GPS World, January 2013

innovation algorithms and Methods GPS antenna LNA Battery Down converter DBSRX2 A D converter USRP N210 OCXO Portable authenticator Gigabit Ethernet Laptop with SSD b a FIGURE 8 a Block diagram of GPS position authenticator b photo of constructed unit Channel 1 PRN 7 Correlation Result window 40 milliseconds 10 05 0 05 10 Time 5117280 microseconds 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Offset time microseconds 10000 a FIGURE 9 Example of cross correlation detection results a without high pass filter and b with high pass filter authenticator described above First we present results that show we can successfully deal with the C A leakage problem using the simple high pass lter We do this by performing a correlation between snapshots of signal collected from the authenticator and a second USRP N210 software de ned radio FIGURE 9a is the correlation result without the highpass lter The periodic peaks in the result have a period of 1 millisecond and are a graphic representation of the C A leakage problem Because of noise these peaks do not have the same amplitude FIGURE 9b shows the correlation result using the same data snapshot as in Figure 9a The difference is that Figure 9b uses the high pass lter to attenuate the false peaks caused by the C A signal residual Only one peak appears in this result as expected and thus con rms the analysis given earlier We performed an experiment to validate the authentication performance In this experiment the authenticator and the supplicant were separated by about 1 mile about 16 kilometers The location of the authenticator was xed The supplicant was then sequentially placed at ve points along a straight line The distance between two adjacent points is about 15 meters The supplicant was in an open area with no tall buildings or structures Therefore a Channel 1 PRN 7 Correlation Result window 40 milliseconds 10 05 0 05 10 Time 1126110 microseconds 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Offset time microseconds 10000 b suf cient number of satellites were in view and multipath if any was minimal The locations of the ve test points are shown in FIGURE 10 The rst step of this test was to place the supplicant at point A and collect a 40 millisecond snippet of data This data was then processed by the authenticator to determine if The signal contained the watermark We call this the signal authentication test It determines whether a genuine GPS signal is being used to form the supplicants position report The supplicant is actually at the position coordinates that they say they are We call this the position authentication test It determines whether or not falsi cation of the position report is being attempted Next the supplicant was moved to point B However in this instance the supplicant reports that it is still located at point A That is it makes a false position report This is repeated for the remaining positions C through E where at each point the supplicant reports that it is located at point A That is the supplicant continues to make false position reports In this experiment we have ve common satellites between the supplicant at all of the test points A to E and GPS World January 2013 www gpsworld com 72