destevez – Page 23 – Daniel Estévez

Galileo outage revisited

A few weeks ago, a presentation by Octavian Andrei, of the Finnish Geospatial Research Institute, appeared in YouTube showing technical details about the Galileo constellation outage that happened between July 12 and 16. In the presentation, Octavian studies the navigation data gathered by a geodetic receiver in Metsähovi, showing anomalies in some of the parameters of the navigation message, such as the \(\text{IOD}_{\text{nav}}\), the SISA (signal in space accuracy) and the DVS (data validity status).

Back in July, I looked at the navigation data from the outage in this post, where I used navigation RINEX files collected by the IGS MGEX to study changes and anomalies in the navigation message. In that post I concentrated on July 16 and 17, to show what happened as the system was coming back online. Octavian has discovered some very interesting anomalies that happened before the incident, on July 10 and 11. Indeed, the first anomaly happened at 13:40 GST on July 10, well before July 11 21:50 GST, when the navigation message stopped being updated.

Thus, in view of Octavian’s discoveries, I have revisited my study, including also data from July 10 and 11, and paying special attention to the \(\text{IOD}_{\text{nav}}\) parameter, which can be seen to have the most interesting behaviour in Octavian’s presentation.

Measuring the gain of a dish

Here I want to show a technique for measuring the gain of a dish that I first learned from an article by Christian Monstein about the Moon’s temperature at a wavelength of 2.77cm. The technique only uses power measurements from an observation of a radio source, at different angles from the boresight. Ideally, the radio source should be strong and point-like. It is also important that the angles at which the power measurements are made are known with good accuracy. This can be achieved either with a good rotator or by letting an astronomical object drift by on a dish that is left stationary.

Trying to find the DSLWP-B crash site

As you may well know, DSLWP-B, the Chinese lunar orbiting Amateur satellite crashed with the Moon on July 31 as a way to end its mission without leaving debris in orbit. I made a post with my prediction, which showed the impact point southeast of Mare Moscoviense, in the far side of the Moon. Phil Stooke was more precise and located the impact point near the Van Gent crater.

Our plan is to get in contact with the LRO team and try to find the crash site in future LRO images. We are confident that this can be done, since they were able to locate the Beresheet impact site a few months ago. However, to help in the search we need to compute the location of the impact point as accurately as possible, and also come up with some estimate of the error to define a search area where we are likely to find the crash. This post is a detailed account of my calculations.

Transmitting through QO-100 with the LimeNET Micro and LimeRFE

A couple weeks ago, I did a demo where I showed the LimeRFE radio frequency frontend being used as an HF power amplifier to transmit WSPR in the 10m band. Another demo I wanted to do was to show the LimeNET Micro and LimeRFE as a standalone 2.4GHz transmitter for the QO-100 Amateur radio geostationary satellite.

The LimeNET Micro can be best described as a LimeSDR plus Raspberry Pi, so it can be used as an autonomous transceiver or remotely through an Ethernet network. The LimeRFE has a power amplifier for 2.4GHz. According to the specs, it gives a power of 31dBm, or a bit over 1W. This should be enough to work QO-100 with a typical antenna.

You may have seen the field report article about the QO-100 groundstation I have in my garden. It is based around a LimeSDR Mini and BeagleBone Black single board ARM computer. The groundstation includes a driver amplifier that boosts the LimeSDR to 100mW, and a large power amplifier that gives up to 100W. The LimeSDR Mini and BeagleBone Black give a very similar functionality to the LimeNET Micro, but the LimeNET Micro CPU is more powerful.

The idea for this demo is to replace my QO-100 groundstation by the LimeNET Micro and LimeRFE, maintaining only the antenna, which is a 24dBi WiFi grid parabola, and show how this hardware can be used as a QO-100 groundstation.

Precise orbit determination for Lucky-7

In one of my previous posts, I used measurements from the GPS receiver on-board the Lucky-7 cubesat in order to find the TLE that best matched its orbit, and help determine which NORAD ID corresponded to Lucky-7.

Now I have used the same GPS measurements to perform precise orbit determination with GMAT. Here I describe the results of this experiment.

More DSLWP-B lunar surface images identified

In my last posts about DSLWP-B, I have been showing all the images of the lunar surface that were taken by the satellite during the last weeks of the mission, and tried to identify to which area of the Moon each image corresponded. For several of them, I was able to give a good identification using Google Moon, but for many of the latest images I was unable to find an identification, since they show few or none characteristic craters.

Thus, for these images I only gave a rough prediction of which area of the Moon was imaged by using GMAT and the published ephemeris from dslwp_dev. This doesn’t take into account camera pointing, orbit or shutter time errors.

Phil Stooke has become interested in this and he has managed to identify many of the images, even some containing very little detail, which I find impressive. No wonder, Phil is the author of several atlases of space exploration of the Moon and Mars, so he knows a lot of lunar geography.

