DSLWP-B clock running late

This Wednesday, a DSLWP-B test was done between 04:00 and 06:00 UTC. During this test, a few stations reported on Twitter that they were able to receive the JT4G signal correctly (they saw the tones on the waterfall) but decoding failed.

It turns out that the cause of the decoding failures is that the DSLWP-B clock is running a few seconds late. Thus, the JT4G transmission starts several seconds after the start of the UTC minute and so the decoding fails, since WSJT-X only searchs for a time offset of a few seconds.

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Lunar orbit perturbations and DSLWP-B

Ever since simulating DSLWP-B’s long term orbit with GMAT, I wanted to understand the cause of the periodic perturbations that occur in some Keplerian elements such as the excentricity. As a reminder from that post, the excentricity of DSLWP-B’s orbit shows two periodic perturbations (see the figure below). One of them has a period of half a sidereal lunar month, so it should be possible to explain this effect from the rotation of the Moon around the Earth. The other has a period on the order of 8 or 9 months, so explaining this could be more difficult.

DSLWP-B eccentricity

In this post I look at how to model the perturbations of the orbit of a satellite in lunar orbit, explaining the behaviour of the long term orbit of DSLWP-B.

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Playing with LilacSat-1

Even though the cubesat LilacSat-1 was launched more than a year ago, I haven’t played with it much, since I’ve been busy with many other things. I tested it briefly after it was launched, using its Codec2 downlink, but I hadn’t done anything else since then.

LilacSat-1 has an FM/Codec2 transponder (uplink is analog FM in the 2m band and downlink is Codec2 digital voice in the 70cm band) and a camera that can be remotely commanded to take and downlink JPEG images (see the instructions here). Thus, it offers very interesting possibilities.

Since I have some free time this weekend, I had planned on playing again with LilacSat-1 by using the Codec2 transponder. Wei Mingchuan BG2BHC persuaded me to try the camera as well, so I teamed up with Mike Rupprecht DK3WN to try the camera this morning. Mike would command the camera, since he has a fixed station with more power, and we would collaborate to receive the image. This is important because a single bit error or lost chunk in a JPEG file ruins the image from the point where it happens, and LilacSat-1 doesn’t have much protection against these problems. By joining the data received by multiple stations, the chances of receiving the complete image correctly are higher.

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Decoding TANUSHA-3

On August 15, during a Russian EVA on the ISS, a total of four Russian nanosatellites were deployed by hand. Although different online sources give incomplete and contradictory information about which satellites were released, it seems that they were SiriusSat 1 and 2, from the Sirius educational centre in Sochi, and Tanusha 3 and 4 from the Southwest State University in Kursk (see also Jonathan McDowell’s space report).

The SiriusSats are using 4k8 FSK AX.25 packet radio at 435.570MHz and 435.670MHz respectively, using callsigns RS13S and RS14S. The Tanushas transmit at 437.050MHz. Tanusha-3 normally transmits 1k2 AFSK AX.25 packet radio using the callsign RS8S, but Mike Rupprecht sent me the other day a recording of a transmission from Tanusha-3 that he could not decode.

It turns out that the packet in this recording uses a very peculiar modulation. The modulation is FM, but the data is carried in audio frequency phase modulation with a deviation of approximately 1 radian. The baudrate is 1200baud and the frequency for the phase modulation carrier is 2400Hz. The coding is AX.25 packet radio.

Why this peculiar mode is used in addition to the standard 1k2 packet radio is a mystery. Mike believes that the satellite is somehow faulty, since the pre-recorded audio messages that it transmits are also garbled (see this recording). If this is the case, it would be very interesting to know which particular failure can turn an AFSK transmitter into a phase modulation transmitter.

I have added support to gr-satellites for decoding the Tanusha-3 phase modulation telemetry. To decode the standard 1k2 AFSK telemetry direwolf can be used. The decoder flowgraph can be seen in the figure below.

TANUSHA-3 gr-satellites decoder

The FM demodulated signal comes in from the UDP source. It is first converted down to baseband and then a PLL is used to recover the carrier. The Complex to Arg block recovers the phase, yielding an NRZ signal. This signal is lowpass filtered, and then clock recovery, bit slicing and AX.25 deframing is done. Note that it is also possible to decode this kind of signal differentially, without doing carrier recovery, since the NRZI encoding used by AX.25 is differential. However, the carrier recovery works really well, because there is a lot of residual carrier and this is an audio frequency carrier, so it should be very stable in frequency.

The recording that Mike sent me is in tanusha3_pm.wav. It contains a single AX.25 packet that when analyzed in direwolf yields the following.

RS8S>ALL:This is SWSU satellite TANUSHA-3 from Russia, Kursk<0x0d>
------
U frame UI: p/f=0, No layer 3 protocol implemented., length = 68
 dest    ALL     0 c/r=1 res=3 last=0
 source  RS8S    0 c/r=0 res=3 last=1
  000:  82 98 98 40 40 40 e0 a4 a6 70 a6 40 40 61 03 f0  ...@@@...p.@@a..
  010:  54 68 69 73 20 69 73 20 53 57 53 55 20 73 61 74  This is SWSU sat
  020:  65 6c 6c 69 74 65 20 54 41 4e 55 53 48 41 2d 33  ellite TANUSHA-3
  030:  20 66 72 6f 6d 20 52 75 73 73 69 61 2c 20 4b 75   from Russia, Ku
  040:  72 73 6b 0d                                      rsk.
------

The contents of the packet are a message in ASCII. The message is of the same kind as those transmitted in AFSK.

