GALILEO System Architecture and Services
Javier Benedicto, Daniël Ludwig
Galileo Project Manager
ESTEC / APP-NG
Keplerlaan 1 – 2201 AZ Noordwijk – The Netherlands
Directorate General "Transports and Energy" (DG/TREN)
Rue de Mot, 24 bureau 7/93 B-1049 Brussels – Belgium
Javier Benedicto obtained his MSc in Telecommunications Engineering from the Polytechnic University of Catalonia (Barcelona, Spain) in 1981 He joined Mier Allende in 1982 to develop satellite microwave TVRO equipment and in 1985 joined the European Space Agency (ESA) where he managed on-board microwave and digital equipment development contracts and supported a number of commercial TV broadcast and mobile satellite programmes. In 1992 he became Head of the Mobile Satellite Services section within the ESA Telecommunications Directorate. Since 1995 he has managed the EGNOS Project from the ESA GNSS-1 Project Office in Toulouse, and since February 2000 is GALILEO Project Manager, leading the ESA GALILEO Project Office located in ESTEC, The Netherlands.
Daniel Ludwig graduated from INSA (Lyon, France) in 1965 and obtained his PhD in Telecommunications Theory and Stochastic processes from Laval University (Quebec, Canada) in 1971. He joined CNES in 1971 as space segment manager in the ARGOS Project, then took up duties as SARSAT Project Manager and in 1996 he joined the ESA GNSS-1 Project Office as EGNOS Ground Segment Manager. He joined the European Commission’s Navigation Unit (DG/TREN) in 1998 and has
since been in charge of with GALILEO Mission definition and overall architectural aspects.
This paper provides a description of the GALILEO system capabilities and related architecture, leading to the definition of services. The contents of this paper results from the conclusions of the studies conducted by the EC and ESA during the GALILEO Definition Phase, and form the basis for the GALILEO Phase B2 which is being conducted during 2001.
THE EUROPEAN SATELLITE NAVIGATION STRATEGY
Satellite navigation, positioning and timing has already found widespread applications in a large variety of fields. Recognizing the strategic importance of its applications, a European approach has been developed over recent years. It started
with a European contribution in the first generation of Global Navigation Satellite Systems (GNSS-1), the EGNOS programme, to continue with the future generation of Global Navigation Satellite Systems (GNSS-2), the GALILEO programme.
EGNOS provides a regional overlay augmentation to the current GPS and GLONASS, to serve the needs of maritime, land and aeronautical transport applications in the European region and beyond. EGNOS can fulfil a range of user service requirements by means of the broadcast, through Geo-stationary satellites, of ranging signals containing GPS and GLONASS integrity and differential corrections.
This first step provides Europe with early benefits but does neither offer a sufficient level of control over GNSS nor a signal of
guaranteed availability and performance. Therefore, upon request of the EU Council in July 1999, preparations for the next step have been undertaken in parallel to the implementation of EGNOS. Those preparations have led to the definition of GALILEO, a satellite constellation providing world-wide coverage which is to become the European contribution to GNSS-2.
Combined use of GALILEO, EGNOS and GPS/GLONASS will increase the overall performance, availability, continuity and the inherent safety of the services achieved from GNSS, and will allow for world-wide acceptability of the exploitation and use of satellite navigation for the benefit of all users.
GALILEO SYSTEM ARCHITECTURE
The GALILEO system architecture is based on a number of components as described below.
A Global Component composed of:
- A constellation of up to 30 satellites in Medium-Earth Orbit (MEO) providing adequate coverage for the provision of the
GALILEO services on a world-wide basis. Each satellite will contain a navigation payload and a search and rescue
- A ground segment in charge of managing the constellation of navigation satellites, controlling core functions of the
navigation mission (orbit determination of satellites, clock synchronisation) and determining and disseminating (via the
MEO satellites) the integrity information (warning alerts within time-to-alarm requirements) at global level.
Implementation aspects of global integrity concept are under evaluation. The Global ground segment will also provide
interfaces with service centres providing value-added commercial services and with the COSPAS-SARSAT Ground
Segment for the provision of S&R services.
Regional Components including:
- Non-European GALILEO Regional Components, made of ground segments dedicated to GALILEO integrity
determination over their specific area if regions choose not to adopt GALILEO’s global integrity.
