As promised in this post, I will now speak about how to demodulate the Tianwen-1 telemetry signal. This post will deal with demodulation and FEC decoding. The structure of the frames will be explained in the next post. In this post I also give a fully working GNU Radio decoder that can store frames in the format used by the orbit state vector extraction Python script.
In my previous post I explained how our problems using the orbit state vectors transmitted in the telemetry of Tianwen-1 were caused by an incorrect interpretation of the timestamps attached to these vectors. The timestamp is a 32 bit counter with a 100us resolution, but the difficulty is that the epoch of this counter is not known. It seems that the epoch is around 2020-07-23 00:00 UTC, which is the day of launch, but not quite because approximately 57 minutes need to be subtracted from the epoch.
In my post I used some sort of experimental procedure to determine the correction that needs to be subtracted from the timestamp, and I obtained 3400.2 seconds, which I believe should be accurate to within a few seconds.
However, I found this correction somewhat unsatisfactory, as I wasn’t able to explain where it comes from. Now I think I have found a reasonable explanation.
Yesterday I published a post explaining how Tianwen-1 is transmitting real time state vectors for its own orbit in its telemetry and how we’ve used those to propagate its orbit and track the spacecraft with the Bochum observatory 20m dish. However, there seemed to be some problem in the way we were interpreting the state vectors, since the ephemerides derived from these had a pointing error of a few degrees when compared with observations from Bochum and other smaller Amateur stations.
As of writing that post, I believe I have found the problem. It has to do with the way that the timestamps from the state vectors are interpreted. After correcting this problem I am getting an orbit that matches the observations well. Here I explain this problem and show some more details about the corrected ephemerides.
Last Thursday 2020-07-23 at 04:41 UTC, Tianwen-1, a Chinese mission to Mars consisting of an orbiter, a lander and a rover, launched from Wenchang. Usually, I would be posting an analysis of a recording of the telemetry signal, made by Paul Marsh M0EYT or another of my Amateur DSN contributors, as I did a few days ago for the Emirates Mars Mission. However, something amazing has happened that has kept me quite busy. Rest assured that the analysis of the signal will come in a future post, but here I’m going to tell a story about Tianwen-1’s orbit.