## 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.

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.

## 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.

## 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 $D(t)-D(t-\delta)$.

## A first look at DSLWP SSDV downlink

The Chang'e 4 is a Chinese lunar mission that will land a rover on the far side of the Moon by the end of 2018. To support this mission, the Chang'e 4 relay satellite will be launched six months before and put into a halo orbit around the Earth-Moon Lagrange L2 point. The relay will provide four 256Kbps links with the rover and lander on X-band and a 2Mbps link with Earth on S-band using a 4.2m dish. Two CE-4 microsatellites will be launched together with the relay satellite. They will be put in a 200km x 9000km lunar elliptical orbit. The main mission of the CE-4 microsatellites is to perform HF interferometry of celestial bodies, using the Moon as a shield from the radiation of the Sun and Earth. The satellites also carry an Amateur radio system called DSLWP, which will provide telecommand, telemetry and image downlink.

A team at Harbin Institute of Technology is currently designing the Amateur radio payload. As it is the case with previous HIT satellites such as BY70-1 and LilacSat-1, the payload will have a camera which can be telecommanded by radio Amateurs, which can use it to take and download pictures. Yesterday, Wei BG2BHC has released some work in progress of the image downlink. Many important parts of the downlink will still change, but releasing the work in progress at this early stage is a very good idea. Probably it is not too late in the development process so that the Amateur community can contribute with ideas and improvements.

The release consists of an IQ recording of the signal containing a full image and a decoder in gr-lilacsat. The IQ recording is at 2ksamp/s, since the signal is FSK at 250baud. Note that the recording is almost 32 minutes long. It takes a while to transmit an image at such a low rate. However, a low baudrate and a good amount of FEC are needed for an effective downlink from the Moon, given the huge path loss of around 197dB in the 70cm band.

The good news about this work in progress is that SSDV is now used to transmit the image. SSDV is a packetised protocol based on JPEG, but which is tolerant to packet loss. In contrast, BY70-1 and LilacSat-1 send JPEG images in 64byte chunks, and a single lost chunk can destroy the image completely. SSDV was originally developed to transmit images from Amateur high altitude ballons, so it is a good idea to use it also for DSLWP.

The bad news is that the way that SSDV has been included into the downlink protocol is not very optimal. In the rest of this post I do an in-depth look at the protocol, point out the main problems and suggest some solutions. Hopefully the protocol can still be modified and improved.

## LilacSat-1 image downlink

Yesterday, Wei BG2BHC posted on Twitter an IQ recording of LilacSat-1 sending an image. LilacSat-1 has an onboard camera and it can send images using the same format as BY70-1. However, one has to keep in mind that in LilacSat-1 the Codec2 frames and the KISS stream with telemetry and image packets are multiplexed as described here, whereas BY70-1 only transmitted the KISS stream with telemetry and image packets. As in the case of BY70-1, the camera is potentially open to telecommand by all Amateurs, although it seems that system is not enabled yet.

The signal in Wei's recording is very strong and stable, about 20dB SNR in its natural bandwidth of 13kHz. Therefore, it is no surprise that the image can be decoded without errors.

When BY70-1 was in orbit, it was quite difficult for an Amateur station to get a perfect decode of the image, since a single fade in the signal would completely corrupt the JPEG file. LilacSat-1 doesn't seem particularly stronger than BY70-1, so the same degree of difficulty can be expected. Of course, a well equipped groundstation such as the one in Harbin Institute of Technollogy will have no problems to get a good decode, as shown by this IQ recording. Amateurs with more modest stations should resort to a collaborative effort to try to combine the different packets that form the image, as received by several stations. Currently this procedure can only be partially automated by software, because the CRC algorithm used in LilacSat-1 is not publicly known, so it is not possible to check the packets for bit errors.

The image transmitted by LilacSat-1 can be seen above. Its size is 13861 bytes and it took 217 camera packets and 1 minute and 26 seconds to transmit. This is pretty good, as it means that several images can be taken and transmitted during a pass.

Recall that the downlink of LilacSat-1 transmits at 4800bps, but 1400bps are taken for Codec2, leaving 3400bps for the KISS stream containing image packets (and telemetry packets). Each camera packet contains a 64 byte JPEG chunk, but taking into account headers it is 87 bytes long. We also need to take into account the overhead of the KISS stream. Assuming that no bytes have to be escaped, we just need to include 2 extra bytes for the frame delimiters, so a camera packet takes 89 bytes from the KISS stream and so it takes 197ms to transmit. This means that the image above could have been sent in only 43 seconds. All the extra time is probably due to the fact that the image was sent interleaved with many telemetry packets, although it would be interesting to examine if the KISS stream was in fact completely busy all the time during the image download.

The complete telemetry log decoded from this recording is in this gist. I have also taken the GPS data from the telemetry and plotted it in the map below. The position of the Harbin Institute of Technology, where the recording was made, is also shown.

