satellites – Page 18 – Daniel Estévez

Imaging times for DSLWP-B eclipse observation

In my last post, I spoke about the possibility of imaging the July 2 solar eclipse using the Inory eye camera on-board DSLWP-B. After discussing the plans for the observations with Wei Mingchuan BG2BHC, we have decided to activate the DSLWP-B Amateur payload during the following intervals:

2019-06-30 05:30 to 07:30
2019-07-01 05:30 to 07:30
2019-07-02 18:00 to 20:00
2019-07-03 06:00 to 08:00
2019-07-04 06:30 to 08:30
2019-07-05 07:30 to 09:30

The camera will be used in 2x zoomed mode, which has a field of view of 14×18.5, degrees. Using the zoomed mode requires careful planning, since part of the Moon needs to appear inside the image, to help the camera auto-exposure algorithm, but the Moon shouldn’t hide the Earth.

The June 30 activation will be used to test the camera, taking images of the Moon in similar positions to those on July 2. The Earth will not be in view of the camera on this day, but these tests will serve to validate camera pointing, exposure, and satellite ephemeris errors.

The following imaging times have been proposed:

2019-06-30 05:51:20
2019-06-30 05:52:20
2019-06-30 05:53:20
2019-06-30 06:29:50
2019-06-30 06:30:50
2019-06-30 06:31:50
2019-07-02 18:56:00
2019-07-02 18:57:00
2019-07-02 18:58:00
2019-07-02 19:31:45
2019-07-02 19:32:45
2019-07-02 19:33:45

The idea for July 2 is to take an image of the Earth and Moon just before the Earth becomes hidden behind the Moon and just after it reappears. Determining these moments accurately is difficult. The Moon will be moving rather fast across the field of view of the camera, since the orbit altitude is rather low. Therefore, the timing of these events is sensitive to the satellite ephemeris and the orbit propagation algorithm. To try to mitigate this effect, we will take a series of three images spaced one minute instead of taking a single image.

On June 30, the same imaging run is mimicked: a series of three images will be taken before the Moon hides the centre of the image (this time the Earth will not be present) and a series of three images will be taken after the centre of the image becomes unblocked again.

The figure below shows the camera view prediction for the June 30 imaging run. The calculations have been done with the 20190630 ephemeris from dslwp_dev.

We note that the second run of three images seems a little early. Wei is doing his calculations with STK and apparently he is getting slightly different orbital predictions compared to my predictions done in GMAT. We haven’t tried to study these differences, but this gives an idea of how sensitive the imaging times are to ephemeris and orbital propagators. Hopefully the series of three images will account for orbital errors. Additionally, after doing the test run on June 30, the results can be compared with the orbital prediction and the imaging times for July 2 can be modified slightly if necessary.

The figure below shows the camera view for the July 2 eclipse imaging run. The 20190630 ephemeris have been used for this plot also. We have the same effect, where the second proposed imaging times seem somewhat early.

Since this time the Earth is also visible in the image, it is convenient to plot the “Earthrise view” plot, which I have used on other occasions. This shows the angular distance between the Earth and the Moon rim, so it can be used to determine if the the Earth is hidden by the Moon (negative distance) or not.

As we can see below, according to my GMAT prediction, the Earth will not be visible in the images around 19:30. It seems these should be taken a few minutes later. However, Wei has obtained different results with STK. In any case, these imaging times can be corrected based on the results obtained on June 30.

The plots in this post have been made in this Jupyter notebook.

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.

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.

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.

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.

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.

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.

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.

Decoding SSDV from JY1SAT

JY1SAT is a Jordanian 1U Amateur cubesat that carries a FUNcube payload by AMSAT-UK. As usual, the FUNcube payload on-board JY1SAT has a linear transponder with uplink in the 435MHz band and downlink in the 145MHz band, and a 1k2 BPSK telemetry transmitter in the 145MHz band. The novelty in comparison to the older FUNcube satellites is that the BPSK transmitter is also used to send SSDV images and Codec2 digital voice data.

Here I show how to decode the SSDV images using gr-satellites.