(1) IRS alignment consists of determining local vertical and initial heading. Both accelerometer and laser gyro inputs are used for alignment. The alignment computations use the basic premise that the only accelerations during alignment are due to the earth's gravity; the only motion during alignment is due to the earth's rotation. Accelerations due to gravity are always perpendicular to the earth's surface and thus define the local vertical. This local vertical is used to erect the attitude data so that it is accurately referenced to vertical. Initially, only a coarse vertical is established. Once vertical is established, the laser gyro sensed earth rate components are used to establish the heading of the airplane. As the alignment continues, both the vertical reference and the heading determinations are fine tuned for maximum accuracy.
(2) The orientation of the vertical axis of the attitude reference relative to the earth's surface is based on airplane position input to the IRU. The initial position entry can be made at any time during the alignment period. Earth rate sensing by the laser gyros allows the IRU to determine initial latitude. This gyro determined latitude is compared to the crew entered latitude. Crew entered longitude is compared to the last stored longitude. These comparisons must be favorable to complete the alignment period. During the alignment period all outputs of the IRU, except for present position, are set to NCD (No Computed Data). The minimum duration of the align mode is 10 minutes.