The ESA “GNSS+” Project. Inter-satellite ranging and inter-satellite communication links in the frame of the GNSS infrastructure evolutions.
Amarillo Fernandez, F.¹; Gerner, J.L.¹; Rodriguez, R.L.¹; Dow, J.^2
1ESA/ESTEC; ²ESA/ESOC

The ESA “GNSS+” Project is devoted to assess the benefits of inter-satellite ranging, inter-satellite communication links and auxiliary on-board orbit-clock determination processing in enhancing satellite broadcast navigation data. It targets the definition of a feasible GNSS system architecture taken advantage of the above technologies and it assesses the performance improvements it brings. The “GNSS+” Project analyzes the achievable performance in terms of broadcast-orbit and broadcast-clock errors, of the proposed GNSS system architecture, under scenarios in which 1) The Space and Ground Segment are fully available 2) The contact with the Ground Segment is interrupted The performances evaluated in the first scenario are named as nominal-mode performances, while those in the second are named as autonomy-mode performances. The target GNSS system architecture is being derived from a detailed evaluation of the adequate solutions for a number of key dimensions of the problem, such as: 1) The set of satellite-to-satellite, satellite-to-ground and ground-to-satellite observables to be processed by the orbit and clock determination processes, which are located both on-board and on-ground 2) The exchanged information via satellite-to-satellite, ground-to-satellite, satellite-to-ground communication links, which includes orbit, clock, observables, telemetry, telecommand and navigation auxiliary data. 3) The characteristics of the orbit and clock determination processes located both on-board and on-ground, as well as the broadcast-orbit and broadcast-clock refresh rates 4) The definition of the navigation system time reference, and the means to ensure its linkage to ground based time references 5) The characteristics of the inter-satellite ranging and inter-satellite communication signals and links 6) The characteristics of the on-board and on-ground transmitting and receiving chains, including antenna subsystems, for both ranging and communication signals The definition of the GNSS system architecture requires of sophisticated SW simulation tools which are developed as part of the GNSS+ project. The simulation environment comprises the orbit determination and clock determination filters, for both on-board and on-ground processing, as well as the simulation of the input observables. The GNSS+ architecture considers: 1) The availability of almost purely geometrical observables (physical clocks free, group delay free and ionospheric free) and almost purely time transfer observables (geometry free, ionosphere free and troposphere free), what allows a high level of isolation between the orbit determination algorithms and the clock determination functions. 2) The distribution of the orbit and clock determination functions amongst on-ground processes and on-board processes, being the first responsible of the nominal-mode performances, and the second of the autonomy-mode performances. 3) That the on-ground orbit determination process has a very high level of sophistication and complexity compared to the on-board one, and that in both cases satellite-to-satellite, ground-to-satellite and satellite-to-ground observables are processed. 4) That the above paragraph is also valid for the clock determination processes, as well as that in addition both the on-ground and on-board clock determination processes consider a system time reference defined from a composite clock based on the on-board clocks. This represents a difference with respect to current Galileo baseline which relies on a precise timing facility (PTF) at the master station (including also a hot redundancy scheme). The selected approach has the advantage of a superior robustness, due to the isolation of each of the individual clock contributions, eliminating single point of failures, although at the price of a higher complexity. The proposed architecture considers all the functionalities required to steer the defined system time reference towards TAI. The paper will present a consolidated and justified architecture, as well as first evaluation of the achievable performances.