Body Mounted Attitude Gyro
The Body Mounted Attitude Gyros, or BMAGs, help to back up to the IMU. The two assemblies are capable of detecting the spacecraft's rate. This information is sent to the Gyro Display Coupler, or GDC, which adds the rate to its current know attitude, updating its knowledge spacecraft's orientation. There are two sets of rate gyros that are referred to as BMAGs one and two. Both of these can be activated or powered off through their rotary switches on MDC-7.
Both gyro assemblies are typically caged and determine only the spacecraft's rate. The rate is then sent to the GDC. The three BMAG switches on MDC-1 determine which set of gyros the GDC receives rate data from. Setting a switch to either Rate 1 or Rate 2 determines which BMAG the GDC receives rate from in that axis. BMAG one is capable of being uncaged to determine attitude errors. The center setting of the BMAG switches, Att 1/Rate 2, tells the GDC to accept rate data from BMAG 2 and attitude data from BMAG 1. In order to do this, BMAG one must be uncaged by the ECA. This is done automatically when setting a BMAG switch to the center position.