[X] My Assistant

[Sandro]

Hi together

During the hot phase of the EDL-sequence with thrusters active, phoenix made automatically many different attitude controls. Among them was the orientation of phoenix's arm to the north. I wondered how phoenix found out his actual orientation and where north was. Since Mars does not have a strong dipol magnetic field like earth, I assume there must have been some other way to figure out the current orientation?!

I thought of maybe using the sun? But this would have been not easy at all!

Does anyone know how this was done?

Many thanks in advance!

Regards,
Sandro

[dvandorn]

Yep -- this is what's called a stable matrix. You can change the stable matrix out from under an inertial system without re-aligning the platform(s) and expect accuracy to remain within less than a tenth of a degree.

Remember, changing the matrix is something that is done in software, not in hardware. The platform(s) remain inertial, with the same angles relative to the spacecraft body, before and after a change to the stable matrix. The only difference is how the "body angles" are intepreted relative to the stable matrix. I would expect greater deviations based on "gyro drift" than due to changing the stable matrix.

In Phoenix's case, they had two platforms, so the gyro drift should have been minimal (and easily predictable) after the final star alignment. Regardless of the amount of movement by the spacecraft. Heck, the deceleration of atmospheric braking probably introduced more drift to the system than motions on 'chute. That's why the event triggers were based on deceleration rates and atmospheric pressure, and why final maneuvers were based primarily on radar data. The landing site stable matrix was likely a relatively crude approximation of the actual landing site/spacecraft geometry, especially in terms of exact location of the ground relative to the spacecraft.

-the other Doug