Extracting AX.25 satellites from SatNOGS DB

In my last post about gr-satellites 3, I announced that gr-satellites would start to support all the AX.25 satellites transmitting in Amateur bands. Historically, gr-satellites didn’t support packet radio (AFSK and FSK AX.25) satellites since there were too many of them and there were already other good decoders such as Direwolf. At one point Rocco Valenzano W2RTV convinced me to add “generic” packet radio decoders to gr-satellites and since then these have been seeing quite some use.

In gr-satellites 3 it is very easy to add new satellites, since this is done with a SatYAML file, which is a brief YAML file describing basic information about the satellite and its transmitters. Therefore, I decided to make a script to get this data from SatNOGS DB and write the SatYAMLs automatically for all the AFSK and FSK AX.25 satellites.

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DSLWP-B whole mission telemetry

Recently, together with people from Harbin Institute of Technology and CAMRAS, we have published in Zenodo the data collected during the DSLWP-B mission. This data release includes all the raw telemetry frames uploaded to the DSLWP telemetry server.

I have made a Jupyter notebook that loads up and parses the telemetry, with the idea of providing a simple way to study the data.

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First tests of a narrowband data modem for QO-100

Since a while ago, I have had the idea to design a data modem for the NB transponder of QO-100 (Es’hail 2). The main design criteria of this modem is that it should fit in a bandwidth of 2.7kHz and be able to work at a signal power equal to that of the transponder BPSK beacon, since these are the bandwidth and power constraints when using the NB transponder.

Currently, the following modes are used for medium speed data (understood as a few kbps) on the NB transponder. First, there are the FreeDV modes, whose use has been covered in this Lime microsystems community post. Most of these modes use OFDM or multi-carrier modems and are designed having HF fading channels in mind. These don’t give good performance over the QO-100 transponder, since the frequency instabilities of the transmitters and receivers give problems with OFDM modems. A single carrier modem is much better. David Rowe VK5DGR has made some modifications to the FreeDV 2020 modem to improve performance over QO-100, and it certainly works quite well, but better results can be obtained with a single carrier modem.

There are some people using DRM for DSSTV. This is also an OFDM modem intended for HF, and the symbol time is quite long, so the frequency instabilities can give problems. Finally, there is KG-STV, which was relatively unpopular before QO-100 but it is seeing a lot of use due to its good performance. It uses a single carrier MSK modem. This is probably the most popular medium speed mode on the NB transponder, but it is only 1200bps.

One important characteristic of the NB transponder is that there is a lot of SNR available. The rule is that no signal should be stronger than the beacons, but the BPSK beacon has a CN0 of around 54dB as received in my station. It is also not difficult (in terms of uplink EIRP) to achieve the same power as the beacon. Therefore, it is a reasonable assumption that stations interested in using a medium speed data modem will adjust their uplink power to be as strong as the BPSK beacon. I already hinted at what is possible with such a strong signal in this post.

I have decided to do some preliminary tests to check the performance of a 2kbaud 8PSK signal over the NB transponder. This post summarizes my results. The material for the post can be found in the qo100-modem Github repository.

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Plotting spectrum measurements by SMOG-P

The SMOG-P 1P PocketQube that was launched recently has an interesting payload: a UHF spectrum monitor that records power spectral density measurements. Lately, I have been adding support in gr-satellites to decode the telemetry frames transmitted by SMOG-P and ATL-1 (which also carries a similar spectrum monitor), using the code published here as a reference.

As a result of this work, now it is possible to save and plot the spectrum data transmitted by SMOG-P and ATL-1 using gr-satellites. This post explains how.

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Oscillations and relativistic effects in Galileo broadcast clocks

A few days ago, Bert Hubert, the creator of galmon.eu, discovered a sinusoidal oscillation in the clock drift \(a_{f1}\) parameter of the broadcast ephemerides of Galileo satellites. This variation has a frequency that matches the orbital period of 14 hours and 7 minutes. At first, I suggested that it might be caused by relativistic effects, which are given by\[-\frac{\sqrt{\mu}}{c^2}e\sqrt{A}\sin E,\]where \(\mu\) is the Earth’s gravitational parameter, \(c\) is the speed of light, \(e\) is the eccentricity, \(A\) is the semi-major axis, and \(E\) is the eccentric anomaly. In fact, the order of magnitude of the oscillations that Bert was seeing seemed to agree with this formula.

However, then I realised that this relativistic effect is not included in the broadcast clock model. It needs to be included back by the receivers. Therefore, it shouldn’t appear at all in the broadcast clock. Something didn’t seem quite right. This post is an in-depth look at this problem.

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QO-100 BPSK beacon frequency measured at Bochum

The experiments about measuring the frequency stability of the local oscillator of the QO-100 NB transponder with a Vectron MD-011 GPSDO I made a few days ago indicated that the Allan deviation of the local oscillator was probably better than \(10^{-11}\) for \(\tau\) between 1 and 100 seconds. The next step in trying to characterize the stability of the local oscillator is to use a reference clock which is more stable than the Vectron.

I contacted Achim Vollhardt DH2VA asking him if it was possible to record the downlink of the BPSK beacon at Bochum, so as to have a recording referenced to the Z3801A GPSDO in Bochum, which is much more stable than the Vectron. He and Mario Lorenz DL5MLO have been very kind and they have taken the effort to make a recording for me. This post is an analysis of this recording made at Bochum.

