Rising edge detection used as TOA estimator for mode S signals with multipath propagation

Local area multilateration (LAM) systems installed at airports suffer from multipath propagation caused by reflections from objects such as buildings, hangars, masts or shelters. Receiving multiple overlapped signals from different radio propagation channels deteriorates the time of arrival (TOA) estimation. In this paper the edge detection algorithm is used to estimate the TOA of 1,090 MHz Mode S downlink signals with overlapping multipath. This algorithm is a fundamental tool in image processing, it detects the zero crossing of the Laplacian signal and is applied on the rising edge of the first pulse. Herein we have shown in a real-world example, that the edge detection leads to better results than the threshold detection algorithm in terms of TOA estimation accuracy when having multipath overlapping. In addition, the low-cost implementation is suitable for deployment in centralized or decentralized time-synchronized LAM systems and requires only one single receive stream.