GPS World, January 2013

Algorithms and Methods innovAtion Location Pass Signal Authentication Pass Position Authentication A Yes Yes B Yes No C Yes No D Yes No E Yes No TABLE 1 Five point position authentication results the authenticator The results of the experiment are summarized in TABLE 1 If we can detect a strong peak for every common satellite we say this point passes the signal authentication test and note Yes in second column of Table 1 That means the supplicants raw IF signal has the watermark signal from every common satellite Next we perform the position authentication test This test tries to determine whether the supplicant is at the position it claims to be If we determine that the position of the supplicant is inconsistent with its reported position we say that the supplicant has failed the position authentication test In this case we put a No in the third column of Table 1 As we can see from Table 1 the performance of the authenticator is consistent with the test setup That is even though the wrong positions of points B C D E are reported the authenticator can detect the inconsistency between the reported position and the raw IF data Furthermore since the distance between two adjacent points is 15 meters this implies that resolution of the position authentication is at or better than 15 meters While we have not tested it based on the timing resolution used in the system we believe resolutions better than 12 meters are achievable Conclusion In this article we have described a GPS position authentication system The authentication system has many potential applications where high credibility of a position report is required such as cargo and asset tracking The system figUrE 10 Five point field test Image courtesy of Google detects a speci c watermark signal in the broadcast GPS signal to judge if a receiver is using the authentic GPS signal The differences between the watermark signal travel times are constrained by the positions of the GPS satellites and the receiver A method to calculate an authentic position using this constraint is discussed and is the basis for the position authentication function of the system A hardware platform that accomplishes this was developed using a software de ned radio Experimental results demonstrate that this authentication methodology is sound and has a resolution of better than 15 meters This method can also be used with other GNSS systems provided that watermark signals can be found For example in the Galileo system the encrypted Public Regulated Service signal is a candidate for a watermark signal In closing we note that before any system such as ours is elded its performance with respect to metrics such as false alarm rates How often do we Àag an authentic position report as false and missed detection probabilities How often do we fail to detect false position reports must be quanti ed Thus more analysis and experimental validation is required Acknowledgments The authors acknowledge the United States Department of Homeland Security DHS for supporting the work reported in this article through the National Center for Border Security and Immigration under grant number 2008 ST 061 BS0002 However any opinions ndings conclusions or recommendations in this article are those of the authors and do not necessarily reÀect views of the DHS This article is based on the paper Performance Analysis of a Civilian GPS Position Authentication System presented at PLANS 2012 the Institute of Electrical and Electronics Engineers Institute of Navigation Position Location and Navigation Symposium held in Myrtle Beach South Carolina April 23 26 2012 Manufacturers The GPS position authenticator uses an Ettus Research LLC www ettus com model USRP N210 softwarede ned radio with a DBSRX2 RF daughterboard ZhEfEng Li is a Ph D candidate in the Department of Aerospace Engineering and Mechanics at the University of Minnesota Twin Cities His research interests include GPS signal processing real time implementation of signal processing algorithms and the authentication methods for civilian GNSS systems DEmoZ gEBrE EgZiABhEr is an associate professor in the Department of Aerospace Engineering and Mechanics at the University of Minnesota Twin Cities His research deals with the design of multi sensor navigation and attitude determination systems for aerospace vehicles ranging from small unmanned aerial vehicles to Earth orbiting satellites MORE ONLINE Further Reading for references related to this article go to gpsworld com and click on Innovation in the navigation bar www gpsworld com January 2013 GPS World 73