Events – Page 7 – Daniel Estévez

DSLWP-B photo planning

As you may already know, most of the pictures taken so far by the DSLWP-B Inory eye camera have been over-exposed purple things. In order to take more interesting pictures, such as pictures of the Earth and the Moon, we have to plan ahead and know when these objects will be in view of the camera.

The picture below shows a diagram of DSLWP-B. The solar panel is behind the main body, pointing towards the back right. The Inory eye camera points in the opposite direction, towards the front left of the diagram. The camera is located in the front panel of the Amateur radio module, which is one of the pink modules.

In the nominal flight configuration, the solar panel is pointed towards the Sun to gather the maximum amount of sunlight. Therefore, the camera should point opposite to the Sun. This is good, because objects in the field of view of the camera are guaranteed to be well illuminated and it also prevents the Sun from appearing in the image, causing lens flare.

In one of the images taken previously by the camera there is a very bright lens flare, possibly caused by the Sun hitting the camera directly. This seems to indicate that the spacecraft is not always in the nominal flight orientation. As the orientation is very important when doing photo planning, here we assume that the spacecraft is always oriented in the nominal flight configuration, with its camera pointing opposite to the Sun.

We use GMAT with the 20181006 tracking file from dswlp_dev for the orbital calculations. The GMAT script can be downloaded here.

To simulate the orientation of the camera in GMAT, I have used an ObjectReferenced coordinate system. The primary is the Sun, the secondary is DSLWP-B, and the axes are chosen as X = R, Z = N. Therefore, the X axis points away from the Sun, in the direction of the camera. The orbit view is centred on DSLWP-B and points towards +X, so it shows what the camera would see (except for the field of view, which is not simulated).

When propagating the orbit in GMAT we see that the Earth passes near the +X axis every Lunar month and that the Moon crosses the image every orbit.

The two images below show the dates when the Earth will pass nearby the +X axis. These dates are good candidates for taking images of the Earth. From the point of view of the camera, the Earth seems to orbit slowly from right to left in these images. Therefore, there should be a tolerance of a couple of days as to when the images of the Earth could be taken.

Thus, we see that the next good dates for attempting to take images of the Earth are October 9 and November 7 (plus/minus a couple days of margin).

The Moon is much larger from the point of view of DSLWP-B and we have seen already that it fills the field of view of the camera completely. Thus, even though in this GMAT simulation the Moon crosses the screen every orbit, we need to wait until the path taken by the Moon is near the centre of the screen.

In the image below we see a good moment to take an image of the Moon. From the point of view of the camera, the Moon crosses from top to bottom of the screen every orbit and its path moves slightly to the right every time, taking it closer to the centre as we progress towards December.

Therefore, a good moment to attempt to take an image of the Moon is late October and all of November. However, the time when the picture is taken is critical, because the Moon crosses the screen quickly. It is near the +X axis only for one or two hours. Therefore, starting in late October, there will be a window of a couple hours each orbit (or each day, since the orbital period is close to one day) where photos of the Moon can be attempted.

Judging by one of Wei Mingchuan BG2BHC’s lasts tweets, he has also been thinking about dates to take good images with the camera. It would be interesting to know if his findings match what I have exposed here.

A good question when doing this sort of planning is what is the field of view of the camera. Probably this can be estimated from some of the existing images.

DSLWP-B talk in IberRadio

As you may already know if you follow me on Twitter, on Saturday September 15 I was in Ávila in the IberRadio Spanish Amateur Radio fair giving a talk about DSLWP-B. The talk was well received and I had quite a full room. I recorded my talk to upload it later to YouTube as I did last year.

Since I know that many international people would be interested in this talk and the talk was in Spanish, I wanted to prepare something in English. As I would be overlaying the slides on the video (since the video quality is quite poor and it is impossible to see the projected slides), I thought of overlaying the slides also in English and adding English subtitles.

Doing the subtitles has been a nightmare. I have been working on this non-stop since Sunday morning (as my free time allows me). I intended to use YouTube’s transcript and auto-sync feature, where you provide a transcript (in the video’s original language) and presumably an AI will auto-sync this to the video’s audio. I say presumably because in my case the auto-sync was a complete failure, so I had to resync everything by hand.