Trying to make the DSLWP-B GMSK decoder more robust

If you’ve being following my latest posts, probably you’ve seen that I’m taking great care to decode as much as possible from the SSDV transmissions by DSLWP-B using the recordings made at the Dwingeloo radiotelescope. Since Dwingeloo sees a very high SNR, the reception should be error free, even without any bit error before Turbo decoding.

However, there are some occasional glitches that corrupt a packet, thus losing an SSDV frame. Some of these glitches have been attributed to a frequency jump in the DSLWP-B transmitter. This jump has to do with the onboard TCXO, which compensates frequency digitally, in discrete steps. When the frequency jump happens, the decoder’s PLL loses lock and this corrupts the packet that is being received (note that a carrier phase slip will render the packet undecodable unless it happens very near the end of the packet).

There are other glitches where the gr-dslwp decoder is at fault. The ones that I’ve identify deal in one way or another with the detection of the ASM (attached sync marker). Here I describe some of these problems and my proposed solutions.

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Report for today’s DSLWP-B SSDV session

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.

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DSLWP-B corrupted SSDV frames

In my previous post I looked at the first SSDV transmission made by DSLWP-B from lunar orbit. There I used the recording made at the Dwingeloo radiotelescope and showed how to decode the SSDV frames and produce a JPEG image.

Only four SSDV frames where transmitted by DSLWP-B, and out of those four, only two could be decoded correctly. I wondered why the decoding of the other two frames failed, since the SNR of the signal as recorded at Dwingeloo was very good, yielding essentially no bit errors (even before FEC decoding).

Now I have looked at the signal more in detail and have found the cause of the corrupted SSDV frames. I have demodulated the signal in Python and have looked at the position where an ASM (attached sync marker) is transmitted. As explained in this post, the ASM marks the beginning of each Turbo codeword. The Turbo codewords are 3576 symbols long and contain a single SSDV frame.

A total of four ASMs are found in the GMSK transmission that contains the SSDV frames, which matches the four SSDV transmitted. However, the distance between some of the ASMs doesn’t agree with the expected length of the Turbo codeword. Two of the Turbo codewords where cut short and not transmitted completely. This explains why the decoding of the corresponding SSDV frames fails.

The detailed analysis can be seen in this Jupyter notebook.

This is rather interesting, as it seems that DSLWP-B had some problem when transmitting the SSDV frames. I have no idea about the cause of the problem, however. It would be convenient to monitor carefully future SSDV transmissions to see if any similar problem happens again.

First SSDV transmission from DSLWP-B

As some of you may know, DSLWP-B, the Chinese lunar-orbiting Amateur satellite carries a camera which is able to take pictures of the Moon and stars. The pictures can be downlinked through the 70cm 250bps GMSK telemetry channel using the SSDV protocol. Since an r=1/2 turbo code is used, this gives a net rate of 125bps, without taking into account overhead due to headers. Thus, even small 640×480 images can take many minutes to transfer, but that is the price one must pay for sending pictures over a distance of 400000km.

On Saturday August 3, at 01:27 UTC, the first SSDV downlink in the history of DSLWP-B was attempted. According to Wei Mingchuan BG2BHC, the groundstation at Harbin managed to command the picture download at 436.400MHz a few minutes before the GMSK transmitter went off at 01:30 UTC. A few SSDV frames were received by the PI9CAM radiotelescope at Dwingeloo.

The partial image that was received was quickly shared on Twitter and on the DSLWP-B camera webpage. The PI9CAM team has now published the IQ recording of this event in their recording repository. Here I analyze that recording and perform my own decoding of the image.

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DSLWP-B long-term orbit evolution

In my last post I commented that one of the motivations for the periapsis raise manoeuvre of DSLWP-B on July 20 was to prevent the satellite from crashing into the Moon in a few months. When Wei Mingchuan BG2BHC told me this, I found it a bit surprising, since I had the impression that the periapsis had been raising naturally since the satellite was injected in lunar orbit on May 25. Thus, I decided to propagate the orbit for a period of 2 years using GMAT and study the long-term effects in the Keplerian elements.

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Trying to decode EQUiSat

EQUiSat is a cubesat from Brown University that was launched to the ISS on May 21 with the Cygnus CRS-9 supply ship. It was released from the ISS on July 13. The payload of EQUiSat is rather interesting: an optical beacon, formed by an array of 4 high power LEDs designed to flash and be visible with the naked eye.

The EQUiSat radio system is also quite interesting and unusual. It uses the PacificCrest XDL Micro transmitter in 4FSK mode. This UHF transmitter is normally used to transmit data between survey GNSS receivers. Unfortunately, there is very little documentation about the radio protocol used by this transmitter.

I am in communication with the satellite team, since they are interested in producing a GNU Radio decoder. However, they don’t know much about the radio protocol either. Here is my first try at trying to decode transmissions from EQUiSat.
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