The following paragraph is technical and can be skipped. Without power applied to the LN16, the inertial platform, made top heavy by the star tracker, would tumble over. When power was applied to the inertial platform a coarse level was achieved by "caging", slewing the gimbals so that the platform was parallel to the top and bottom of the inertial platform case. This case was mounted on the centerline of the aircraft over the undercarriage. So if the aircraft was on flat ground, the case was flat and when the platform "caged" the platform vertical axis would nearly align with the direction of gravity. I had observed that when RC 357 was on a pad at Eielson that within 5 minutes of being "caged" the platform had leveled. Thus I could knock 10 minutes off the 15 minutes of the time allocated to leveling. The direction to north can be determined from a magnetic compass. However, a magnetic compass points to a magnetic north that is some distance from true north, the North Pole. This offset in direction is corrected by knowing the magnetic variation for the location of the compass. At northern latitudes magnetic variation can be large. So the direction to true north is imprecisely determine from a magnetic compass. The Gyrocompass process refines the north direction determination by monitoring the gyros. Gyros are sensitive to the spin of the earth. The LN16 determines north by monitoring the north gyro mounted on the level platform. The angle that the platform heading is off north is proportional to the amount of earth spin that the north gyro senses. The LN16 allocated 10 minutes for this process. However, the length of this process could be shortened if a good guess of the direction of north was known when the gyrocompass process started. When Rivet Amber was in the hangar I had the LN16 gyrocompass yielding the direction to north. I compared it with the compass reading. The difference would be a calibrated magnetic variation. I did this just once. Subsequently before flight I had the navigator dial in the predetermined magnetic variation into the LN16, gyrocompass for about a minute and then put the LN16 into the navigation mode. This would work if the aircraft had the same heading every time. In order to ensure this after the gyrocompass run that was used to calibrate the magnitude variation I placed a piece of tape on the wall of the hangar that the Amber faced. This tape was on a line with the cockpit front window separator and the vertical stabilizer. The success of the shortened gyrocompass depended on how well the yuke operator could maneuver Amber to be on this line. Amber was never more than a half of a degree off the line.
- Control & Display Unit
- Turn On Sequence
- Platform Schematic Diagram
- Stellar Inertial Reference Unit