# Does Tianwen-1 have a near-repeat ground track orbit in order to scan the same general area on each of its low altitude passes?

Though it's not as easy1 to get details on Chinese missions in English, it seems that Tianwen-1 is in a 12,000 x 256 km highly eccentric orbit around Mars, with its hi res cameras able to resolve 0.5 meters during "periMars"2. If so that makes the semimajor axis $$(256 + 3396 + 12,000 + 3396)/2 = 9524$$ km and with $$GM=4.283 \times 10^{4} \ km^3/s^2$$ that makes the period 7.8 hours, or not quite Mars' sidereal day of 24.62 hours divided by three.

Question: Does it turn out that the period is a little longer than the number above and the orbit is much closer to repeat ground track3 over the proposed landing site than this envelope-back estimate suggests?

Or are the weeks or months of orbit before landing enough to scan the area over time by "filling in the blanks" over successive passes?

• I presume this higher period, will make ground track regress east to west. So one will have 1 more try over some 20 days I guess over the landing zone Feb 28, 2021 at 7:01
• @Prakhar I see what you mean, good point! it's an unofficial estimate only, an answer will need to find the actual orbit before it can speculate on the exact strategy strategy. The problem is very sensitive to the period; If it's 7.5 or 8 hours instead of 7.8 that "20 days" changes by lot!
– uhoh
Feb 28, 2021 at 7:18

## 1 Answer

Plugging in a periapsis altitude of of 265km, apoapsis of 12,000km, inclination of 86.9 degrees, and right ascension of -10 degrees (so the groundtracks would pass over Utopia Planitia) yields these groundtracks for the satellite with respect to the IAU_MARS body fixed reference frame:

The parts of the groundtrack where the dots are more spread out signify larger relative velocity with the surface, which are the close approaches, and where the points are closer together are the apoapsis areas. This plot is for 5 periods. Judging from this it looks like they can align the orbit such that periapsis points will pass over the landing site, but it wouldn't be repeating everyday with these orbital elements. Also with these orbital elements, they can get near global coverage of Mars. Although from what I saw on wikipedia, looks like they have multiple orbits for different phases of the mission.

• Excellent first post, welcome to Stack Exchange! If we can track down more accurate information on the orbital parameters at some point in the future you can update this analysis accordingly.
– uhoh
Mar 2, 2021 at 3:37
• Is there any chance of an update to this? Can "Is it possible to arrive at a yes or no answer to "Tianwen-1 have a near-repeat ground track orbit in order to scan the same general area on each of its low altitude passes?" Thanks!
– uhoh
Jul 18, 2021 at 0:04