Regional integrity data can be up-linked directly from each region or, alternatively, routed to the Ground segment of the
Global Component for up-linking to the satellites together with the SAR and service provider’s data.
- EGNOS providing integrity and differential correction for GPS and GLONASS through Geo-stationary satellites.
Some classes of user have local area requirements more demanding than those that will be available from the global system (accuracy, integrity time-to-alarm, signal acquisition/reacquisition, etc). These special services will be met through the use of
augmentations provided by local components.
In addition to providing differential levels of accuracy and stringent integrity time-to-alarm requirements (within 1 second) Local Components can provide services such as:
- Commercial data (corrections, maps, databases)
- Additional navigation signals (pseudolites)
- Enhanced positioning data in areas of poor signal reception (underground car parks) from GSM/UMTS station-based
assisted position calculations
- Mobile communication channels
The design of the GALILEO signal is conditioned to support the operation of local components. An optimum use of those features and capabilities, together with the deployment of complementary local components under the responsibility of the respective service providers, will provide for the commercial exploitation of the GALILEO signals.
Consisting of different types of user receivers, with different capabilities related to the different GALILEO signals in order to
fulfil the various GALILEO services.
The GALILEO system provides an interface to users and value added service providers (including local component value added services) through Service Centres. Where appropriate for the different positioning, timing and navigation service categories, these centres perform functions such as providing:
- Information and warranty on performances and data archiving;
- Subscription and access key management;
- Insurance, liability; legal and litigation management;
- Certification and license information management;
- Commercial interfaces;
- Support to application development and possible elaboration of R&D approaches.
The precise functions depend on the nature of services provided, where appropriate, Service Centres can also play a role in collecting fees.
GALILEO will also provide interfaces with External Complementary System (GPS, GLONASS, LORAN-C, etc…) for the
provision of combined services.
GALILEO LOCALREGIONALCOMPONENTSCOMPONENTS GLOBAL COMPONENT
IULSLocal DATA IMSMEO CONSTELLATION MSLINKNetwork?ICC……S-band
L - band TTCL-bandIULS IMS S-bandLocal DATANetwork MSLINKOSS ICC NSCCNetwork ICCIULSIMS Network GEO NLES RIMSINTEGRITY DETERMINATIONNAVIGATION CONTROL &Network& DISSEMINATION CONSTELLATION MANAGEMENTUMTSMCCUMTS L-bandUHFEGNOSNAVS&R
COSPAS-SARSATGROUND SEGMENTExternal ComplementaryUSER SEGMENTSystems
Figure 1: GALILEO System Architecture
The GALILEO services are the result of the combination of the system capabilities of each of the components and segments of the GALILEO architecture: global, regional, local and the user segment. GALILEO also provides services resulting from the use of other existing GNSS systems.
GALILEO Signals and Data: The GALILEO constellation provides the capability of broadcasting globally a set of five navigation signals supporting the open, commercial, safety-of-life and public regulated services. Each navigation signal is composed of one or two ranging codes and navigation data as well as, depending on the signal, integrity, commercial and search and rescue data. Satellite-to-user distance measurements based on ranging codes and data are used in the GALILEO user receivers to fulfil the different GALILEO services.
Encryption: Ranging codes and data can be open or encrypted in order to exercise control over the service access. Encryption is also a capability which could be activated permanently or temporarily, therefore allowing a dynamic allocation of signals and data to services in order to respond to the evolution of the user needs and markets or other general considerations, such as
Service Denial: Denial is the capacity of the system to deny access to a system capability, in order mainly to prevent misuse by unauthorized users. Denial of access to encrypted ranging codes or data can be fulfilled by an adequate management of keys.
OPEN, COMMERCIAL AND SAFETY-OF-LIFE NAVIGATION SERVICES
Two open navigation signals are commonly used by the open, commercial and safety-of-life services. The two signals are separated in frequency in order to allow the fulfilment of precise ionospheric measurements by differentiation of the ranging measurements made at each frequency. Each navigation signal will consist of two ranging codes. Data are added to one of the ranging codes while the other ranging code is dedicated to supporting the fulfilment of more precise and robust navigation measurements. In principle no ranging code will be encrypted since the signals are intended to support open and safety-of-life services, however, it may be considered the encryption of one of the data-less ranging codes for commercial applications . In this case, the intention would be to offer this commercial encrypted data-less ranging code to value-added service providers developing local contributions such as wireless communication networks (UMTS-GSM) station-based position location for urban environments. It is to be analyzed whether this option would still be compatible with the performance requirements of the safety-of-life service.
