A software defined radio receiver for ILS localizer signal-in-space parameters inspection using Chirp-Z transform and self-adaptive AGC

Abstract

In aviation navigation, a localizer of Instrument Landing System (ILS) provides lateral guidance to keep aircrafts on a runway centerline during approaching and landing. A Very High Frequency (VHF) signal is transmitted on ILS localizer channel to allow the airborne avionics equipment to determine its position from the centerline. In compliance with International Civil Aviation Organization (ICAO) standards, this ILS localizer signal must be inspected to assess the transmitted signal-in-space parameters. In this paper, we have developed a Software Defined Radio (SDR) receiver for preflight inspection stage to assist ground personnel to inspect the ILS localizer signal. The SDR receiver uses a novel design to measure the ILS localizer signal. Firstly, chirp-z transform algorithm is proposed to estimate frequency magnitudes of the ILS localizer signal for modulation index calculation. This algorithm aims to enlarge the input spectrum around a selected center frequency for more accurate frequency magnitude estimation. Secondly, the iterative searching is presented to handle a signal level control of hardware front-end. This algorithm iteratively adjusts a software controllable gain of Automatic Gain Control (AGC) for more extensive range of SDR input power. In the experiments, performance evaluation was carried out through three commercial off-the-shelf (COTS) ILS generators and a COTS ILS receiver to benchmark our SDR-based receiver. Test results showed that our approach achieves accurate measurement and wide range of the received ILS localizer signal.