A 48kHz WAV file extracted from the recording has been included in satellite-recordings. It can be fed directly to the gr-satellites LilacSat-1 decoder.

## Viterbi decoding for NanoCom U482C

The NanoCom U482C is a a transceiver made by GOMspace intended for cubesats and other small satellites. Currently, it seems to be out of production, since it has been superseded by the newer NanoCom AX100, but nevertheless the U482C is being flown in new satellites, such as the QB50 AU03 INSPIRE-2. The U482C is also used in GOMspace's cubesat GOMX-1, so we may say that GOMX-1 is the reference satellite for U482C.

My gr-satellites project includes a partially reverse-engineered U482C decoder which is able to decode GOMX-1 and several other satellites. It does CCSDS descrambling and Reed-Solomon decoding. Recently, Jan PE0SAT made a recording of INSPIRE-2. I tried to decode it with gr-satellites and although the signal was very good, the Reed-Solomon decoder failed. The history behind this recording is interesting. After being released from the ISS near the end of May, INSPIRE-2 wasn't transmitting as it should. The satellite team got in contact with Amateurs having powerful stations to try to telecommand the satellite and get it transmitting. Eventually, the CAMRAS 25m dish was used to telecommand and activate INSPIRE-2. Later, Jan made a recording from his groundstation.

After exchanging some emails with the satellite team, I learnt that the U482C also supports an $r=1/2$, $k=7$ convolutional code, which is used by INSPIRE-2 but not by other satellite I've seen. I have added Viterbi decoding support for the U482C decoder in gr-satellites, so that INSPIRE-2 can now be decoded. Here I describe some details of the implementation.

## Testing LilacSat-1 Codec2 downlink and GPS telemetry

Today I've finally had some time to test the LilacSat-1 Codec2 downlink on the air. I've been transmitting and listening to myself on the downlink during the 17:16 UTC pass over Europe from locator IN80do. The equipment used is a Yaesu FT-2D for the FM uplink, a FUNcube Dongle Pro+ and my decoder from gr-satellites for the downlink, and a handheld Arrow satellite yagi (3 elements on VHF and 7 elements on UHF). Here I describe the results of my test.

## Waterfalls from QB50

In the previous post, I analysed a QB50 recording. Now I have prepared some waterfalls from my recording using the procedure I already described a while ago. The image above is obtained from a 1600x1024 waterfall with a resolution of 2.93kHz or 0.86s per pixel. I have labelled all the satellites and cropped it to a 1600x900 image that now I'm using as my desktop wallpaper.

I have also made a large 14120x16384 image with a resolution of 183.1Hz or 0.1s per pixel. The image can be downloaded here (142MB). I have found the following interesting crops within the large image. Remember that you can click on each image to view it in full size.

The fast fading that I detected in nSIGHT is clearly visible below. Note that the beacon period is almost, but not quite, an integer multiple of the fading period.

In the image below, we can see that SpaceCube is not very stable in frequency. The carrier frequency tends to rise rapidly each time that the transmitter goes on. Also, the overall trend is a frequency increase, counteracting the frequency decreasing effect of Doppler. This excerpt is near the end of SpaceCube's pass, so the change in Doppler is not so large. The other French satellite, X-CubeSat, also shows a similar behaviour.

AAUSAT-4 usually transmits in 4k8 FSK using CCSDS FEC, but it also transmits a CW beacon sometimes. Both can be seen below.

Finally, a couple of CW satellites with interesting behaviour. On the upper part of the image below we can see BeEagleSat with fading. On the lower part, we can see Aalto-2 with its characteristic sidebands.

## A tour of QB50

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.

## Decoding AO-40 uncoded telemetry

AO-40 is an Amateur satellite that was active between 2000 and 2004. It had several transponders and beacons covering many bands from HF to microwave and its position on a HEO orbit provided several consecutive hours of coverage each day and allowed long distance contacts. Since then, many interesting things have happened with Amateur satellites, particularly the high increase of the number of cubesats that is happening over the last few years, but even so, we haven't seen again any other satellite with the characteristics of AO-40 nor it is to be expected in the near future.

I was quite young when AO-40 was operational, so for me this is all history. However, Pieter N4IP has posted recently on Twitter some IQ recordings of AO-40 that he made back in 2003. I have been playing with these recordings to see how AO-40 was like. One of the things I've dong is to write my own telemetry decoder using GNU Radio.

AO-40 transmitted telemetry using 400bps BPSK. There were two modes: an uncoded mode which used no forward error correction and an experimental FEC mode proposed by Phil Karn KA9Q. The FEC mode was used later in the FUNcube satellites, and I've already talked about it in a previous post. The beacon in Pieter's recordings is in uncoded mode. Here I describe this mode in detail and how my decoder works. The decoder and a small sample taken from Pieter's recordings have already been included in gr-satellites.