Accommodating Direction Ambiguities in Direction of Arrival based GNSS Spoof Detection

Abstract

Using direction of arrival (DOA) for GNSS spoof detection has several desirable properties. First, DOA-based spoof detection makes any spoofing from a single antenna very detectable regardless of how sophisticated its generation. It is difficult for a GNSS spoofer to create different DOAs as it generally requires transmitting from different locations, simultaneously. Thus, it forces a spoofer to utilize a much more complicated transmission system than a single antenna to create signals that can deceive DOA-based spoof detection. Second, it is complementary to and independent of other commonly used GNSS spoof detection methods thus providing additional layer of protection and certitude to detection. To utilize this method, we need means of getting DOA measurements of GNSS signals, preferably one that is both simple and has small form factor equipment. Controlled reception pattern antenna (CRPA) and dual polarization antenna (DPA) are two means of making such measurements [1][2]. While simple, low-profile methods such as the DPA and a two-element antenna are preferred, these methods result in ambiguity in measured direction of arrivals. DPA measurements have a 180degree ambiguity while two-element antennas have a symmetric ambiguity in DOA along the axis between the two antennas. The ambiguity can affect detection performance and limit the utility of such a system. This paper examines the ambiguity issue, focused on the DPA. It examines and develops a processing method to handle the ambiguity. First, we create two separate cases from the ambiguous measurements – a best genuine and a best spoof case. From these cases, we develop tests to examine each case and their likelihood. We use the processing results of both cases to manage the ambiguity. The processing method is tested and demonstrated using simulations and data from on-air tests in both nominal and spoofing conditions.