Space – Page 23 – Daniel Estévez

Recovering an image transmitted by DSLWP-B

The image accompanying this post has a nice story to it. It was taken by the Amateur camera in DSLWP-B, the Chinese microsatellite in lunar orbit. On February 27, a download of this image was attempted by transmitting the image in SSDV format in the 70cm band and receiving it in the Dwingeloo radiotelescope, in the Netherlands.

The download was attempted twice, but due to errors in the transmission, a small piece of the image was still missing. Today, the Amateur payload of DSLWP-B was active again, and the plan was to download the missing piece, as well as other images. However, after the payload turned on and transmitted its first telemetry beacons, we discovered that the image had been overwritten.

The camera on-board DSLWP-B has a buffer that stores the last 16 images taken. Any of these images can be selected to be transmitted (completely or partially) while the Amateur payload is active. An image can be taken manually by issuing a command from ground. Besides this, every time the Amateur payload powers on, an image is taken. Of course, taking new images overwrites the older ones.

This is what happened today. The image we wanted to download was the oldest one in the buffer and got overwritten as soon as the payload turned on. This is a pity, especially because there was another activation of the payload last Friday, but a large storm in Germany prevented Reinhard Kuehn DK5LA’ from moving his antennas safely, so the satellite couldn’t be commanded to start the download.

After seeing that the image had been overwritten, Tammo Jan Dijkema suggested that I try to recover manually the missing chunk in the recording made on February 27. As you can see, I was successful. This is a report of how I proceeded.

Changes to the QO-100 NB transponder settings

Yesterday, AMSAT-DL announced that the narrowband transponder of QO-100 was under maintenance and that some changes to its settings would be made. This was also announced by the messages of the 400baud BPSK beacon. Not much information was given at first, but then they mentioned that the transponder gain was reduced by 6dB and a few hours later the beacon power was increased by 5dB.

Since I am currently doing continuous power measurements of the transponder noise and the beacons, when I arrived home I could examine the changes and determine using my measurements that the transponder gain was reduced by 5dB (not 6dB) at around 15:30 UTC, and then the uplink power of the beacons was increased by 5dB at around 21:00 UTC, thus bringing the beacons to the same downlink power as before. In what follows, I do a detailed analysis of my measurements.

New dish for Es’hail 2 reception

I have replaced the dish I had for receiving Es’hail 2 by a new one. The former dish was a 95cm offset from diesl.es which was a few years old. I had previously used this dish for portable experiments, and it had been lying on an open balcony for many months until I finally installed it in my garden, so it wasn’t in very good shape.

Comparing with other stations in Spain, I received less transponder noise from the narrowband transponder of QO-100 than other stations. Doing some tests, I found out that the dish was off focus. I could get an improvement of 4dB or so by placing the LNB a bit farther from the dish. This was probably caused by a few hits that the dish got while using it portable. Rather than trying to fix this by modifying the arm (as the LNB couldn’t be held in this position), I decided to buy a new dish.

Es’hail 2 frequency measurements

After being busy with other projects, I have resumed my frequency measurements of the Es’hail 2 beacons. The last measurement I performed was made when the satellite reached its operational slot at 26ºE. After manoeuvering to this spot, the Doppler was very small, on the order of 0.8ppb peak-to-peak, indicating a very accurate geostationary orbit. Now Es’hail 2 has been two months in its operational slot, inaugurating its Amateur transponders on February 14 and entering commercial service on March 7.

I am curious about studying again the Doppler at this point in the mission, to see how accurate the GEO orbit is. I am also interested in collaborating with other Amateurs to perform differential Doppler measurements, as I did with Jean Marc Momple 3B8DU. Here I detail the first results of my measurements.

New decoders for Astrocast 0.1

A couple months ago, I added a decoder for Astrocast 0.1 to gr-satellites. I spoke about the rather non-standard FX.25 protocol it used. Since then, Mike Rupprecht DK3WN and I have been in contact with the Astrocast team. They noticed the mistake about using NRZ instead of NRZ-I, and in February 13 they sent a software update to the satellite to use NRZ-I instead of NRZ. However, the satellite has some failsafe mechanisms, so sometimes it is seen transmitting in the older NRZ protocol.

