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note: possibly helpful:

@karthikeyan's excellent answer links to The Rotation and Interior Structure Experiment on the InSight Mission to Mars (Folkner, W.M., Dehant, V., Le Maistre, S. et al. Space Sci Rev (2018) 214: 100. https://doi.org/10.1007/s11214-018-0530-5), and includes this image from there.

The antennas need to point towards Earth when InSight is near the edge of Mars' disk as seen from Earth so that the Doppler shift of the returned signal will be sensitive to small changes in Mars' rotation. The antennas are back-to-back so that the signals will be received and rebroadcast coherently both when moving towards Earth and away from Earth, corresponding to the East-pointing and West-pointing antenna, respectively.

This will need to be done over a period of at least two years in order to get a sufficient amount of data to detect the very tiny changes in Mars' rotation due to things like the polar caps evaporating and re-forming for example.

Considering all of this, I can't understand why it looks like the two horn antennas have been rotated slightly clockwise as viewed from above, slightly south of east, and north of west. Why exactly?

enter image description here

uhoh
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  • Somewhat related During its final descent, how will the InSight lander know cardinal directions in order to land with proper orientation? – uhoh Nov 24 '18 at 04:40
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    Exact details from the paper will be as follows :"One antenna points 15.5◦ south of due east and the other point 6◦ north of due west. These angles were designed to provide good viewing geometry for RISE while also allowing low-rate telemetry from Earth to the lander throughout the mission". Still the why question remains unanswered – karthikeyan Nov 24 '18 at 06:03
  • @karthikeyan wow, the plot thickens. That's quite interesting. – uhoh Nov 24 '18 at 06:15
  • I am guessing that this asymmetric arrangement might be helpful to tolerate a larger misaligned landing. Considering a symmetrically arranged antennas, a misaligned landing will render both useless. Having an asymmetry might help in asymmetric landing with at least one antenna pointing to earth – karthikeyan Nov 24 '18 at 09:44
  • @karthikeyan If you have Doppler data from both sides of Mars, it tends to cancel the uncertainties in the orbital velocity of Mars and Earth which is much larger (~50 to 100x) than the rotational velocity. Without both-side data, you have an enormous problem with subtraction. I don't think RISE can yield the kind of data needed without both sides working well. – uhoh Nov 24 '18 at 09:56
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    There's also the 25° tilt of Mars axis that changes the pointing direction of antennas throughout the year. The opening angle of the antennas might not be as large as the required 50°. – asdfex Nov 24 '18 at 14:12
  • @asdfex good point. This is a real puzzle. I wish I could read the paper karthikeyan has mentioned, perhaps one of the references cited where those angles are mentioned could be tracked down. – uhoh Nov 24 '18 at 14:17
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    @uhoh I just checked the paper. There are no references to the angle numbers! – karthikeyan Nov 24 '18 at 18:20
  • @karthikeyan oh! then perhaps they are movable. That would address what asdfex mentions; that the range of required angles over a Martian year is probably much larger than the antenna's beam width. – uhoh Nov 24 '18 at 23:49
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    Because of the difference in the orbital plane inclinations, Earth can appear up to about 4 degrees south or north of the Sun on Mars. But that would be a uniform offset on both sides, and changes during the year. – Bob Jacobsen Jan 31 '20 at 23:49

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The two antennas need to point at Earth during morning and evening on Mars. They are oriented in opposite directions so that one can be used in the morning and one in the evening. The science goal requires continuous contact over several years, so all the relative orientation changes of the antennas due to the movement of Mars need to be taken into account.

Mars has an axial tilt of 25°, so that the orientation of the two antennas varies by up to 50° throughout a year. In addition to that Mars' orbit is inclined by 2°. In the extreme case (Mars is far from the ecliptic and distance to Earth is minimal) this results in a change of 6° in the direction to Earth which is 3 times closer to Mars than the Sun.

Adding these up results in a needed coverage of the antennas of 62°. (Note that this does not take into account whether the extremes are actually reached during the planned duration of the mission, but is the theoretical maximum)

Both antennas have an opening angle of 50° which is less than the required 62°, not taking into account a small additional contingency for a possible misalignment of the lander. Tilting them by 15.5° and -6°, respectively, guarantees that at least one of the antennas can be used for communication once each Sol. On the other hand, precision of measurements can be increased if both antennas can be used during the same day, e.g. to cancel out effects of orbital speed. This requires the tilt to be as small as possible.

The precise tilting angles are likely given by some constraints of the vehicle construction. Unfortunately I wasn't able to find more detailed construction considerations of Insight.

asdfex
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  • Okay I was able to muster enough 3D-fu to briefly picture all of this at once and I'm convinced. – uhoh Mar 30 '20 at 02:07