satellites – Page 19 – Daniel Estévez

Diffraction in DSLWP-B lunar occultations

In February this year, the orientation of the orbit of DSLWP-B around the Moon was such that, when viewed from the Earth, it passed behind the Moon on every orbit. This opened up the possibility for recording the signal of DSLWP-B as it hid behind the Moon, thus blocking the line of sight path. The physical effect that can be observed in such events is that of diffraction. The power of the received signal doesn’t drop down to zero in a brick-wall fashion just after the line of sight is blocked, but rather behaves in an oscillatory fashion, forming the so called diffraction fringes.

The signal from DSLWP-B was observed and recorded at the Dwingeloo 25m radiotelescope for three days in February: 4th, 13th and 15th. During the first two days, an SSDV transmission was commanded several minutes before DSLWP-B hid behind the Moon, so as to guarantee a continuous signal at 436.4MHz to observe the variations in signal power as DSLWP-B went behind the Moon. On the 15th, the occultation was especially brief, lasting only 28 minutes. Thus, DSLWP-B was commanded to transmit continuously before hiding behind the Moon. This enabled us to also observe the end of the occultation, since DSLWP-B continued transmitting when it exited from behind the Moon. This is an analysis of the recordings made at Dwingeloo.

An overview of IARU R1 interim meeting proposals

The IARU R1 interim meeting is being held in Vienna, Austria, on April 27 and 28. This post is an overview of the proposals that will be presented during this meeting, from the point of view of the usual topics that I treat in this blog.

The proposals can be found in the conference documents. There are a total of 64 documents for the meeting, so a review of all of them or an in-depth read would be a huge work. I have taken a brief look at all the papers and selected those that I think to be more interesting. For these, I do a brief summary and include my technical opinion about them. Hopefully this will be useful to some readers of this blog, and help them spot what documents could be more interesting to read in detail.

Detecting the Sprites from KickSat-2

The Sprites chipsats are tiny satellites built on a 3.5×3.5cm PCB with the bare minimum electronics to do something useful: a CC430 microcontroller with integrated FSK transceiver, an IMU, and solar cells.

The Sprites have been developed as part of the KickSat project, led by Zac Manchester, from Stanford University. The idea is to carry up to 128 Sprites in a cubesat and deploy them in a swarm once the cubesat is in orbit. The first test of this concept was done by the KickSat 3U cubesat in 2014. The test was a failure, since the Sprites couldn’t be deployed before KickSat reentered.

The second test was made this year with the KickSat-2 3U cubesat, a reflight of the KickSat mission carrying 104 Sprites. KickSat-2 was launched to the ISS onboard Cygnus NG-10 in November 2018 and deployed into orbit in February 2019.

On March 19, the Sprites were successfully deployed from KickSat-2, as Zac announced in Twitter, requesting help from the Amateur radio community to receive the signals from the Sprites at 437.240MHz. On March 22, Cees Bassa and Tammo Jan Dijkema tried to detect the Sprites by doing a planar scan with the Dwingeloo 25m radiotelescope. They were successful, detecting several transmissions from the Sprites in the waterfall. At that moment, the Sprites were up to 5 minutes ahead KickSat-2, due to their much higher drag to mass ratio. They all probably reentered a few days after this.

All the Sprites transmit in the same frequency using CDMA, so further analysis is required to identify which Sprites were observed by Dwingeloo. Zac said he was working on decoding the recording, however, I haven’t seen any results published yet. Here I show my analysis of the recording made at Dwingeloo. I manage to detect 4 different Sprites.

Weekend maintenance to QO-100 NB beacons

This weekend, the beacons of the Es’hail 2 narrowband transponder have undergone maintenance. The beacons have been off for several periods of a few hours on Friday and Saturday. After the maintenance, there are two main changes: the phase noise of the beacons has been fixed, and the beacons are now approximately 3dB stronger.

Since the opening of the transponder on February 14, some phase noise on the two beacon signals was appreciable slightly above the noise floor, and with the latest increase in power of the beacons, the phase noise was more evident. Now the problem is fixed and the transponder is clear of phase noise.

The figure below shows the power of the beacons and transponder noise (measured in 2kHz bandwidth). You can see that the beacon power has daily fluctuations of up to 2dB, but despite of this fact it is clear that the beacons are now approximately 3dB stronger than before (maybe even 4dB).

The figure below shows the CN0 of the beacons, measured both at the transponder and at my receiver (where it is lower due to system noise). The CN0 is now extremely high: 56dB for the BPSK beacon. In a previous post I thought about what could be done with 45dB of CN0. The conclusion was that if you want to fit a digital signal in an SSB channel bandwidth, you are much more bandwidth-limited than SNR-limited. This is now even more true.

With the increased beacon power, it should be fairly easy to decode the beacons with a bare LNB, even despite the fact that the transponder gain has been reduced twice. Also, now that the SNR of the beacons is so high, there is no excuse for being louder than the beacon. Anyone who is stronger than the beacon is most likely using too much power. Their mode of choice probably works equally well with several dB less of SNR.

Measuring QO-100 beacons frequency

Continuing with my frequency measurements of Es’hail 2, I have now been measuring the frequency of the beacons of the QO-100 narrowband transponder for several days. The main goal of these frequency measurements is to use Doppler to study the orbit of Es’hail 2. Previously, I had been doing frequency measurements on the engineering beacons at 10706MHz and 11205MHz. However, these beacons are currently being transmitted on a MENA beam, so I’m quite lucky to be in Spain, as they can’t be received in many other parts of Europe.

During the in-orbit tests of Es’hail 2, the engineering beacons were transmitted on a global beam, and I performed some differential Doppler studies with Jean Marc Momple 3B8DU, in Mauritius. The engineering beacons are no longer any good for these kind of studies, since their area of coverage is small. Thus, I have started to measure the beacons in the narrowband transponder, which covers all the satellite footprint.

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.

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.