Phil tells me that he has used Quickmap, which is a very nice tool that I didn’t know of. It is much more powerful than Google Moon. He recommends to switch to an equidistant cylindrical projection and set as a basemap layer the “WAC mosaic (no shadows) map”, which contains images with the sun directly overhead. This resembles the images taken by DSLWP-B better, since these are always taken with the sun at a high elevation, because the camera always points away from the sun. It is interesting to see how the appearance of the surface changes between the “no shadows” and “big shadows” maps.

In this post I show the locations of the images identified by Phil.

DSLWP-B last activities and end of mission

As probably you all know, the Chinese Amateur lunar orbiting satellite DSLWP-B was expected to collide with the Moon on July 31 at 14:20 UTC, so this is the last report about the DSLWP-B activities. The collision was planned since January this year, and was done as a means to end the mission without leaving debris in lunar orbit.

The activation slots for the Amateur payload on-board DSLWP-B for this week were the following:

29 Jul 00:15 to 02:15
29 Jul 04:30 to 06:30
29 Jul 20:00 to 22:00
30 Jul 05:30 to 07:30
30 Jul 16:20 to 18:20
31 Jul 06:30 to 08:30
31 Jul 13:24 to 15.24
1 Aug 05:30 to 07:30

I had calculated a periapsis height of -62km for the July 31 orbit, so the collision with the Moon was quite certain, even taking orbit errors into account. However, a slot was set on August 1 just in case the collision didn’t happen.

This post summarizes the activities done this week with DSLWP-B and the end of the mission.

WSPR with the LimeRFE

A few days ago, I received a LimeRFE from Andrew Back of Lime Microsystems. He was kind enough to send me a unit so that I can test it and make some usage demos during the ongoing crowdfunding campaign at Crowd Supply.

The LimeRFE is intended to work as an RF frontend for the LimeSDR family, although it can work coupled with any other SDR or conventional radio. As such, it has power amplifiers, filters and LNAs designed to cover the huge frequency range of these SDRs. It is designed to cover all the Amateur radio bands from HF up to 9cm, and a few cellular bands.

As anyone will know, designing broadband RF hardware is often quite difficult or expensive (Amateur radio amplifiers and LNAs are usually designed for a single band), so packing all this into a single unit is a considerable feat. The output power on most bands is around a couple watts, which is already enough for many experiments and applications. The block diagram of the LimeRFE can be seen below.

In this post I show a brief overview of the LimeRFE and demonstrate its HF transmission capabilities by showing a WSPR transmitter in the 10m band, using a LimeSDR as the transmitter.

DSLWP-B activities for the fourth week of July

During the fourth week of July, the Amateur payload on-board DSLWP-B was active in the following slots.

22 Jul 06:14 to 08:14
22 Jul 22:40 to 23 Jul 00:40
23 Jul 23:20 to 24 Jul 01:20
25 Jul 00:30 to 02:30
26 Jul 10:55 to 12:55
27 Jul 02:30 to 04:30
28 Jul 03:30 to 05:30

Additionally, Wei Mingchuan BG2BHC shared on Twitter the 10 minute slots for the activations of the X band transmitter. This transmitter uses a frequency of 8478MHz (in the Deep Space X band) and 2Mbps BPSK with CCSDS standards. The transmit power is 2W and the gain of the small X-band dish is 22dBi. The signal is detectable with small stations (as shown here), but to demodulate the data a large dish is needed. The Chinese DSN uses 35m and 50m antennas to receive this signal.

DSLWP-B mission end prediction

Back in May, I spoke about the future collision of DSLWP-B with the lunar surface. It would happen on July 31, thus putting and end to the mission. Now that the impact date is near, I have run again the calculations with the latest ephemeris in order to have an accurate simulation of the event.

The ephemeris I’m using consist of a Moon centred ICRF Keplerian state vector which has been shared by Wei Mingchuan BG2BHC. In GMAT, this state vector is as follows:

DSLWP_B.Epoch = '25 Jul 2019 02:30:00.000';
DSLWP_B.CoordinateSystem = LunaICRF;
DSLWP_B.SMA = 8708.404;
DSLWP_B.ECC = 0.747921;
DSLWP_B.INC = 44.157;
DSLWP_B.RAAN = 52.405;
DSLWP_B.AOP = 86.261;
DSLWP_B.TA = 165.00062091131025;

Using this GMAT script, I have obtained that the impact will happen on 2019-07-31 14:19:57 UTC, near Mare Moscoviense, in the lunar far side. This result is quite close to the calculations I did in May, which predicted an impact at 14:47 UTC.

The images below show the impact simulation in GMAT. Since the impact happens on the far side of the Moon, it will not be visible from Earth. There is an activation of the Amateur payload onboard DSLWP-B for 2019-07-31 13:24 to 15:24 UTC. The satellite will hide behind the Moon around 14:08 UTC. If the Moon was not solid, DSLWP-B would reappear around 14:35 UTC. The absence of radio signals after this moment will confirm that the impact has occurred.