Midcourse navigation sightingc

Figure 5.3

Use of landmark line and star line to take navigational fixes or alignment sightings with Apollo guidance optical system. (Redrawn by the author from Draper et al., ''Spacecraft Navigation Guidance and Control,'' 2-74.)

Figure 5.3

Use of landmark line and star line to take navigational fixes or alignment sightings with Apollo guidance optical system. (Redrawn by the author from Draper et al., ''Spacecraft Navigation Guidance and Control,'' 2-74.)

the star fix. Hence the two had to be mounted together on a ''navigation base'' that tied the optics to the inertial platform with a rigid frame made out of the stiff metal beryllium (figure 5.5). But North American simply could not find space for all the equipment near where the crew's eyes would be when strapped to their couches. Instead North American engineers created a ''lower equipment bay'' for the inertial unit, well below the couches. During flight, an astronaut would unstrap himself and float down below the instrument panel to align the platform. This arrangement meant that the astronauts would not be doing star sightings in real-time as they ''flew'' the vehicle (or during launch or reentry), but would rather have to rearrange themselves to take a fix in a special operation. When Michael Collins likened his command module to a cathedral, he called the navigation station the altar.65

Questions surrounding the mounting of the optics stressed the relationship between the IL and North American. Somewhere in the lower equipment bay the optics had to penetrate the wall of the vehicle, letting a light path through so the astronaut could see the stars. As it turned out, the optics in the lower bay penetrated on the ''hot side'' of the vehicle—the side that would face down during reentry and absorb most of the heat. The IL designed a door that would open on the outside of the vehicle. The optical assembly would extend out on a swiveling head so the optics could rotate around for a wide field of view. This made a ''deployable'' package, configured by an astronaut with a crank, like that on a bathroom window.

Landmark sighting angles

Landmark sighting angles

IMU angles

2) Astronaut uses tracking and space sextant to generate acquisition and tracking signal

3) Inertial measurement unit (IMU) provides spatial direction reference

IMU angles

4) Astronaut enters mark" when sight is on landmark

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