Also, since I have listened to the video over and over, I have gotten a bit bored of my own voice and noted some expressions and words that I tend to use a lot. I think this is good, because now I am more conscious when talking in public and can try to avoid using these expressions so much.

In any case, here is the final result. First you have the video with English slides:

And also you have the video with Spanish slides:

Remember that you can use either Spanish or English subtitles with any of the videos. Also, I have the translation contributions enabled, so feel free to provide subtitles in your own language if you wish.

The PDF slides for the talk can be downloaded in Spanish here and in English here.

Playing with LilacSat-1

Even though the cubesat LilacSat-1 was launched more than a year ago, I haven’t played with it much, since I’ve been busy with many other things. I tested it briefly after it was launched, using its Codec2 downlink, but I hadn’t done anything else since then.

LilacSat-1 has an FM/Codec2 transponder (uplink is analog FM in the 2m band and downlink is Codec2 digital voice in the 70cm band) and a camera that can be remotely commanded to take and downlink JPEG images (see the instructions here). Thus, it offers very interesting possibilities.

Since I have some free time this weekend, I had planned on playing again with LilacSat-1 by using the Codec2 transponder. Wei Mingchuan BG2BHC persuaded me to try the camera as well, so I teamed up with Mike Rupprecht DK3WN to try the camera this morning. Mike would command the camera, since he has a fixed station with more power, and we would collaborate to receive the image. This is important because a single bit error or lost chunk in a JPEG file ruins the image from the point where it happens, and LilacSat-1 doesn’t have much protection against these problems. By joining the data received by multiple stations, the chances of receiving the complete image correctly are higher.

Report for today’s DSLWP-B SSDV session

Today an SSDV transmission session from DSLWP-B was programmed between 7:00 and 9:00 UTC. The main receiving groundstation was the Dwingeloo radiotelescope. Cees Bassa retransmitted the reception progress live on Twitter. Since the start of the recording, it seemed that some of the SSDV packets were being lost. As Dwingeloo gets a very high SNR and essentially no bit errors, any lost packets indicate a problem either with the transmitter at DSLWP-B or with the receiving software at Dwingeloo.

My analysis of last week’s SSDV transmissions spotted some problems in the transmitter. Namely, some packets were being cut short. Therefore, I have been closely watching out the live reports from Cees Bassa and Wei Mingchuan BG2BHC and then spent most of the day analysing in detail the recordings done at Dwingeloo, which have been already published here. This is my report.

First SSDV transmission from DSLWP-B

As some of you may know, DSLWP-B, the Chinese lunar-orbiting Amateur satellite carries a camera which is able to take pictures of the Moon and stars. The pictures can be downlinked through the 70cm 250bps GMSK telemetry channel using the SSDV protocol. Since an r=1/2 turbo code is used, this gives a net rate of 125bps, without taking into account overhead due to headers. Thus, even small 640×480 images can take many minutes to transfer, but that is the price one must pay for sending pictures over a distance of 400000km.

On Saturday August 3, at 01:27 UTC, the first SSDV downlink in the history of DSLWP-B was attempted. According to Wei Mingchuan BG2BHC, the groundstation at Harbin managed to command the picture download at 436.400MHz a few minutes before the GMSK transmitter went off at 01:30 UTC. A few SSDV frames were received by the PI9CAM radiotelescope at Dwingeloo.

The partial image that was received was quickly shared on Twitter and on the DSLWP-B camera webpage. The PI9CAM team has now published the IQ recording of this event in their recording repository. Here I analyze that recording and perform my own decoding of the image.

DSLWP-B and the lunar eclipse

As you may well know, last Friday 27th July there was a total lunar eclipse. This is an interesting event for lunar-orbiting spacecraft such as DSLWP-B. In fact, depending on the spacecraft’s orbit, it may also pass through the Earth’s umbra or penumbra. Here I look at the trajectory taken by DSLWP-B during the eclipse.

