Guidance, Navigation, and Control
Section 4.3.1.
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Star Tracker Orientation

The star trackers don't really have to be at 90-degree angles to each other. Mathematically, any set of angles will do as long as none of them are parallel. But to place 3 of them at mutual right angles, imagine a pyramid formed by 3 vectors coming together at right angles. Then balance it on its point. Then the three telescopes could be looking out the same side of the Lunar Transfer Vehicle service module. None of the star trackers have to point precisely along the major axes of the vehicle. If need be, we can rotate the vehicle to several different attitudes to update the inertial platform, picking up stars hither and yon around the sky. Or even have a single star tracker with a wide enough field of view to see at least three guide stars.

             ==== === === ====  Vehicle skin, with holes for star trackers
                  \  |  /
     Star Tracker  \ | /    Image this is a right-angle, 3-sided pyramid;
     Cones          \|/     a regular tetrahedron

It's hard to do a 3-D drawing in text graphics, but that's sort of what it would look like. Incidentally, the star trackers need not all be in the same location in the service module, and probably should not be.

Guidance, Navigation, and Control

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