Calculations and analysis of signal processing by various navigation receivers architectures

Abstract

Calculations of receiver response based on the predicted instrument landing system (ILS) and VHF omnirange (VOR) signals at an observation point are presented. The information contained in the radiated signals are detected by an airborne receiver and processed to provide guidance indications. This is accomplished through the use of heterodyne amplitude modulation (AM) receivers. The ILS signal in space is a composite of the direct and scattered carrier-plus-sidebands and sidebands-only signals. For dual-frequency arrays, this includes both the course and clearance signals. The receiver antenna is designed to pick up only the horizontally polarized component of the electric field available at the antenna. The received signal is then amplified at the radio frequency. The guidance information is computed from the audio tones at the output of the respective audio filters. Several factors which affect the received signal are incorporated in the computation of the detected audio signal. These include the velocity of the aircraft [Doppler effects], multipath, and other spurious effects. Four receiver algorithms are defined and the results from these processing algorithms are compared with actual flight measurements. The VOR receiver operates essentially in the same manner as the ILS receiver, except in the audio filtering stage. Rather than compute a DDM, the VOR receiver employs a phase comparator to compare the phases of two 30 Hz audio signals: the reference audio, and the variable audio. One of the two audio signals is obtained from an amplitude detector while the other is obtained through a frequency discriminator. Two algorithms are implemented, one for the Doppler VOR and the other for the conventional VOR.