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More frequency measurements of the QO-100 NB transponder

This post is a follow up to my experiments about measuring the stability of the QO-100 NB transponder local oscillator. I am now using the Vectron MD-011 GPSDO that Carlos Cabezas EB4FBZ has lent me to reference all my QO-100 groundstation (see more information about the Vectron GPSDO in this post).

The Vectron MD-011 has an Allan deviation of \(10^{-11}\) at \(\tau = 1\,\mathrm{s}\) and \(2\cdot10^{-11}\) at \(\tau = 10\,\mathrm{s}\) according to the datasheet, so it is an improvement of an order of magnitude compared to my DF9NP TCXO-based GPSDO. I have made more measurements with the Vectron MD-011 as in my previous experiments, measuring the phase of the BPSK beacon transmitted from Bochum and a CW tone transmitted with my station. This post summarizes my results and conclusions.

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Measuring the Allan deviation of a GPSDO with an SDR

A few days ago I tried to measure the QO-100 NB transponder LO stability using my DF9NP 10MHz GPSDO. It turned out that my GPSDO was less stable than the LO, so my measurements showed nothing about the QO-100 LO. Carlos Cabezas EB4FBZ has been kind enough to lend me a Vectron MD-011 GPSDO, which is much better than my DF9NP GPSDO and should allow me to measure the QO-100 LO.

Before starting the measurements with QO-100, I have taken the time to use the Vectron GPSDO to measure the Allan deviation of my DF9NP GPSDO over several days. This post is an account of the methods and results.

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DSLWP-B crash site found

Back in August, I posted about my calculations of the site where DSLWP-B impacted with the lunar surface on July 31. The goal was to pass the results of these calculations to the Lunar Reconnaissance Orbiter Camera team so that they could image the location and try to find the impact crater.

Yesterday, the LROC team published a post saying that they had been able to find the crash site in an image taken by the LRO NAC camera on October 5. The impact crater is only 328 metres away from the location I had estimated.

This is amazing, as in some way it represents the definitive end of the DSLWP-B mission (besides all the science data we still need to process) and it validates the accuracy of the calculations we did to locate the crash site. I feel that I should give due credit to all the people involved in the location of the impact.

Wei Mingchuan BG2BHC from Harbin Institute of Technology was the first to take the orbital information from the Chinese Deep Space Network, perform orbit propagation and compute the crash location assuming a spherical Moon, thus obtaining an approximate position in Van Gent X crater. Cees Bassa from ASTRON refined Wei’s calculations by including a digital elevation model. Phil Stooke from Western University first suggested to use a digital elevation model, helped us contact the LROC team, and filled in an observation request for the camera. And of course the LROC team and the Chinese DSN, since the quality of their ephemeris for DSLWP-B allowed us to make a rather precise estimate.

The LROC team has posted the images shown below, where in a comparison between an image taken in 2014 and the image taken in October the small crater can be seen.

DSLWP-B crash site image

The image of the crash is M1324916226L, an image taken by the left NAC camera. However, I can’t find this image yet in the LROC archive, so it seems this image hasn’t been made public yet.

The small crater, which the LROC team estimate to be 4×5 metres in diameter, is visible more clearly if we compute the difference between the before and after images (an idea of Phil Stooke). The figures below show this difference both as a signed quantity and as an absolute value.

Difference in the before and after images
Absolute value of the difference between before and after images

Though my eye fails to see it, the LROC team says that the long axis of the crater is oriented in a southwest-northeast direction. This is consistent with the direction of the impact, since DSLWP-B was travelling towards the northeast.

For the comparison with the October 5 image, the LROC team has chosen an image taken with a similar illumination angle. In fact, the lunar phase in both images only differs in 10º, so the shadows are very similar, with the sun located towards the southwest. In fact, the newest image of the area was taken on 2018-10-16, but the one from 2014 probably gave the most similar illumination conditions.

In my post in August I included a link to Quickmap showing the estimated area of the impact. Now I have marked in red the location of the crash. For a sense of scale, the large crater northwest of the crash is some 50 metres in diameter. You can see both points in Quickmap here.

Location of DSLWP-B crash (red) and estimate (blue)

It is good to go back to all the simulations I did to have an idea of what the 328m error represents. My final simulation was done with the ephemeris from July 25, so they were 6 days old at the moment of impact. When I used the ephemeris from July 18, the position of the impact changed by 231m, while the ephemeris from June 28 yielded a change of 496m. Therefore, it seems that an error of 300m is well in line with what we could expect of the precision of the Chinese DSN ephemeris.

The impact location computed by Cees Bassa was 2786m away from my estimate. The main problem with Cees’s estimate is that the orbital model he used considered spherical gravity for the Moon, while my studies showed that it was important to consider non-spherical gravity.

I did most of my simulations with a 10×10 spherical harmonic model for the Moon gravity, but to assess whether this was enough, I also made a simulation with a 20×20 spherical harmonic model. This yielded an impact point which was 74m away from the impact computed with the 10×10 model.

According to my Monte Carlo simulations with a 1km ephemeris error, the 1-sigma ellipse semi-axes of the impact position were 876m in the northeast direction and 239m in the southeast direction. With this information, I gave an educated guess of the position error of 600m in the northeast direction and 200m in the southeasth direction. The actual impact point is 328m northwest of my estimate, so somewhat higher than my error estimate but still within the 2-sigma ellipse. This leaves me quite happy with the quality of my estimate.