Receiver-Autonomous Spoofing Detection: Experimental Results of a Multi-antenna Receiver Defense Against a Portable Civil GPS Spoofer

In this work we demonstrate the use of a dual antenna receiver that employs a receiver-autonomous angle-ofarrival spoofing countermeasure. This defense is conjectured to be effective against all but the most sophisticated spoofing attempts. The technique is based on observation of L1 carrier differences between multiple antennas referenced to a common oscillator. We first employ a moderately sophisticated spoofer to "fool" a single-antenna civil receiver. We then deploy the same attack after augmenting the receiver with an additional antenna and with receiver-autonomous spoofdetection software. The work discusses the experimental results together with various issues related to sensitivity, probability of false alarm, impact of carrier multipath, line-bias-calibration, and physical setup and security. We suggest that this work is important to the community as it provides experimental validation of a low-cost technique for receiver-autonomous spoofing detection. Furthermore, the technique, when combined with physical security of the antenna installation, provides a strong defense against even a sophisticated attack. The receiver employed is an L1-only civil GPS receiver with multiple antenna capability. The GPS chipset employed is the venerable GP2015/GP2021 that has been freely available for over a decade. As such, this receiver is representative of many civil receivers in use today for a variety of applications. Multiple antennas are enabled either through multiple independent RF front ends and correlators or via antenna multiplexing into a single RF front end and correlator bank. INTRODUCTION In 2001, the U.S. Department of Transportation assessed the U.S. Transportation infrastructure's vulnerability to civil GPS disruption [1]. Their report, known as the Volpe report, warned that “as GPS further penetrates into civil infrastructure, it becomes a tempting target that could be exploited by individuals, groups or countries hostile to the U.S.” Among other type of interference, the report considers civil GPS spoofing, an intentional interference whereby a GPS receiver is fooled into tracking counterfeit GPS signals. Spoofing is more 124 ION 2009 International Technical Meeting, January 26-28, 2009, Anaheim, CA sinister than intentional jamming because the targeted receiver cannot detect a spoofing attack and so can be fooled into generating data that is hazardously misleading. Previous work into spoofing countermeasures has been carried out, notably in an internal memorandum from the MITRE Corporation in which the author, Edwin L. Key appears to have examined spoofing and spoofing countermeasures in detail [2]. The memorandum recommends the following techniques for countering spoofing: 1.Amplitude discrimination 2.Time-of-arrival discrimination 3.Consistency of navigation inertial measurement unit (IMU) cross check 4.Polarization discrimination 5.Angle-of-arrival discrimination 6.Cryptographic authentication This paper concerns an implementation of angle-of-arrival discrimination for spoofing detection and employs two or more antennas. Of the proposed techniques, angle-ofarrival discrimination coupled with physical security of the antennas provides significant protection and is relatively easy to implement with inexpensive single frequency receiver technology.