DSLWP-B and the solar eclipse

On July 2, there will be a total solar eclipse observable from parts of the Pacific Ocean, Chile and Argentina. This gives the opportunity to image the eclipse with the Inory eye camera on-board DSLWP-B, the Chinese lunar orbiting Amateur satellite. Wei Mingchuan BG2BHC has already started planning for the eclipse observation, and I have run my usual calculations using the 20190618 ephemeris from dlswp_dev.

The main interest in trying to do an imaging session during the eclipse is to photograph the shadow of the Moon on the surface of the Earth. The camera doesn’t have a large resolution, and the Earth looks small in the image, but perhaps it will be possible to distinguish the shadow clearly.

Besides this, it is also interesting to try to get the Moon in the image, as it has been done in other occasions. This not only gives a more interesting picture, but also helps the camera auto-exposure algorithm by providing a large bright object in the field of view. Past attempts to image the Earth alone have all yielded over-exposed images. It turns out that the orbit of DSLWP-B is ideal to image the eclipse, partly by chance and partly because of the nominal satellite attitude.

Recall that the camera of DSLWP-B is always pointing away from the Sun, because the satellite aims its solar panel towards the Sun. Since DSLWP-B orbits the Moon, this means that the Earth will be in the centre of the camera field of view whenever a solar eclipse happens. However, the satellite could be at any point of its orbit. It might happen that the Moon is between the satellite and the Earth, hiding the view, or, more likely, that the Moon is outside of the field of view of the camera.

The total eclipse is observable between 18:01 and 20:45 UTC, with the maximum happening at 19:23. The two figures below show the positions of the Moon and Earth within the field of view of the camera. As explained above, the Earth is near the centre of the image during the eclipse. In the bottom figure we see that the Earth is hidden by the Moon until 19:27.

Therefore, it seems that this is an optimal chance to image the eclipse. The Earth will emerge behind the Moon very near to the eclipse maximum. Since DSLWP-B is orbiting at a lower altitude in comparison with other imaging sessions, the Moon will move rather quickly through the camera field of view and disappear in a matter of 10 minutes.

Thus, my recommendation is that instead of taking images every 10 minutes, as it has been done in other occasions, a smaller interval of 2 minutes is used instead. A series of 9 images starting at 19:20 is shown in the plot above as green lines. This gives good coverage of the eclipse and the Earth appearing behind the Moon.

The figure below shows the simulation of the view in GMAT. Note that the field of view of the camera is smaller than what this image shows.

GMAT simulation of the eclipse view

Astrometry with DSLWP-B camera

In my last post, I spoke about the images taken by DSLWP-B, the Chinese lunar orbiting Amateur satellite, during the first week of June. One of these images was the picture of stars shown below, taken on 2019-06-07 08:00 UTC.

Image 0xD5, taken on 2019-06-07 08:00, downloaded on 2019-06-07 09:30 – 09:45

Although it may seem that this image is not very interesting in comparison with the other awesome images of the Earth and the Moon, the stars that appear in the image can be used as a reference to compute where the camera was aiming. In a previous post, I used an image of stars to compute the field of view of the camera. In this post, I will assess the accuracy in the camera pointing. Currently, there exist only two images of stars taken by DSLWP-B, so it is interesting to try to study these as much as possible.

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Report for DSLWP-B June imaging

In my previous post, I spoke about the opportunity to take images of the Moon and Earth using the Inory eye camera on DSLWP-B during the first week of June. All the tentative plannings for programming the image taking and downloading the images listed in that post were eventually made final, so the observation runs have been done without any modifications to the schedule.

On June 3, a series of 9 images with 10 minutes of spacing was taken starting at 03:05 UTC. This gives a nice sequence of the Earth hiding behind the Moon and reappearing. One of the images was partially downloaded during the same 2 hour activation of the Amateur payload on June 3. Several of the remaining images were downloaded between June 4 and June 6. On June 7, the station of Reinhard Kuehn DK5LA, which is normally used as the uplink station, wasn’t available, so a single image outside of the Moon series was downloaded using Harbin as uplink station.

This is a report of the images taken and downloaded during this week.

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DSLWP-B June lunar imaging and VLBI

Yesterday, Wei Mingchuan BG2BHC sent an email to the team of DSLWP-B collaborators saying that the first week of June would give good opportunities both to take images of the Moon and Earth (as it has been done in other occasions) and to perform VLBI sessions involving Dwingeloo, Shahe, Harbin, and perhaps Wakayama University, which has a 12m dish. Here I show the preliminary plan proposed by Wei and a few graphs useful for camera and VLBI planning.

