Data Integrity for GPS and Galileo Signals used by Civil Aviation

Abstract

GNSS signals coupled with control mechanisms at the receiver end must guarantee that the information transmitted through the navigation message is provided unaltered to the final end user. From this premise the concept of Data Integrity is presented in this paper -measure of the trust that can be placed in the invariance of the received message information with respect to the transmitted information- and is quantified through 3 parameters, probability of erroneous message, probability of undetected error and Data Integrity Risk (DIR). The Data Integrity concept is special important in Civil Aviation where the provision of altered Clock offset correction and satellite Ephemeris Data (CED) to the end user can provoke an out of tolerance position error which would directly impact the Integrity Risk (IR) set by the civil aviation standards, IR=?10?^(-7)/operation. Therefore, this paper discusses a requirement for the DIR of CED information for the civil aviation application, ?DIR?_CA as well as presents the principle of calculation of the CED information DIR, ?DIR?_CED, (and any other generic field, ?DIR?_F) of any couple GNSS signal/control mechanism. Several DIR calculation expressions are provided and commented in this paper. Moreover, they are applied, with their required modifications, to two GNSS signals, Galileo E1 OS and GPS L1 C/A for two different control mechanisms – CED information is only read once and the same CED information must be read twice in a row to be provided to the final end user. The C/N0 thresholds for which both GNSS signals/control mechanisms couples are compliant with the civil aviation DIR requirement, ?DIR?_CA, are provided and compared.