DSLWP-B first JT4G test

Yesterday, between 9:00 and 11:00, DSLWP-B made its first JT4G 70cm transmissions from lunar orbit. Several stations such as Cees Bassa and the rest of the PI9CAM team at Dwingeloo, the Netherlands, Fer IW1DTU in Italy, Tetsu JA0CAW and Yasuo JA5BLZ in Japan, Mike DK3WN in Germany, Jiang Lei BG6LQV in China, Dave G4RGK in the UK, and others exchanged reception reports on Twitter. Some of them have also shared their recordings of the signals.

Last week I presented a JT4G detection algorithm intended to detect very weak signals from DSLWP-B, down to -25dB SNR in 2500Hz. I have now processed the recordings of yesterday’s transmissions with this algorithm and here I look at the results. I have also made a Python script with the algorithm so that people can process their recordings easily. Instructions are included in this post.

First results of DSLWP-B Amateur VLBI

In March this year I spoke about the Amateur VLBI with LilacSat-2 experiment. This experiment consisted of a GPS-synchronized recording of LilacSat-2 at groundstations in Harbin and Chongqing, China, which are 2500km apart. The experiment was a preparation for the Amateur VLBI project with the DSLWP lunar orbiting satellites, and I contributed with some signal processing techniques for VLBI.

As you may know, the DSLWP-B satellite is now orbiting the Moon since May 25 and the first Amateur VLBI session was performed last Sunday. The groundstations at Shahe in Beijing, China, and Dwingeloo in the Netherlands performed a GPS-synchronized recording of the 70cm signals from DSLWP-B from 04:20 to 5:40 UTC on 2018-06-10. I have adapted my VLBI correlation algorithms and processed these recordings. Here are my first results.

DSLWP-B’s journey to the Moon: part III

This is a follow-up on the series about DSLWP-B’s orbital dynamics (see part I and part II). In part I we looked at the tracking files published by Wei Mingchuan BG2BHC, which list the position and velocity of the satellite in ECEF coordinates, and presented basic orbit propagation with GMAT. In part II we explored GMAT’s capabilities to plan and perform manoeuvres, making a tentative simulation of DSLWP-B’s mid-course correction and lunar orbit injection. Now we turn to the study of DSLWP-B’s elliptical lunar orbit.

In this post we will examine the Keplerian elements of the orbits described by each of the tracking files published so far. We will also use Scott Tilley VE7TIL’s Doppler measurements of the S-band beacon of DSLWP-B to validate and determine the orbit.

DSLWP-B’s journey to the Moon: part II

This forms parts of a series of posts showing how to use GMAT to track the DSLWP-B Chinese lunar satellite. In part I we looked at how to examine and validate the tracking files published by BG2BHC using GMAT. It is an easy exercise to use GMAT to perform orbit propagation and produce new tracking files. However, note that the available tracking files come from orbit planning and simulation, not from actual measurements. It seems that the elliptical lunar orbit achieved by DSWLP-B is at least slightly different from the published data. We are already working on using Doppler measurements to perform orbit determination (stay tuned for more information).

Recall that there are three published tracking files that can be taken as a rough guideline of DSLWP-B’s actual trajectory. Each file covers 48 hours. The first file starts just after trans-lunar injection, and the second and third files already show the lunar orbit. Therefore, there is a gap in the story: how DSLWP-B reached the Moon.

There are at least two manoeuvres (or burns) needed to get from trans-lunar injection into lunar orbit. The first is a mid-course correction, whose goal is to correct slightly the path of the spacecraft to make it reach the desired point for lunar orbit injection, which is usually the lunar orbit periapsis (the periapsis is the lowest part of the elliptical orbit). The second is the lunar orbit injection, a braking manoeuvre to get the spacecraft into the desired lunar orbit and adjust the orbit apoapsis (the highest part of the orbit). Without a lunar orbit injection, the satellite simply swings by the Moon and doesn’t enter lunar orbit.

In this post we will see how to use GMAT to calculate and simulate these two burns, so as to obtain a full trajectory that is consistent with the published tracking files. The final trajectory can be seen in the figure below.