Operational modes, which may be used during flight, consists of two vertical modes (NORM, AUX) and three heading modes, (TRUE, GRID and GRID –1 to –5, +1 to +5). The third heading mode is subdivided into ten divisions, manually set as required to compensate for residual transport wander. The operation modes are as follows:

a. Vertical. When the AUX/NORM/TEST switch on the controller is selected to the NORM position, the system operates to erect the GRU at a low erection rate which permits a highly accurate vertical (hence horizontal) to be sensed by the GRU. When selected to AUX the system operates to erect the GRU at a faster rate but with less accurate vertical sensibility. The AUX position may be used as a backup vertical erection loop. The TEST position is used for ground maintenance only.
b. Heading. When the TRUE/GRID switch on the gyro controller is selected to TRUE, the gyro reference unit will be compensated for earth’s rate, which varies with the sine of the latitude (15.04 × sin lat). Also if the doppler is on and operating, the GRU will be compensated for Transport Wander (TW), which is an apparent gyro drift due to transporting the gyro over the earth’s surface (i.e., meridian change). TW = Earth convergency (meridian change °long) × sin latitude. When operating beyond ±70 degrees latitude, the GHARS shall be operated in GRID, as the GRU cannot be adequately torqued (compensated) for the rapid meridian changes in east-west directions. In GRID, the problem of Earth Convergency or meridian change is eliminated because GRID navigation is based on heading reference from a fixed meridian on alignment as selected by the navigator (Greenwich or local). The settings GRID –1 to –5, +1 to +5 are to compensate for Residual Transport Wander (RTW). RTW is the error or apparent wander caused by the mathematical difference between Earth Convergency (change longitude × sine mid-latitude) and Map Convergency (change longitude × Constant of the Cone [CC] of the map/chart). See C-12-124-A00/MB-001, Section 3, paragraph 20., for RTW calculations and settings. Magnetic heading, presented on the BDHI after selection, is processed by the gyro controller. The gyro controller is designed to indicate variation/grivation by measuring the difference between the orientation of the GRU and the flux valve heading signals. By definition, variation is the difference between magnetic and true north; and grivation the difference between magnetic and grid north.