743 - Abstract:
The success of SBAS services attracts more and more users in all areas of application. The aviation community, driver of the mission requirements for Safety-of-Life applications, is considering new and improved services (e.g. beyond Cat-I). These will demand more stringent Required Navigation Performance (RNP) parameters, including the Time-to-Alert. In the Security domain, the advent of more rigorous security requirements into the regional augmentation systems presents a challenge to the current architectures. A simpler infrastructure, modernisation of the architecture, use of latest technology towards more automated operations are also targets that contribute to lower life-cycle costs.
These are challenges that future SBAS systems will face. They invite to re-consider the current SBAS architectures, based on a large ground infrastructure and transparent (bent-pipe) payloads on board geostationary satellites. There is fast progress in satellite on-board computing in the last years, achieved by the satellite telecomm industry. In navigation, it opens a window to an alternative way of generating and broadcasting augmentation signals. On-board processors are more performant, with less mass, volume and lower power consumption. Within a short time, they could host the core of the SBAS algorithms.
The European Space Agency proposes the Space-Based Architecture for GNSS Augmentation Systems as an innovative architecture concept, designed for future SBAS navigation systems. The objective is to generate navigation augmentation signals in compliance with the ICAO SARPS. The ground infrastructure is much simplified compared to the “classical” SBAS architectures. The system communications and the core of the SBAS real-time processing functions are supported by the space segment. The Monitoring Stations (MS) send the data collected directly to the GEO satellites. There is no need of a real-time ground communications network. On-board the GEO, the MS data is processed and the integrity is checked; the SBAS message is computed; the augmentation signal is generated and broadcast to the users. There is no need of uplink stations.
Figure 1 represents the schematic space-based architecture, where redundancies are not shown. The working principle is similar to the one used by the classical architecture, with the key difference that the communication and navigation functions are highly integrated, and the core SBAS functions are carried out on board the GEO satellite.
Figure 2 zooms in on the payload high-level architecture.
On board the GEO satellite, the COMM payload receives the raw data from the Monitoring Stations (MS). Outputs from independent receivers at each MS are routed to the SBAS Processing function or to the Integrity Monitoring function, according to a pre-defined scheme.
The SBAS Processing function computes the SBAS Network Time and the data augmentation messages based on the data observables collected by the MS receivers allocated to the Processing.
The Integrity Monitoring function checks the integrity of the messages generated by the Processing function based on Integrity data collected by the MS receivers allocated to Integrity. It detects unwanted conditions in the data that could compromise the Safety of the users. In those cases, it generates alerts to warn the users.
SBAS data augmentation messages are delivered to the NAV P/L for encoding, signal modulation and broadcast to the users in the assigned L-band frequencies.
Innovations and advantages:
The advantages of the Space-Based architecture are:
- Mission performances beyond Precision Approach Category-I (shorter Time-to-Alert).
- Improved security against interference, jamming, spoofing.
- Improved security against cyber-attacks.
- Reduced ground infrastructure, less assets, less sites, simplified maintenance, highly automated operations.
- Simplified system upgrade and migration concept.
- Enabled integration of services concept: communications network, GEO and SBAS services under a single provider.
- Reduced exploitation costs.
Domain of application:
The main field of application envisaged is the Dual Frequency Multi-Constellation SBAS services, e.g. EGNOS in Europe.