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Results of DSLWP-B Amateur VLBI experiment on 2018-11-21

The first Amateur VLBI experiment with DSLWP-B was performed on 2018-06-10. In that experiment, the 250baud GMSK beacons at 435.4MHz and 436.4MHz were recorded in the 25m PI9CAM radiotelescope in Dwingeloo, The Netherlands, and a 12m repurposed Inmarsat C-band dish in Shahe, Beijing. These synchronized recordings were processed later to obtain delta-range and delta-velocity measurements. Due to the low baudrate, the noise of the delta-range measurements was quite high, on the order of 20km. Since the beacons were short transmissions of 15 seconds, making accumulated phase measurements was not possible.

Another Amateur VLBI experiment was performed on 2018-11-21. The novelty of this experiment was that 500baud GMSK SSDV transmissions were made on 436.4MHz. These long transmissions, lasting around 30 minutes each, allow us to make accumulated phase measurements. Also, the higher baudrate reduces the noise in the delta-range measurements. Another novelty was that a third station, the Harbin Institute of Technology Amateur Radio Club BY2HIT groundstation also joined the experiments, so observations from three stations are available.

This post is an account of the results I have obtained processing the observations from 2018-11-21.

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DSLWP-B lunar impact location

A few days ago, I spoke about the future impact of DSLWP-B on the lunar surface, which will happen in the far side of the Moon around the end of July, and how the spacecraft could be manoeuvred to make the impact point fall on the near side of the Moon instead, so that it can be observed from Earth.

Philip Stooke made a very good remark in the comments saying that the impact might have been planned on the far side of the Moon deliberately in order to avoid Apollo landing sites and other heritage sites. This is a very valid concern. By all means, the crash should be planned to avoid disturbing heritage sites or other areas of specific interest.

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DSLWP-B lunar impact prediction

In my last post, I spoke about the future lunar impact of DSLWP-B on July 31. Edgar Kaiser DF2MZ asked over on Twitter if the impact would be visible from Earth. As I didn’t know the answer, I have made a simulation in GMAT to find this out.

The figure below shows the orbit of DSLWP-B between July 28 12:00 UTC and the moment of impact, on July 13 14:47 UTC. The orbital state used for DSLWP-B is the 20190426 tracking file from dslwp_dev. The reference frame is arranged so that the +X axis points towards the Earth, and the Y axis lies on the Earth-Moon orbital plane. As we can see, unfortunately, the impact will happen on the far side of the Moon, where it is not observable from Earth.

Future impact of DSLWP-B on the far side of the Moon

However, it is possible to arrange a manoeuvre to modify the orbit slightly and make the impact point fall on the near side of the Moon, where it is visible from Earth. In the previous post we observed that, ignoring the collision with the lunar surface, the periapsis radius would continue to decrease after July 31, until reaching a minimum value in January 2020.

Therefore, it is possible to raise the periapsis radius slightly in order to delay the collision approximately half a lunar month, so that the periapsis faces the Earth at the moment of impact. The delta-v required to make this manoeuvre is small, as the adjustment to the orbit is subtle.

For instance, performing a prograde burn of 7m/s at the first apoapsis after July 1 delays the collision until August 13, producing an impact in the near side of the Moon. The resulting orbit can be seen in the figure below, which shows the path of DSLWP-B between July 28 and the moment of impact.

Impact of DSLWP-B on the near side of the Moon if a correction manoeuvre is applied

Adjusting the delta-v more precisely would make it possible even to control the time of the impact, so as to guarantee that the Moon will be in view of the groundstations at China and The Netherlands when the collision happens. However, this adjustment requires a very precise delta-v and is quite sensitive to the orbital state, so perhaps it is not feasible without performing a precise orbit determination and maybe some smaller correction manoeuvres following the periapsis raise.

Another possible problem that can affect the prediction of the impact point are the perturbations of the orbit caused by the lunar mascons, which can be noticeable when the altitude of the orbit starts getting small, and which haven’t been considered very carefully in this simulation (the non-spherical gravity of the Moon was only simulated up to degree and order 10).

The GMAT script used for this post can be found here.

DSLWP-B deorbit and mission end

On January 24, the periapsis of the lunar orbit of DSLWP-B was lowered approximately by 500km, so that orbital perturbations would eventually force the satellite to collide with the Moon. This was done to put an end to the mission and to avoid leaving debris in orbit. It is expected that the collision will happen at the end of July, so there are only three months left now for the DSLWP-B mission. Here I look at the details of the deorbit.

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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.

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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.

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