GPS World, January 2013

innovation algorithms and Methods GPS satellite Message packet Time stamp Location User vehicle with GPS receiver GPS signal Message channel Monitoring center FIGURE 1 A typical asset tracking system modes which can lead to erroneous tracking of the asset The rst scenario occurs when an incorrect position solution is calculated as a result of GPS RF signal abnormalities such as GPS signal spoo ng The second scenario occurs when the correct position solution is calculated but the tracking message is tampered with during the transmission from the asset being tracked to the monitoring center The rst scenario is a falsi cation of the sensor and the second scenario is a falsi cation of the transmitted position report The purpose of this article is to examine the problem of detecting sensor or report falsi cation at the monitoring center We discuss an authentication system utilizing the white noise like spreading codes of GPS to calculate an authentic position based on a snapshot of raw IF signal from the receiver Using White Noise as a Watermark The features for GPS position authentication should be very hard to reproduce and unique to different locations and time In this case the authentication process is reduced to detecting these features and checking if these features satisfy some time and space constraints The features are similar to the welldesigned watermarks used to detect counterfeit currency A white noise process that is superimposed on the GPS signal would be a perfect watermark signal in the sense that it is impossible reproduce and predict FIGURE 2 is an abstraction that shows how the above idea of a superimposed white noise process would work in the signal authentication problem The system has one transmitter T x and two receivers R s and R a R s is the supplicant and R a is the authenticator The task of the authenticator is to determine whether the supplicant is using a signal from T x or is being spoofed by a malicious transmitter T m R a is the trusted source which gets a copy of the authentic signal V x t that is the signal transmitted by T x The snapshot signal V s t received at R s is sent to the trusted agent to compare with the signal V a t received at R a Every time a veri cation is performed the snapshot signal from R s is compared with a piece of the signal from R a If these two pieces of signal match we can say the snapshot signal from R s was truly transmitted from T x For the white noise signal match detection is accomplished via a cross correlation operation see Further Reading The crosscorrelation between one white noise signal and any other signal is always zero Only when the correlation is between the signal and its copy will the correlation have a nonzero value So a non zero correlation means a match The time when the correlation peak occurs provides additional information about the distance between R a and R s Unfortunately generation of a white noise watermark template based on a mathematical model is impossible But as we will see there is an easy to use alternative An Intrinsic GPS Watermark The RF carrier broadcast by each GPS satellite is modulated by the coarse acquisition C A code which is known and which can be processed by all users and the encrypted P Y code which can be decoded and used by Department of Defense DoD authorized users only Both civilians and DoD authorized users see the same signal To commercial GPS receivers the P Y code appears as uncorrelated noise Thus as discussed above this noise can be used as a watermark which uniquely encodes locations and times In a typical civilian GPS receivers tracking loop this watermark signal can be found inside the tracking loop quadrature signal The position authentication approach discussed here is based on using the P Y signal to determine whether a user is utilizing an authentic GPS signal This method uses a segment of noisy P Y signal collected by a trusted user the authenticator as a watermark template Another users the supplicants GPS signal can be compared with the template signal to judge if the users position and time reports are authentic Correlating the supplicants signal with the authenticators copy of the signal recorded yields a correlation peak which serves as a watermark An absent correlation peak means the GPS signal provided by the supplicant is not genuine A correlation peak that occurs earlier or later than predicted based on the supplicants reported position indicates a false position report System Architecture FIGURE 3 is a high level architecture of our proposed position authentication system In practice we need a short snapshot of the raw GPS IF signal from the supplicant This piece of the signal is the digitalized down converted IF signal before the tracking loops of a generic GPS receiver Another piece of information needed from the supplicant is the position solution and GPS Time calculated using only the C A signal The raw IF signal and the position message are transmitted to the authentication center by any data link using a cell phone data network Wi Fi or other means GPS World January 2013 www gpsworld com 68