LDACS1-Based Passive MIMO Radar

Abstract

In civil aviation, ADS-B is expected to become the primary surveillance technology. To ensure that accurate surveillance services can be provided even during possible ADS-B outages or when the localization of a non-cooperative aircraft is of interest, a backup system is required. Motivated by the Alternative Positioning, Navigation, and Timing (APNT) technologies currently developed to serve as a GPS backup for aeronautical users, we propose using LDACS1 as a non-cooperative surveillance alternative and backup. To this end, we reuse the signals of the future L-band digital aeronautical communications system type 1 (LDACS1) as signals of opportunity for target detection and parameter estimation. LDACS1 is a multi-carrier system that uses frequency-division duplexing and is designed to operate in the lower part of the L-band. Its main advantage is that LDACS1 channels are placed in the spectral gaps between the channels of the distance measuring equipment - a legacy radio navigation system currently used in civil aviation. The LDACS1 surveillance functionality is enabled by the forward link signals which, continuously transmitted by the LDACS1 ground stations, are reflected off the aircraft and towards the dedicated surveillance receivers within the radio horizon of the reflecting aircraft. An appropriate processing of the received reflections allows for the detection of the aircraft and estimation of its position on the ground. This implementation resembles a passive radar system, where commercial transmitters are used as illuminators of opportunity to detect and estimate airborne targets. The main difference to the proposed surveillance technology is the availability of the transmitted signals at the receivers. While, in general, a passive radar system does not have any access to the transmitted signals, which need to be therefore reconstructed, for the LDACS1-based application the transmitted signals can be readily made available since the same infrastructure is used for both communication and surveillance purposes. Moreover, the LDACS1 infrastructure enables the implementation of a MIMO radar with widely distributed antennas when using the LDACS1 ground stations as MIMO radar transmit antennas and augmenting the infrastructure with dedicated surveillance receivers. The challenges associated with the LDACS1-based surveillance application arise from the relatively low transmit power and small bandwidth characteristics of the LDACS1 signal which make the practical implementation of such a surveillance concept challenging.