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The rendering of InSight shows the RISE antennas, the two microwave horns pointed just above the horizontal over each of the solar array paddles.

If I understand correctly the experiment works when signals from these horns are received on Earth when Mars' rotation puts InSight on the edge of the planet as seen from Earth, maximizing the Doppler shift and its sensitivity to changes in planet rotation.

According to this answer and the linked source there:

...it's job is to stay on the lander deck and trade X-band radio signals back and forth with Earth for an hour or so each day.

That suggests there is some significant directionality of the antennas, perhaps 15 degrees, which is consistent with the apparent aperture shown in the image considering the X-band wavelength will be about 3.5 cm (I'm guessing about 8.6 GHz).

This works if the antennas point roughly East/West (really, in the plane of the ecliptic) and would probably be hopeless if they ended up pointing North/South.

From this short conference paper:

For RISE, Doppler measurements will be made at times when the Earth is at low elevation, when the Doppler signature due to the rotation of Mars is largest. Two fixed medium-gain antennas, one pointed to the east and one pointed to the west, will be used to provide adequate gain for RISE.


Question: How will InSight's RISE antennas end up pointed in the right direction?


Related to the RISE instrument and its operation:

enter image description here

"The Mars InSight lander as depicted in an illustration with its instruments deployed on the surface of Mars. Image: NASA/JPL-Caltech" Cropped from Source

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  • $\begingroup$ What if they are on motorised beds?! So that it can be rotated to align towards earth. Or while landing, could make use of magnetometer to align to east/west direction! Need to lookup $\endgroup$ – karthikeyan Nov 23 '18 at 21:04
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    $\begingroup$ @karthikeyan I believe that Mars has no useful, planet-wide magnetic field. There are locally magnetized areas due to ferrous rock formations, but there's no spinning core. This is believed to be part of the reason why Mars' current atmosphere is so thin. $\endgroup$ – uhoh Nov 24 '18 at 0:17
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    $\begingroup$ oh yeah! I didn't consider that. I was taking a guess. But the navigation system must be having star sensory/sun sensor which might help in determining the orientation! $\endgroup$ – karthikeyan Nov 24 '18 at 2:10
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The landing systems seem to be the key.

In this paper 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), it refers to using the Insight's landing system.

The InSight landing system will control its azimuth during landing so the instruments will be deployed to the south.

For clarity on what "instruments" are, they are the HP3 and SEIS, as displayed in the picture Top view of InSight lander after landing

Image source

I guess the landing site inclination has also be taken care by design.


From Spaceflight 101:

The 12 Terminal Descent Thrusters are installed around the bottom panel of the deck and are tasked with delivering InSight from the point of separation from the backshell and parachute to a gentle landing on the Martian surface at a speed of less than 3 meters per second.

They are operated in pulse mode to arrest the craft’s horizontal and vertical velocity and also keep it in the proper orientation for landing, achieved through differential pulsing of the thrusters to actively control the pitch, yaw and roll of the lander.

Chosen for the Terminal Descent Thruster was the Aerojet Rocketdyne MR-107N, capable of providing a nominal thrust of 170 Newtons with a range of 109 to 296 Newtons. With the 12 engines at nominal throttle, InSight has a total thrust of 3,516 Newtons (capable of decelerating the craft at 2.65 Gs).

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