Galileo Model of Group Delay Accuracy for Advanced RAIM users

Abstract

Group delays (GD) are errors affecting GNSS measurements due to no-idealities of the satellite or the receiver hardware. They depend on the signal characteristics and are in principle different for each signal component and each signal combination. The satellite contribution can be partially mitigated applying the corrections transmitted in the navigation message, but their residual errors can still harm safety critical applications where integrity requirements are at major or even hazardous safety level. For the definition for example of aviation standard, such as the Dual Frequency Multi-Constellation Minimum Operational Performance Specification (DFMC MOPS [8]), it is in fact essential to properly model and bound the accuracy of the GD broadcast by the Constellation Service Providers (CSPs). In this paper GPS and Galileo cases are analyzed highlighting in particular their differences. In fact Galileo navigation message is generated by the ground system processing the same frequencies, L1 and L5, used by aviation users, while GPS uses L1 and L2 frequencies, being L2 outside the aviation bandwidth. This characteristic implies that GPS has to include in the error bounds, that is in the broadcast User Ranging Accuracy (URA), the contribution of the GD errors for all aviation users, for single frequency L1 and L5 as well as for dual frequency L1-L5 users. Galileo instead can use a different approach for modeling the GD accuracy, exploiting the advantage of its different architecture. The GD accuracy bound needs to be applied only by single frequency users and can be in fact separated from the broadcast Signal In Space Accuracy (SISA). In this way, SISA can refer only to dual frequency users. This has the advantage to reduce SISA values with respect to URA ones and improve availability for DF users. Based on a real measurements characterization, the paper describes the method and shows its advantages focusing on Advanced RAIM availability improvement with respect to the GPS approach ([1]).