Mike has also spotted Astrocast 0.1 transmitting sometimes in 9k6, instead of the usual 1k2. This is used to download telemetry, and it is only enabled for certain passes. The coding used for this telemetry download is different from the FX.25 beacon. The team has published the following information about it. The coding follows CCSDS, using five interleaved Reed-Solomon encoders. A CCSDS scrambler is also used.

Following this variety of protocols, I have added new decoders for Astrocast 0.1 to gr-satellites. The astrocast.grc decoder does NRZ-I FX.25, and should be used for the beacon. The astrocast_old.grc decoder implements NRZ FX.25, and should be used for the beacon when the satellite is in failsafe mode. The astrocast_9k6.grc decoder serves to decode the 9k6 telemetry downloads. Sample recordings corresponding to these three decoders can be found in satellite-recordings.

QO-100 beacons power

In the QO-100 (Es’hail 2) narrow band transponder, the recommendation for the adjustment of your downlink signal power is not to be stronger than the beacon. This was also the recommended usage of the old AO-40. Since the transponder has two beacons marking the transponder edges: a CW beacon marking the lower edge and a 400baud BPSK beacon marking the upper edge, there has been some debate on Twitter about which beacon does this recommendation refer to and what does “stronger” mean.

Of course, more formally, signal strength means power, which is a well defined physical concept, so there should be no argument about what does power mean. However, there are two different power measurements used for RF: average power and peak envelope power. I will assume that the recommendation refers to average power, not to peak envelope power. This makes more sense from the point of view of the power budget of the satellite amplifier (The total average power it needs to deliver is just the sum of the average powers of the signals of all the users, while the behaviour of the peak envelope power is much more complicated).

Also, I think that using peak envelope power for this restriction would be a very strict requirement on high PAPR signals. Note that the PAPR of CW is 0dB and the PAPR of BPSK is between 2 and 3dB, depending on the pulse shaping, so these are rather low PAPRs. For comparison, a moderately compressed SSB voice signal has a PAPR of 6dB.

In my opinion, the main problem with these discussions about “signal strength” is that many people are trying to judge power by looking at their waterfall or spectrum display and seeing what signal looks “higher”. This kind of measurement is not any good, because it doesn’t take signal bandwidth into account, depends on the FFT size, the window function, etc. It doesn’t help that many popular SDR software don’t have a good signal meter displaying the average power of the signal tuned in the passband.

In any case, I was curious about whether the power of the two beacons is the same and whether there is any interesting change over time. I have made a GNU Radio flowgraph that measures the power of each of the two beacons and of the transponder noise, and saves them to a file for later analysis.

Decoding the QO-100 beacon with gr-satellites

On February 14, the Amateur transponders on Es’hail 2 (which now has the AMSAT designation QO-100) were inaugurated. Since then, two beacons are being transmitted by the groundstation in Doha (Qatar) through the narrowband transponder. These beacons mark the edges of the transponder.

The lower beacon is CW, while the upper beacon is a 400baud BPSK beacon that uses the same format as the uncoded beacon of AO-40. I have already talked about the AO-40 uncoded beacon in an older post, including the technical details.

Based on my AO-40 decoder in gr-satellites, I have made a decoder for the QO-100 beacon. Patrick Dohmen DL4PD has been kind enough to write some instructions about how to use the old ao40_uncoded decoder with the BATC WebSDR. I recommend that you use the new qo100 decoder. You just have substitute ao40_uncoded by qo100 in Patrick’s instructions

As additional hints, I can say that for the best decoding, the beacon must be centred at 1.5kHz into the SSB passband. The centre of the signal is easy to spot because there is a null at the centre, due to the use of Manchester encoding. Frequency stability is somewhat important with this decoder, so if your LNB drifts too much you may run into problems.

The SNR of the beacon over the transponder noise floor is rather high, so you should achieve a clean decoding unless you are using a very small station and you have the transponder noise way below your receiver noise floor.

