The inertial reference system (IRS) Theory of Operation

A. General

The IRS provides basic heading and attitude reference accomplished through computations based on accelerometer and laser gyro sensed signals. Three accelerometers and three laser gyros are used. The accelerometers and laser gyros are of the strap-down type and are positioned in the inertial reference units so that they are oriented along each of the three axis of the airplane. This orientation allows the IRU to sense accelerations along and rotation about each of the three axis. Computer manipulation of the signals from all six sensors provide the basic heading and attitude reference signals along with present position, accelerations, ground speed, drift angle and attitude rate information. The first requirement which must be met for proper IRS operation is alignment. IRS alignment basically consists of determination of local vertical and initial heading.

B. Alignment

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.

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.

C. Navigate Mode

In the navigate mode the IRUs provide outputs for attitude, heading, present position, accelerations, track angle, drift angle, ground speed, and wind data. These outputs are all derived from gyro and accelerometer strap down sensor data. The initial attitude, heading and velocity signals are modified by inputs from the sensors to establish real time present parameters through integration and computer calculations. Additional calculations by the computer establish such parameters as present position, ground speed and drift angle. Inputs from the air data computers are used for inertially smoothed altitude and altitude rate (baro altitude) and wind speed/direction (true airspeed).[readmore in]