GPS World, September 2010

RECEIVER DESIGN Security ÅFIGURE 4 Skyplots of available satellites a spoofing signals from Spirent generator b authentic signals from rooftop antenna ÅFIGURE 5 Normalized amplitude value of the signal amplitude for different PRNs a generated from the same antenna b Authentic GPS signals GNSS code If for each PN sequence there is more than one strong peak above the acquisition threshold the system goes to an alert state and declares a potential spoofing attack Then the receiver starts parallel tracking on each individual signal The outputs of the tracking pass to the discriminator to measure the correlation coefficient among different PN sequences As shown in Figure 3 if is greater than a predefined threshold the receiver goes to defensive mode As the spoofer attempts to pull the tracking point off the authentic signals the spoofer and authentic signals for a period of time will have approximately the same code offset and Doppler frequency Hence it may not be possible to detect more than one peak in the acquisition mode However after a while the spoofer tries to pull tracking mode off The outputs of the parallel tracking can be divided into two groups the J group is the data set that is highly correlated and the A group is the set that is uncorrelated It is necessary that the receiver antenna trajectory be of sufficient length a few tens of the carrier wavelengths such that M is moderately large to provide a reasonable estimate of the pairwise correlation The A group will be constrained in size based on the number of observable satellites Usually this is known and L can be set The receiver has control over this by setting the bank of despreaders If an SV signal is known to be unobtainable due to its position in the sky it is eliminated by the receiver Hence the A group can be assumed to be constrained in size to L There is the possibility that a spoofer will generate a signal that is clear while the SV signal is obscured by shadowing obstacles Hence a spoofing signal can inadvertently be placed in the A group However as this signal will be correlated with other signals in the J group it can be transferred from the A to the J group When the spoofing navigation solution pulls sufficiently away from the authentic solution then the navigation solution can create two solutions one corresponding to the authentic signals and the other corresponding to the spoofing signals At this stage the despreading code delay and Doppler will change such that the authentic and spoofing signals corresponding to the same GNSS signal will appear to be orthogonal to each other Proper placement of the members in the J and A groups can be reassessed as the set of members in the A group should provide the minimum navigation solution variance Hence in general there will be a spoofing and authentic signal that corresponds to the GNSS signal of index i If the spoofing signal in group J appears to have marginal correlation with its peer in group A and when interchanged with its corresponding signal in group A the latter generates a lower solution variance then the exchange is confirmed Experimental Measurements We used two data collection scenarios in experiments of spoofi ng detection based on utilizing a single antenna that is spatially translated to demonstrate the practicality of spoofi ng signal detection based on spatial signal correlation discrimination In the fi rst scenario the GPS World September 2010 www gpsworld com 32