Elimination of VOR Signal Rotor Modulation

Rotor modulation of VOR signals is a problem that must be addressed in every helicopter avionic installation, and there are cases in which filtering and antenna placement do not solve it. It is possible however to eliminate rotor modulation completely bv detecting the rotor modulation envelope on the received subcarrier and using the signal thus obtained to cancel the rotor modulation component in the variable phase channel of the VOR receiver. This induces an error into the VOR, and for that reason cannot be used in operational equipment. It does demonstrate, however, that methods can be found to reduce even very severe rotor modulation, and it may lead to the eventual elimination of this problem.