As I have mentioned in several posts, the trajectory in which Tianwen-1 was launched had a closest approach to Mars of around 3 million km, so at some point a large correction manoeuvre was needed. We had already seen small manoeuvres TCM-1 and TCM-2, but we were expecting soon enough this large correction, which is called DSM (deep space manoeuvre). Some sources placed the DSM around the beginning of October, while others were more precise and mentioned that it would happen right after the Mid-Autumn festival holiday season, which this year ended on October 8.
This information turned out to be pretty accurate, since yesterday we heard news that Tianwen-1 had done its DSM at 15:00 UTC on 2020-10-09. This information quickly appeared in the news.
After the Bochum 20m antenna was able to acquire the spacecraft’s signal and gather telemetry at 19:09 UTC, we were able to confirm the manoeuvre in the state vectors transmitted by the spacecraft. Paul Marsh M0EYT was also able to receive telemetry confirming this, starting at 18:18 UTC. This post is an analysis of the data.
On 2020-09-04, China launched a “reusable experimental spacecraft” of which very little is publicly known. The most popular hypothesis is that this is a robotic spaceplane similar to the X-37B. The spacecraft spent two days in orbit and landed back at Earth, most likely near Lop Nur nuclear test site. Marco Langbroek has a nice post detailing all we know about the mission.
During the time it spent in orbit, the spacecraft released an object which has been catalogued as 2020-063G and is commonly known as “Object A”. On 2020-09-14, Dmitry Pashkov R4UABdetected an S-band signal coming from Object A at around 2280 MHz. This was verified later by Scott Tilley VE7TIL, who received a strong signal with lots of fading, suggesting that the object is tumbling. Marco also did some optical observations of the object.
Scott has sent me a recording of the S-band signal that he did on 2020-09-15 so that I can analyse it and we can learn more about this mysterious object. This post shows the results of my analysis.
Today, the Chinese National Space Agency and the Chinese Lunar Exploration Program have released some “selfies” taken by Tianwen-1 to celebrate China’s National Day. Apparently, the images have appeared originally in this press release (Google translation), and many media in English haveechoed the news.
The best image can be seen below. This kind of image might show us something about Tianwen-1’s attitude, or maybe not? Let’s try to find out.
A few days ago, Paul Marsh M0EYT told me that he had seen something interesting in his recording of Tianwen-1‘s signal during the previous night. Paul usually does some nice videos of the signal waterfall by doing screen captures of Spectrum Lab. These clearly show events such as frequency jumps due to groundstation locking, and any high-speed data transmissions. You can see some of these in his Twitter feed.
The waterfall that Paul talked me about can be seen in this video. It corresponds to the night between September 20 and 21. Note that the data sidebands show some horizontal stripes. For a while they disappear altogether, and at other times their density changes. This is what caught Paul’s eye.
Tianwen-1 waterfall between 2020-09-20 and 2020-09-21 by Paul Marsh M0EYT
Paul was convinced that when the stripes disappeared, more data was being transmitted in the signal. I was a bit sceptical about it, but clearly there was something changing in the signal. I thought about the “dancing” related to the subcarrier and data clocks sliding. However, after analysing the signal, I saw that Paul was completely right, as I will explain here. After knowing exactly what happens, I’m still quite amazed that this shows up in the Spectrum Lab waterfall.
This is a post I had announced since I first described Tianwen-1’s modulation. Since we have very high SNR recordings of the Tianwen-1 low rate rate telemetry signal made with the 20m dish in Bochum observatory, it is interesting to make detailed measurements of the modulation parameters. In fact, there is something curious about the way the modulation is implemented in the spacecraft’s transmitter. This analysis will show it clearly, but I will reserve the details for later in the post.
Here I will be using a recording that already appeared in a previous post. It was made on 2020-07-26 07:47:20 UTC in Bochum shortly after the switch to the high gain antenna, so the SNR is fantastic. The recording was done at 2.5Msps, and the spectrum can be seen below. The asymmetry (especially around +1MHz) might be due to the receive chain.
