Robust automatic aircraft lateral beam guidance control using gain-phase margin tester method

For the very high frequency omni-range (VOR) coupled automatic lateral guidance system, the variation of the distance, from the airplane to the transmitting station it is approaching, nearly always generates a stability problem. One method for eliminating such instability is to adequately switch the adjustable gain constants as the airplane proceeds. In this paper, the gain-phase margin tester method is exploited to obtain sufficient information that enables some adjustable system parameters to be altered to compensate for the increasing instability of the beam guidance system as it approaches the VOR transmitter. Some performance specifications of the controlled system can also be satisfied. A practical model of an airplane in flight is studied to help understand the resulting performance of the system. It is proved that one can have acceptable closed-loop stability with the pre-specified safety margins by using the proposed gain-phase margin tester method.