An update about Tianwen-1’s remote sensing orbit

Tianwen-1, the Chinese Mars orbiter, entered its remote sensing orbit on November 8 2021. In a previous post, I gave an overview of the orbit using one month of state vector data collected from the spacecraft’s telemetry by AMSAT-DL using the 20 m antenna at Bochum observatory. AMSAT-DL has continued receiving telemetry almost every day, so in this post we can now look at nearly 4 months of data for the remote sensing orbit.

This orbit is a polar elliptical orbit with 86 deg inclination, a periapsis altitude of 275 km and an apoapsis radius of 14140 km. The orbital period is approximately 2/7 Mars sidereal days plus 170 seconds. This makes the ground track drift slowly towards the west, allowing the spacecraft to scan all the planet’s surface. Additionally, due to orbit perturbations, the argument of periapsis (and hence its latitude) keeps slowly changing with time. This makes possible to scan all of Mars from a low altitude.

One month of Tianwen-1 remote sensing orbit

In a previous post, I described the remote sensing orbit into which Tianwen-1 had moved on November 8. Now it has been in this orbit for more than one month, and AMSAT-DL has been collecting telemetry almost daily with the 20 metre antenna at Bochum obseratory. Therefore, it is a good moment to review the state vector data and look at how the orbit has evolved with time.

Tianwen-1 remote sensing orbit

On November 8, the Tianwen-1 orbiter made a manoeuvre to move itself to the remote sensing orbit, as reported by Chinese media. This orbit is the final orbit in the mission, as depicted in this figure from Wikipedia. The main goal of this orbit is to study the geophysical properties of Mars with all the orbiter instruments (see this paper) and to continue acting as a communications relay for the rover Zhurong.

As usual, AMSAT-DL has been collecting telemetry from Tianwen-1 with the 20 metre antenna at Bochum observatory, including spacecraft state vectors. This allows us to study the orbit change manoeuvre and the properties of the remote sensing orbit. This post is a first look at the state vector data.

Tianwen-1 landing

Yesterday, May 14th, at around 23:18 UTC the Tianwen-1 rover Zhurong safely landed on the Utopia Planitia region of Mars. To follow this event, AMSAT-DL made a 7 hour livestream of the orbiter signals as received by the 20m antenna in Bochum observatory. In this livestream we could see the signal losses caused by the manoeuvres of the deorbit burn and collision avoidance burn. Analysis of the telemetry decoded at Bochum shows more details about these manoeuvres. This post is a detailed report of the landing.

Tianwen-1 reconnaissance orbit

In my last post about Tianwen-1, I explained how on 2021-02-23 the spacecraft would enter an orbit with a period of 2 Mars sidereal days. This would give a repeating ground track with periapsis passages over the intended landing site in Utopia Planitia. Almost one month has passed since then and AMSAT-DL has continued to receive telemetry state vectors every day with the Bochum 20 meter antenna. This data allows us to study the orbit in detail, including orbit perturbations and any station-keeping manoeuvres that are done to maintain the orbit. This post is my first analysis of the current orbit.

Tianwen-1 phasing orbit

Last Saturday 2021-02-20 at 11:46:42 UTC Tianwen-1 passed the periapsis of its elliptical polar orbit at Mars and made a retrograde burn to reduce its apoapsis radius. The trajectory planning of the spacecraft can be seen in its Wikipedia page: the spacecraft first arrived into a low inclination elliptical orbit by making a Mars orbit insertion at periapsis, then coasted to apoapsis, where it performed a plane change, and then it arrived at periapsis, performing the manoeuvre described in this post.

Over the next few days the spacecraft should move into a reconnaissance orbit, which is given in Wikipedia to be a 265 x 60000 km orbit (having a period of 2 days) with an inclination of 86.9 degrees. However, the last burn hasn’t lowered the apoapsis that much. The current orbit is approximately 280 x 84600 km (3.45 day period) with an inclination of 87.7 degrees. A possible reason for using the current orbit, which has been described as a phasing orbit, will be explained in this post after reviewing the data we have about the burn.

