North to South orbit. This means that D-SAT's camera was pointing more or less in a direction normal to the orbit.

This image is a 352x288 pixels JPEG image with a size of 13057 bytes. It took 43 seconds to transfer using D-SAT's 4k8 AF GMSK downlink (yes, the overhead is around 100%, more on that later). In the rest of this post, I detail the protocol used to transmit the images.

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IPv6 for Amateur radio

Back in September 26 2016, I posted an email in Spanish to the Hamnet.es mailing list detailing my proposal for an IPv6 Amateur radio network, and trying to engage people into some preliminary tests. In October 1 2016, I posted a summary (in English) of my message to the 44net mailing list. There was some discussion afterwards in the list, but no real actions were taken. Since then, no much interest in IPv6 for Amateur radio seems to have sprung. Still, I think that the time for IPv6 will come. I have collected my IPv6 for Amateur radio proposal in a page here for future reference. At least I hope that this pops up on Google searches for IPv6 and Amateur radio, since there is not much material about this on the Internet, and most of what one can find is quite dated.

D-SAT support added to gr-satellites

D-SAT is an Italian cubesat that will demonstrate a new deorbit hardware. Apparently this system uses dedicated propulsion to make the satellite re-enter from a 500km orbit in 30 minutes. It also carries three more experiments and it was launched in June 23 together with several other small satellites. According to the information from the team, it transmits 4k8 telemetry in the 70cm band. It is not stated explicitly, but we read attentively, we see that it uses a NanoCom U482C transceiver from GOMspace.

Recently, I have seen Mike DK3WN decode very nice images from D-SAT and I have investigated a bit to see what software he is using.

The satellite team provides some decoding software through their forum, which requires registration. Version 2 of their software can be downloaded directly here using the password dsatmission. Its software is based on GNU Radio and it uses a few components from gr-satellites, namely the U482C decoder and some KISS and CSP blocks. These have been incorporated into their decoder from before gr-satellites was restructured. They include a note thanking me in the README, but I didn't ever hear from them that they were using gr-satellites. It would have been nice if they had contacted me, since this opens up many possibilities for collaboration.

Apart from that, they include a groundstation software which performs telemetry decoding and so on. Unfortunately, the groundstation software is closed-source, distributed only as an x86_64 Linux executable. This is not good for Amateur Radio. We should strive for open source software and open specifications for everything that transmits in our bands. The groundstation software is also distributed in a quite ugly manner as the remains of an Eclipse project (source code stripped, of course). However, it is interesting because it seems that this software is the same they use in their groundstation, and it supports sending commands to the satellite. Naturally, the command transmission is not implemented in the software they distribute, but it is still very interesting to have a peek and see what kinds of commands the satellite supports.

I have added a D-SAT decoder to gr-satellites. The decoder supports sending frames to their groundstation software. Here I describe how to set everything up.

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WSJT-X and linear satellites: part I

Several weeks ago, in an AMSAT EA informal meeting, Eduardo EA3GHS wondered about the possibility of using WSJT-X modes through linear transponder satellites in low Earth orbit. Of course, computer Doppler correction is a must, but even under the best circumstances we cannot assume a perfect Doppler correction. First, there are errors in the Doppler computation because the TLEs used are always measured at an earlier time and do not reflect exactly the current state of the satellite. This was the aspect that Eduardo was studying. Second, there are also errors because the computer clock is not perfect. Even a 10ms error in the computer clock can produce a noticeable error in the Doppler computation. Also, usually there is a delay between the time that the RF signal reaches the antenna and the time that the Doppler correction is computed for and applied to the signal, especially if using SDR hardware, which can have large buffers for the signal. This delay can be measured and compensated in the Doppler calculation, but this is usually not done.

Here we look at errors of the second kind. We denote by D(t) the function describing the Doppler frequency, where t is the time when the signal arrives at the antenna. We assume that the correction is not done using D(t), but rather D(t - \delta), where \delta is a small constant. Thus, a residual Doppler D(t)-D(t-\delta) is still present in the received signal. We will study this residual Doppler and how tolerant to it are several WSJT-X modes, depending on the value of \delta.

The dependence of Doppler on the age of the TLEs will be studied in a later post, but it is worthy to note that the largest error made by using old TLEs is in the along-track position of the satellite, and that this effect is well modelled by offsetting the Doppler curve in time. This justifies the study of the residual Doppler D(t)-D(t-\delta).

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