In a bank-to-turn autopilot, increased accuracy and insensitivity to tracking noise and glint are achieved by using a yaw LOS rate to drive the roll rate loop and by sending only a body coordinate pitch LOS rate to a pitch channel so as to not introduce errors in the cross plane. By uncoupling the pitch and roll loops, pitch/yaw/roll coupling, large roll transients, and cross plane maneuvers can be minimized. Additionally, body LOS rates are transformed to inertial LOS rates and subsequently back to body LOS rates such that guidance filtering may be performed in a non-rolling frame of reference. This allows the guidance filtering to be separately optimized without regard to destabilizing effects of such filtering due to roll-to-yaw coupling path lags. In other words, servo dynamic path lags are reduced since the guidance filters do not have to track the high frequency roll rates. They only need to process a slowly varying inertial pitch and yaw rates which are then mathematically resolved back to body coordinates. Thus heavier filtering may be performed by the guidance filters while maintaining the same phase margin obtainable using simpler guidance filters in a rolling reference frame. Finally, a minimum-roll implementation is obtained by correcting the sign of the roll rate command with the pitch acceleration command. Roll is thereby constrained within .+-.90 as opposed to .+-. 180 otherwise.
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