An Algorithm for Finding the Direction of Arrival of Counterfeit GNSS Signals on a Civil Aircraft

The purpose of this paper is to present and discuss the results of a Spoofing Detection and Direction Finding procedure usable for GNSS spoofing monitoring on board of a commercial aircraft. The targeted use case is an evolution of the GNSS equipment used on board of civil aircrafts, characterized by the availability of more GNSS constellations (multi-constellation) and two carrier frequencies (dual-frequency). The GNSS evolution in the civil aviation domain also foresees the progressive acquisition of a prominent role for GNSS in the aircraft navigation system, while today this role is taken by the traditional terrestrial NavAids. In such a scenario, the safe management of the Radio Frequency (RF) interference risk on GNSS becomes a primary task; nonetheless, the strict regulatory and certification procedures of the civil aviation domain impose a smooth technological evolution for the on-board avionic systems and consequent constraints in the design of new functionalities. The Spoofing Detection and Direction Finding procedure presented in this paper has been developed in this context. It consists of a detection module that employs the Dispersion of Double Difference (D-3) algorithm to identify which signals tracked by the receiver are counterfeit, if any; the detection module is followed by a direction finding module that implements an efficient Direction-Of-Arrival estimator, with the purpose of providing information to a ground control center for the localization of the spoofing source (this ground segment is out of the scope of the paper). The necessary on board equipment consists in three GNSS antennas and the same number of receivers, time-synchronized with a common clock, plus a signal processor that implements the detection and DOA estimation algorithms. The paper presents the design of the chain of algorithms and their preliminary tests in a laboratory setup, with the simulation of a number of spoofing attacks, assumed realistic in a civil aviation scenario. The obtained results confirm the reliability of the spoofing detection and direction finding procedure, in terms of both correct detection rate and DOA estimate accuracy. They also show that the quality of the carrier phase measurements is a key element of the whole procedure, therefore an interesting indication we obtain is that the choice of the on-board multi-constellation GNSS receivers should take into account also the accuracy and continuity of the provided carrier phase measurements.