EGNOS GEO Replenishment : Planning for the Future

EGNOS the European Geostationary Navigation Overlay Service is being developed by the European Tripartite Group and will become operational in 2004. EGNOS is designed as a satellite-based augmentation to GPS (and GLONASS). It will significantly improve the performance available from these two systems, facilitating precision applications for a wide range of users. EGNOS relies on the availability of Geostationary satellites equipped with navigation payloads to broadcast a GPS look -alike signal to users containing integrity and wide-area differential corrections. The operational system will use three satellites to disseminate these data to users: Inmarsat III Atlantic Ocean Region–East (AOR-E) at 15.5oW; Inmarsat III Indian Ocean Region (IOR) at 64oE; and ESA ARTEMIS at 21.5oE. EGNOS currently has Contract leases on the navigation payloads on the Inmarsat AOR-E and IOR satellites. The European Space Agency (ESA) has implemented a third navigation payload on its ARTEMIS Geostationary satellite, to be launched in 2001. Considering the specifications of these three GEOs, they are likely to end their nominal operational lifetime between 2007 and 2009. EGNOS, on the other hand, is currently planned to have fifteen-year lifetime starting in 2004. Hence it follows that arrangements have to be put in place to ensure navigation payload availability from 2004 through to 2018. This paper aims at identifying and discussing a number of possible options to guarantee GEO continuity during EGNOS operational life. 1 – Introduction The EGNOS System the European Geostationary Navigation Overlay Service is being developed in Europe to provide GPS (and GLONASS) regional augmentation services to aviation, maritime and land users [1]. The EGNOS System is being designed to serve the needs of all modes of transport in the European Region, namely: civil aviation; maritime and inland water navigation and docking; and rail and road transport and traffic monitoring systems. EGNOS is being developed by ESA, on behalf of the European Tripartite Group and will become operational in 2004. 1 The material presented here is only for technical discussion and does not reflect in any way EGNOS position for the EGNOS GEO replenishment, nor the position of any of the institutions for which the authors work. The EGNOS Advanced Operational Capability (AOC) relies on navigation payloads on the following three satellites to disseminate ranging, integrity and correction data to users: 1. Inmarsat III Atlantic Ocean Region–East (AOR-E) at 15.5oW. 2. Inmarsat III Indian Ocean Region (IOR) at 64oE. 3. ESA Advanced Relay and Technology Mission Satellite (ARTEMIS) at 21.5oE. Considering the specifications of these three GEOs, there are likely to end their nominal operation lifetime between 2007 and 2009, although the two current Inmarsat leases are due to expire in 2001. EGNOS is currently planned to have fifteen-year lifetime starting in 2004. Hence it follows that arrangements have to be put in place to ensure navigation payload availability from 2004 through to 2018. This paper aims at opening this debate by presenting a number of possible options to guarantee GEO continuity during EGNOS operational life. Within this paper, "EGNOS GEO replenishment" refers to a strategy to ensure the availability of navigation transponders at or near the current locations. Three options are addressed in this paper, namely: q the use of leased transponders (i.e. next generation of Inmarsat Satellites); q piggy-back payloads on satellites “of opportunity”; q and dedicated satellites. A brief review of the EGNOS system is provided in Section 2. Section 3 summarises the current EGNOS AOC GEO configuration, and considers the system requirements for GEO satellites. The various GEO replenishment options are presented in Section 4, before summarising the findings of this paper in Section 5. 2Overview of the EGNOS system This section provides a summary description of the EGNOS AOC architecture [1]. The EGNOS Reference architecture is depicted in Figure 1. It is composed of four segments: ground segment, space segment, user segment and support facilities. The EGNOS Ground Segment consists of GNSS (GPS, GLONASS, GEO) Ranging and Integrity monitoring Stations (called RIMS) which are connected to a set of redundant control and processing facilities called Mission Control Centre (MCC). The system will deploy 34 RIMS located in mainly in Europe and 4 MCCs located in Torrejon (E), Gatwick (UK), Langen (D) and Ciampino (I). RIMS are equipped with external oscillators and dual frequency GPS/GLONASS/GEO receivers to track the three satellite constellations. These tracked data are then transmitted over the EGNOS Wide Area Network (EWAN) to the Master Control Centres (MCC). The MCCs produce the so-called EGNOS data products – Wide Area Differential (WAD) corrections and int egrity information for each satellite. The data products are then transmitted to the Navigation Land Earth Stations (NLES) where they are scheduled to meet time-to-alarm requirements, packed into the standard message form, modulated on a GPS look-alike signal, and uplinked to the EGNOS GEOs. The system will deploy 2 NLESs (one primary and one back up) per GEO navigation transponder and an NLES for Test and Validation purposes, located in Torrejon (E), Fucino (I), Aussaguel (F), Raisting (D), Goonhilly (UK), and Sintra (P) respectively. Each GEO broadcasts a GPS lookalike signal that is modulated with the EGNOS data products. This is beneficial because each GEO ranging signal can be included in the navigation solution. However, it is also important to emphasise that the use of this signal is just one convenient solution for delivering the SBAS data products to the users. The EGNOS AOC space segment consists of three navigation payloads on Geostationary Earth Orbit (GEO) satellites. Two of these are on Inmarsat’s third generation satellites stationed at 15.5o W (Atlantic Ocean Region East (AOR-E)) and 64o E (Indian Ocean Region (IOR)). The third is on the European Space Agency’s ARTEMIS satellite that will be stationed at 15o E and launched in 2000/2001. The navigation payloads on all these satellites are essentially bent-pipe transponders such that a message uploaded to the satellite is broadcast to all users in its Geostationary Broadcast Area (GBA) (see Figure 2). Those three satellites are expected to be operational up to 2007-2009. A GEO replenishment policy needs, therefore, to be defined to guarantee 15 years of operational life for EGNOS The EGNOS User Segment consists of an EGNOS Standard receiver, to verify the Signal-InSpace (SIS) performance, and a set of prototype User equipment for civil aviation, land and maritime applications. That prototype equipment will be used to validate and eventually certify EGNOS for the different applications being considered. Finally, the EGNOS support facilities include the Development Verification Platform (DVP), the Application Specific Qualification Facility (ASQF) located in Torrejon (Spain) and the Performance Assessment and System Checkout Facility (PACF) located in Toulouse (France). Those are facilities needed to support System Development, Operations and Qualification. 3EGNOS AOC – Current GEO Configuration and Requirements The EGNOS Advanced Operational Capability (AOC) relies on navigation payloads on the following three satellites to disseminate ranging, integrity and correction data to users. q Inmarsat III Atlantic Ocean Region–East (AOR-E) at 15.5oW. q Inmarsat III Indian Ocean Region (IOR) at 64oE. EWAN GPS GLONASS GEO AOR-E IOR ARTEMIS