GPS World, September 2010
Security RECEIVER DESIGN of despreaders can be defined by where A i p m T m T and J i p m T m T are the mth time sample of the ith despreader channel for the authentic and jamming GNSS signals Pairwise Correlation The central tenet of the spoofi ng detection is that the array gain vector denoted here as the array manifold vector for the jammer signals J will be the same for all of the L spoofi ng signals while the array manifold vector for the authentic signals A will be different for each of the L authentic signals If the random antenna trajectory is of suffi cient length then the authentic signal array manifold vectors will be uncorrelated On the other hand as the jammer signals emerge from the same source they will all have the same array manifold vector regardless of the random antenna trajectory and also regardless of the spatial fading condition This would indicate that a method of detecting that a spoofer is present to form the M 2L matrix of all of the despreader output vectors denoted as r and given as where it is assumed that M 2L Basically what can be assumed is that if there is a spoofer from a common source that transmits more than one GNSS signal simultaneously there will be some residual spatial correlation of the observables of J i with other despreader outputs of the receiver Therefore if operations of pairwise correlations of all of the 2L despreader outputs result in high correlation there is a likelihood of the existence of spoofing signals These pairwise correlations can also be used to distinguish spoofing from authentic signals Note that even during the time when the spoofing and authentic signals have the same Doppler and code offset the superposition manifold vector of A i and J i will be correlated with other spoofing manifold vectors The pairwise correlation of the various spoofing signals can be quantified based on the standard numerical estimate of the correlation coefficient given as where r i is the ith column vector of r defi ned in Equation 3 and the superscript H denotes the complex conjugate operator Toward Spoofing Detection FIGURE 3 shows the spoofi ng attack detection and mitigation methodology The receiver starts with the acquisition process of a given www gpsworld com September 2010 GPS World 31
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