The following data is being currently transmitted on the beacon (the timestamps and packet numbers are added by gr-satellites):

2019-02-19 21:56:27
Packet number 68
K HI de QO-100 (DL50AMSAT BOCHUM
UPT: 3d 0h 29m CMD: 91 LEI_REQ: 0 LEI_ACT: 0
TEMP: 56 C VOLTAGES: 1.0 1.8 1.0 1.0 1.8 1.5 1.3 0.0 0.5 Volts
TFL: 0 TFE: 0 TFH: 0 HFF: 0 HTH: 0 HR: 0

2019-02-19 21:56:53
Packet number 69
L HI de QO-100 (DL50AMSAT BOCHUM
EXPERIMENTAL MODE. Measurements and tests being conducted,
experimental transponder use OK, but expect ground station tests
Watch this space and www.amsat-dl.org for further announcements

DSLWP-B camera planning for February 3 and 4

As you may know, I am on a scientific expedition in Antarctica until mid-February. Currently I am in the Spanish base Gabriel de Castilla, where we have relatively good satellite internet access. As I have some free time here, I have updated the DSLWP-B camera planning to reflect the upcoming observations announced by Wei Mingchuan BG2BHC on 2019-02-03 14:30 and 2019-02-04 08:20.

As we can see in the figure below, the Earth will be very near to the centre of the image, since there is a new Moon on February 4 (recall that the DSLWP-B camera points away from the Sun, so the Earth is visible on the camera when there is a new Moon, as the Earth is then opposite to the Sun, as seen from the Moon).

The observation times have been selected taking into account the orbit around the Moon, so that the Moon is also visible on the image. On February 3 the Moon should be completely visible inside the camera field of view. On the contrary, on February 4, the Moon will only be partially visible inside the frame.

The figure below shows the angular distance between the centre of the Earth and the rim of the Moon. This kind of graph can be used to compute the times when the Earth crosses the Moon rim, allowing us to take an “Earthrise” image. There is an Earthrise event on February 4, during the time when the Amateur payload is active. Generally, an image is taken whenever the Amateur payload powers up, but in this case it could be possible to command the payload manually to take an image near the Earthrise event.

The figure below shows in detail the Earthrise event, with both edges of the Earth plotted. It seems that a good time to take the Earthrise image is on 2019-02-04 10:00 UTC.

Proposal for IARU R1 about spectrum regulations for Amateur satellites

Together with URE and AMSAT-EA, I have presented a proposal for the upcoming IARU R1 Vienna interim meeting 2019 addressing the problems derived from the use of Amateur satellite spectrum by universties, research institutions and private companies. The title of the proposal is “Satellites transmitting on Amateur bands with undocummented protocols or without permission”, and the text of the proposal can be found here.

This proposal addresses the problems caused both by satellites that are using undocummented protocols for their telemetry, so Amateurs are unable to decode their signals, and satellites that are transmitting without having obtained the appropriate IARU frequency coordination.

The number of small satellites that are launched each year seems to be growing steadily, and many satellite developers are resorting to the Amateur satellite service as a way to obtain free spectrum access, regardless of whether their mission fits the requirements for an Amateur satellite. I think that we must try to defend our Amateur bands and ensure that proper use is made of them. Hopefully this proposal is a first step in making people aware of the problem and trying to find solutions.

A related document was written by the Open Research Institute to the FCC on July 2018. This is a recommended reading.

Please help spread this proposal to give visibility to the current problems with the Amateur satellite spectrum.

Es’hail 2 stationed in 26ºE

If you’ve been following my posts about Es’hail 2, you’ll know that shortly after launch Es’hail 2 was stationed in a test slot at 24ºE. It remained in this slot until December 29, when it started to move to its operational slot at 26ºE. As of January 2, Es’hail is now stationed at 26ºE (25.8ºE, according to the TLEs).

The new GEO orbit at 26ºE is much more perfect than the orbit it had at 24ºE. This is to be expected for an operational orbit. Since December 30, I’ve been recording Doppler data of the satellite moving to its operational slot, and I have found some interesting effects of orbital dynamics in the data. This post is an account of these.