tianwen – Daniel Estévez

2024’s update of Tianwen-1’s remote sensing orbit

Last year I wrote a post on July 23, which is the anniversary of Tiawen-1‘s launch. The post was essentially an updated plot of the orbital parameters of Tianwen-1’s remote sensing orbit. As I explained in that post, AMSAT-DL is using the 20 meter antenna in Bochum observatory to receive telemetry from Tianwen-1 almost every day (this can be followed in the YouTube livestream). Since Tianwen-1 includes its state vector (position and velocity with respect to Mars) in its telemetry, this allows us to monitor its orbit, which is of interest because no other public detailed information is available.

This year I completely forgot to do the same again for July 23, but I have remembered now. Here is the updated plot of the orbital parameters of Tianwen-1 since 8 November 2021, when the remote sensing orbit began. The plot includes data until 2 August 2024. During most of August, AMSAT-DL is not tracking Tianwen-1, since Mars has a very similar right ascension to STEREO-A, and tracking STEREO-A has priority. Tracking of Tianwen-1 will resume as the two objects drift apart in right ascension.

All the changes in the orbital parameters are due to perturbations by the Sun’s gravity and the oblateness of Mars, since as far as I know there have been no manoeuvres in this orbit. The main change in orbital parameters is a steady change in the latitude of the periapsis. The orbit is designed on purpose to exploit this effect. Over time, all the surface of Mars can be observed from a low altitude. This perturbation is related to a change in eccentricity, which is minimal when the periapsis is over the north pole and maximal when the periapsis is over the south pole.

Now it is quite apparent that there is also a slow but steady increase in inclination. This was not so evident last year, due to a sinusoidal perturbation that is also present, but now it is clear that the inclination has increased by about 0.05 deg since November 2021. It seems that this increase in inclination is related to a small increase in the semi-major axis.

The code for the updated plot can be found in this Jupyter notebook.

Tianwen-1 third anniversary

Today is the third anniversary of Tianwen-1, which was launched in 23 July 2020. During the first year of the mission I was tracking it with great detail and writing a lot of posts. A fundamental part of this work was the help of AMSAT-DL. Using its 20 metre antenna in Bochum, they tracked the spacecraft, decoded telemetry and provided live coverage of many of the key mission events in their Youtube livestream. At some point the reception of Tianwen-1’s telemetry at Bochum got completely automated, as we described in a paper in the GNU Radio Conference 2021 proceedings.

To this date, the reception continues in Bochum almost every day, when the dish is not busy with other tasks such as tracking STEREO-A. This means that we get good coverage of the spacecraft orbital data, via the state vectors transmitted in its telemetry.

To celebrate the anniversary, I have updated my plot of the orbital parameters, which I made back in March 2022. The plot covers all the remote sensing orbit, from when it started on 8 November 2021, until the present day. To my knowledge, no manoeuvres have happened in this time (perhaps small station-keeping burns would be unnoticed without more careful analysis), so the changes in the orbit are just due to perturbations by forces such as the Sun’s gravity and the oblateness of Mars.

The updated plot can be seen below. We see that the periapsis latitude changes at a steady rate of 0.598 deg/day. The remote sensing orbit was designed so that the periapsis precessed in this way, which allows the spacecraft to cover all the surface of Mars from a low altitude in 301 days. The surface has now been covered twice, since the periapsis has moved from near the north pole to the south pole and back again to the north pole.

There is also an interesting change in eccentricity, which seems to be correlated with the latitude of the periapsis. The eccentricity is largest when the periapsis is over the south pole. In this case, the altitude of the periapsis decreases by 60 km, compared to when the periapsis is over the north pole. The inclination has remained mostly steady, although there seems to be a small perturbation with an amplitude of 0.1 deg.

The updated Jupyter notebook in which this plot was made can be found here.

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 communications relay orbit

As you may have seen in my last post, lately I have been reviewing some of data we have from Tianwen-1. In the days following the landing of Zhurong, back in May, we had so much data in our hands that I couldn’t post about it in a timely manner. We were wondering if we could use this data to plan for a number of experiments with the 20 meter antenna at Bochum observatory. These included trying to receive data from the rover relayed by the orbiter, and trying to detect the rover’s direct X-band link to Earth. We didn’t manage to do any of these, unfortunately, as they had a great deal of luck involved.

During the summer I’ve been involved in several activities such as collaborating with the SETI Institute and BSRC REU summer student programmes by teaching some GNU Radio lessons, and preparing material for GRCon21 (a talk, a workshop and paper). Now I have more time at hand, so it’s good to revisit this data. In this post I’ll look at Tianwen-1’s orbit after the release of the lander.

Tianwen-1 attitude in Mars orbit

It has been a while since the last time I wrote an update about Tianwen-1’s attitude. In that post I showed that Tianwen-1 had changed to a sun angle of 0 degrees with respect to the spacecraft’s X axis on 2020-10-22. Since then, nothing changed, even after the spacecraft arrived to Mars orbit on February 2021, so I have been sloppy and ignored the ADCS telemetry for quite a while.

As you may have heard, the 20 metre antenna at Bochum observatory had a failure at the beginning of June due to a thunderstorm. It has been under repair until the end of August, when it became operational again. As AMSAT-DL has now returned to receive telemetry from Tianwen-1 at Bochum, I am now reviewing the new data to see if anything interesting might have happened in the last months.

I have updated my attitude study Jupyter notebook, since the previous version assumed that the spacecraft’s state vectors were heliocentric coordinates, so it didn’t work in Mars orbit, due to the change to Mars-centric coordinates.

When plotting the new data I have learned that Tianwen-1 changed attitude during the release of the lander on May 14, and has maintained this new attitude until the present day.

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.