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Over the next years, SBAS Augmentation systems will face a modernization process driven by the availability of new GNSS constellations and signals. This is the case of the finalization of the Galileo constellation deployment as well as the introduction of the GPS L2C and L5 signals. The GNSS users in general and SBAS users in particular will benefit from the better performances achievable through the use of a greater number of GNSS satellites and signals with a lower tracking noise. Current SBAS standard defined by ICAO and RTCA covers the augmentation of a single frequency user covering a maximum of 51 GNSS satellite. Following this standard, most of the operational SBAS systems chose to provide augmentation exclusively to the GPS constellation and, in some cases, its own GEO dissemination satellites. In addition to the satellite information the SBAS systems provides ionosphere corrections and integrity on their service coverage region. The amount of ionospheric data to be transmitted depends on the size of the area to be covered, which, on the other hand, has to be properly monitored with a dense network of SBAS reference stations. On the contrary, the future SBAS services will target Dual Frequency Multi Constellation augmentation (DFMC) to benefit from all the GNSS satellites available including GPS, Galileo, GLONASS or BeiDou signals. The processing of several GNSS constellation means that the information for a greater number of satellites should be broadcast. Also, when considering the dual-frequency user processing, the ionospheric information could be transmitted or not depending on whether a single-frequency fallback service should be made available through the DFMC signal. Because the SBAS message is limited to only 250 bits per second, the differences between the single frequency and the DFMC augmentation approaches have important consequences on the format of the SBAS information broadcast to the user. Consequently, the single frequency message format needs to be evolved towards a new DFMC message capable of supporting the augmentation of a higher number of satellites, still maintaining the requirements of a six seconds “time to alert” and the capability of updating the integrity status of all the satellites at the same time if needed. For this reason, during the last years, different efforts have been made by several entities addressing to the definition and consolidation of an interoperable SBAS DFMC interface. The main differences between the current single frequency standard and the proposed DFMC format can be summarized as follows: • Extension of the Satellite Mask from a maximum of 51 satellites to a maximum of 92 satellites. • Redefinition of the satellite long term corrections with a lower scale error but also with a lower updating rate and incorporation of the satellite covariance matrix. • Removal of satellite fast corrections. • Changes on the satellite integrity bound concept and information scheduling. • New approach for the user management of inter-constellation biases and time scale offsets. • Different error budget associated to the single-frequency and dual-frequency solutions and the new signals processed. The present paper aims at discussing the consequences of the new message definition in the generation of the SBAS correction and integrity information, and at presenting an analysis of the performances achievable with the DFMC standard. For this purpose, the GMV’s magicSBAS platform has been evolved to implement the DFMC interface so that a real-data, real-time evaluation is feasible: • magicSBAS can be used to generate both SBAS L1 single frequency and DFMC SBAS augmentation solutions with a common scenario of reference network data. On one hand, it is possible compute these solutions through the post-processing of real or synthetic data, for example, archived in RINEX format. On the other, it also allows the real time processing of EDAS and NTRIP real time GNSS receivers. • Eclayr can be used to perform Signal in Space performance analysis associated to both single frequency and DFMC messages. Firstly, this allows the evaluation of the SBAS LPV200 or CAT-I service availability over a given area. Secondly, it provides an evaluation of the SBAS pseudorange accuracy and integrity margins, together with its projection to the user by comparing the SBAS information with reference post-processed IGS orbits and clocks. • magicGemini tool can be used to perform user level performance analysis associated to both single frequency and DFMC messages through the application of the SBAS augmentation solution to a specific receiver raw data archived in RINEX format, or received through an NTRIP or EDAS real time interface. • EETES tool can be used to generate synthetic scenarios including characteristics not available at the present time in real data, as for example a full L2C, L5 GPS constellation, or the future Galileo satellites. It also allows calibrating the receiver’s error budget. The paper compares the different SBAS message characteristics and will focus on the comparison of the single frequency and DFMC service and user performances obtained in the following cases: • GPS only and GPS+GAL real time and post-processing performances over Europe based on the available receivers, satellites and signals at the beginning of 2016. • GPS and GAL synthetic data post-processing analysis representing future fully deployed Galileo E1/E5 and GPS L1/L5 constellations. The conclusion of this analysis, as well as the platform developed, shall support and reinforce the next steps in the definition of the future DFMC SBAS interface. |