Tianwen-1 plane change manoeuvre

Today at 9:00 UTC Tianwen-1 made its plane change manoeuvre, as reported by Xinhua. Yesterday I showed my planning for this manoeuvre. Shortly after the spacecraft returned to the high gain antenna after the manoeuvre, the Bochum 20m antenna operated by AMSAT-DL received state vectors with the new trajectory. These state vectors allow us to calculate the timestamp of the burn and the delta-V vector, as I have done in other occasions. It is convenient to remark that the state vectors that we are seeing right now are probably a prediction. In the next few days we will see updates in the trajectory as the Chinese DSN measures the effects of the actual burn and updates the onboard ephemerides.

Tianwen-1 plane change planning

Today, the Chinese media published a short piece of news stating that tomorrow, 2021-02-15, Tiawen-1 will make make a plane change to a polar orbit. The post is accompanied by an short video, which includes an animation depicting the manoeuvre. A screenshot of the video is shown below. As the spacecraft arrives to apoapsis, it effects a plane change into an ascending polar orbit.

Tianwen-1 plane change manoeuvre. Source 人民日报 in weibo.com

This is a good moment to review the maths behind a plane change manoeuvre and compute what the manoeuvre will look like.

Emirates Mars Mission MOI burn observed in Bochum

A few days ago, Emirates Mars Mission (Hope), and Tianwen-1 performed their Mars orbit injection burn (MOI). AMSAT-DL made a livestream for each of the two events, showing the X-band signals of the spacecraft as received with the 20m antenna at Bochum.

In the case of Tianwen-1 the signal was pretty strong even while the spacecraft was on the low gain antenna, and we could clearly see the change in Doppler rate as the thrusters fired up. However, in the case of Emirates Mars Mission the signal disappeared as soon as the spacecraft switched to the low gain antenna. In fact DSN Now reported a received power of -155 dBm with the 34m DSS55. That was a large drop from the -118 dBm that it was reporting with the high gain antenna. Therefore, nothing could be seen in the livestream waterfall until the spacecraft returned to the high gain antenna, well after the manoeuvre was finished.

Nevertheless, a weak trace of the carrier was still present in the livestream audio, and it could be seen by appropriate FFT processing, for example with inspectrum. I put up a couple of tweets showing this, but at the moment I wasn’t completely sure if what I was seeing was the spacecraft’s signal or some interference. After the livestream ended, I’ve been able to analyse the audio more carefully and realize that not only this weak signal was in fact the Hope probe, but that the start of the burn was recorded in perfect conditions.

In this post I’ll show how to process the livestream audio to clearly show the change in drift rate at the start of the burn and measure the acceleration of the spacecraft.

Tianwen-1 Mars-centric state vectors

Since launch, Tianwen-1 has transmitted as part of its telemetry some state vector data, giving its position and velocity vector every 32 seconds. This has allowed us to propagate, track and study its trajectory. We noticed the presence of the state vector data a few hours after launch, and since then we have received and decoded this data using the 20m antenna at Bochum observatory, which is operated by AMSAT-DL. This has allowed us to supply accurate orbit information to JPL HORIZONS, so that Amateur observers (and also some professional ones, for which Tianwen-1 is a useful and strong X-band beacon) can easily get ephemerides for the spacecraft.

Until now, the state vector data has encoded the spacecraft’s Cartesian position (in km) and velocity (in km/s) in a heliocentric reference frame. It is not completely clear if the frame is supposed to be ICRF or MJ2000, since the difference between the two is very small (see Section 3.5 in this paper by Kaplan) to be able to distinguish them with the data at hand, but we have always been using ICRF so far for consistency.

Today we have noticed that starting at some point on 2021-02-08, Tianwen-1 is now transmitting state vectors using a different, Mars-centric frame of reference. We don’t have the exact moment of the change. The last heliocentric vector we received was

2021-02-07 23:23:03.744100 18791639.655712113 211029173.8782428 96492674.05965108 -21.108400067542537 4.768376820024702 1.8445381918644286

This vector was received with one of the antennas at Allen Telescope Array, which I used as a backup since Bochum was unable to track that day due to a big snowfall.

The first Mars-centric state vector was received by Bochum the next day, and is

2021-02-08 22:14:25.049300 -345203.0840200648 103420.7793506239 -15761.456419116437 2.409386271990221 -0.7794198288828312 0.12118319008153547

The change in the frame of reference is clear from the change in magnitude of the position vector. Ensuring that the Mars-centric state vectors are interpreted correctly is important to continue using the data accurately. In this post I give the assessment of the appropriate reference system to use.