The signal is residual carrier phase modulation, with 16348 baud BPSK data on a 65536Hz square wave subcarrier. There is also a 500kHz ranging tone.
Mars 2020, NASA’s latest mission to Mars, was launched a couple weeks ago. However, with all the Tianwen-1 work down the pipeline, until now I haven’t had time to dedicate an appropriate post to this mission (though I showed some sneak peek on Twitter). This mission consists of a rover and helicopter (a real novelty in space exploration). Both were launched with the cruise stage and the entry, descent and landing system on July 30 from Cape Canaveral, an are currently on their transfer orbit to Mars, as Tianwen-1 and Emirates Mars Mission.
In this post I will be working with some recordings made by AMSAT-DL using the 20m radio telescope at Bochum’s observatory. These feature the low rate safe mode telemetry, which was very strong and caused some anecdotes as it saturated some NASA DSN receivers, and the nominal 10kbps telemetry signal that was switched on later. Here I will describe the modulation and coding, giving GNU Radio decoders, and also take a look at the data. r00t.cz has also written a post where he shows similar information.
In this post I analyse part of the telemetry transmitted by Tianwen-1 over high-speed data done on 2020-08-03. The decoded frames were kindly provided by Paul Marsh M0EYT. r00t.cz has made an analysis of this data in his Tianwen-1 page, which also includes some more information not shown here. This telemetry contains a full playback of some channels spanning 2020-07-25 to 2020-08-02, including gyroscope data, as described in this post.
In a previous post I talked about how the high data rate signal of Tianwen-1 can be used to replay recorded telemetry. I did an analysis of the telemetry transmitted over the high speed data signal on 2020-07-30 and showed how to interpret the ADCS data, but left the detailed description of the modulation and coding for a future post.
Here I will talk about the modulation and coding, and how the signal switches from the ordinary low rate telemetry to the high speed signal. I also give GNU Radio decoder flowgraphs, tianwen1_hsd.grc, which works with the 8192 bit frames, and tianwen1_hsd_shortframes.grc, which works with the 2048 bit short frames.
Yesterday I reported about Tianwen-1’s first trajectory correction manoeuvre, TCM-1. In that post I commented the possibility that the updated state vectors that we saw on the telemetry after TCM-1 might come from a prediction or planning rather than take into account the actual performance of the burn.
The figure below shows the error between the state vectors collected after TCM-1 over the last two days, and a trajectory propagated in GMAT, using the following state vector, which is one of the first received after TCM-1.
We see that on the UTC night between August 1 and 2 the state vectors deviate very little from the GMAT trajectory. However, on the UTC night between August 2 and 3 we see a slightly different trajectory in the state vectors. We have no data in between, as the spacecraft is not visible in Europe, so we don’t know the precise moment of change. The gap in telemetry around 2020-08-03 00:45 UTC is due to a transmission of high-speed data.
It seems reasonable to think that after TCM-1 the Chinese DSN performed precise ranging of the spacecraft to determine the new orbit accurately and then uploaded a correction to the state vectors on-board Tianwen-1.
The state vectors from last night all describe the same trajectory, as shown in the plot below which uses
to propagate a trajectory in GMAT. There is a small jump of a few hundred meters at some point. We usually see one or two these jumps per day, but we don’t understand well why they happen.
The trajectory according to the state vectors from 00:17:06 and from 21:03:08 are very similar. For example, at the closest approach to Mars they only differ in 1197km. For comparison, the difference between the new trajectory and the pre–TCM-1 trajectory is 126529km (again, at the closest approach to Mars).
Since Tianwen-1 transmits its own real time orbit state vectors in the telemetry, by comparing the vectors transmitted before and after TCM-1, and also by studying the Doppler observed by groundstations on Earth, we can learn more about the manoeuvre.
