APNT for GNSS Spoof Detection

Global Navigation Satellite Systems (GNSS) are well known to be accurate providers of position, navigation, and time (PNT) information across the globe. With capable receivers and well-populated satellite constellations, GNSS users typically believe that the position and time information provided by their GNSS receiver is perfectly accurate. More sophisticated users look beyond accuracy and are also concerned with the integrity of the GNSS information. Advances in electronics technology have enabled the creation of malicious RF interference of GNSS signals. Inexpensive jamming devices overpower or distort the GNSS receivers input so as to completely deny the GNSS user of PNT information. A second threat to GNSS integrity is spoofing, the creation of counterfeit GNSS signals. This type of attack is considered more dangerous than a jamming attack since an erroneous PNT solution is often worse than no solution at all. The detection of spoofing is the subject of this paper. A variety of approaches have been proposed in the literature to recognize spoofing; many of these are based on the RF signal alone, including multi-antenna and multi-receiver methods. Another class of spoof detection algorithm is to compare the GNSS result to data from another, non-GNSS (hence, non-spoofed) sensor. In this paper we imagine that the trusted signal is the output of an Alternative PNT (APNT) receiver. APNT refers to stand alone, non-GNSS systems that are intended to provide PNT information during periods in which GNSS is unavailable The wide recognition of the vulnerabilities of the GPS in the Volpe report spurred the search for APNT systems; examples include the development of eLoran in the U.S. and Europe, general work on signals of opportunity ranging, DME-DME positioning, and, quite recently, R-Mode in Europe (we note that none of these systems is currently operational). The intent is that an integrated receiver, either loosely or tightly coupled, would merge the two systems’ observables to yield the best PNT information possible; in practice, since the APNTs’ solutions are typically of lower accuracy than the GNSS solutions, the combined result is nearly equal to the GNSS-alone solution. The goal of this paper is to show that these APNT solutions should be used at ALL times; as a substitute for GNSS PNT when GNSS is unavailable and as an integrity check (e.g. spoof detector) when GNSS is available. At a cursory level spoof detection using APNT appears simple; just compare the two position outputs to see if they are close. This paper looks deeper, considering the questions: How can we use the time estimates to detect position spoofing? How close is close enough in this context? What is the probability of error in the decision? How do the geometries of both systems impact the test itself and its resulting performance? What happens if the receivers are providing different information