A third navigation signal separated in frequency from the two above signals is intended for supporting the development of precise local area elements, based on the use of Three Carrier phase Ambiguity Resolution techniques (TCAR). Encryption of the ranging code on this third carrier is a built-in capability on the satellites, which could be activated or not, if it proves to be
useful for the development of TCAR as a commercial service or as an open capability within the GALILEO global component.
Integrity data, required for safety-of-life applications, will be provided The integrity data in the open signal can be encrypted,
thus opening the possibility of providing also the integrity data to develop commercial services.
Commercial data (e.g. corrections, maps, …) can be disseminated through the open signals for the provision of commercial services.
PUBLIC REGULATED NAVIGATION SERVICES
Two regulated navigation signals with encrypted ranging codes and data. These two signals will nominally occupy separate frequency spectrum with respect to the signals used for open, commercial and safety-of-life services described above.
Local components provide specific signals and data which enhance the performance of the services achieved directly from the satellite signals where satellites are not in view (in-door, underground, tunnels, …). They are considered for commercial,
safety-of-life and public regulated services. It is to be noted that commercial services can be developed by local components, even on the basis of open navigation signals. This is the case of TCAR applications, for which the encryption of the ranging code on the third navigation signal does not appear to be strictly required, taking into account that for users to exploit this
TCAR capability, they require local measurements. Those local measurements could therefore be encrypted providing namely a means of user access control.
Also, commercial and open local services could co-exist if the service providers are separated geographically. The local nature
of these services guarantees an adequate protection of the commercial data.
The local component will provide the interface between GALILEO and wireless communications network.
EUROPEAN GEOSTATIONARY NAVIGATION OVERLAY SERVICE (EGNOS)
EGNOS will provide the GPS/GLONASS (GNSS-1) augmentation services. EGNOS is currently under development and is planned to provide its advanced operational capability (AOC) by end of 2003. EGNOS is being built according to international (ICAO) standards and will be interoperable with other equivalent regional SBAS systems, WAAS in North America and MSAS in Japan.
SEARCH-AND RESCUE SERVICES
The Search and Rescue Transponder of the GALILEO satellites supports the provision of an enhanced COSPAS-SARSAT Search and Rescue service through GALILEO. When a user sends a distress message from his/her COSPAS-SARSAT beacon, this message is received by the S&R transponder and down-linked to ground for reception by a COSPAS-SARSAT ground station which will forward it to a rescue centre for further processing. These transmissions are made in frequency bands already allocated for this service. When action has been taken, the COSPAS–SARSAT ground segment sends feedback
message (an acknowledgement message) or co-ordination message to the GALILEO Ground Segment. This message is to be sent to the originator of the alarm. This is achieved by up-linking the message to a satellite in visibility of the user. The message is then included in the navigation signal, which will then be received by the user, if equipped with a GALILEO
receiver. Enhanced Search and Rescue services would be possible allowing limited message exchange between the user and the Search and Rescue centre. This is an aspect to be further investigated. Moreover, the implementation approach for the GALILEO Search and Rescue Service is under co-ordination with COSPAS-SARSAT parties.
A Communications Transponder on the GALILEO satellites is currently under evaluation. This transponder would provide the capability to support Navigation-Related communications services.
SIGNAL AND FREQUENCY PLAN
The navigation signals defined above are made available to the users by modulating the ranging codes and data in radio-frequency carriers which are then transmitted by the navigation payload on-board each satellite.
The GALILEO frequency plan shall respect the radio-regulations in force as they are discussed and agreed at the International Telecommunications Union (ITU) forums such as the World Radio-Communication Conference (WRC).
The available spectrum which can be used for the development of Radio-Navigation Satellite Systems is shown in Figure 2. In this figure, a number of frequency bands are identified for GALILEO. Out of the definition studies, four frequency bands have been retained for the setting up of the GALILEO signals. Those are presented below. A tentative allocation of the five GALILEO navigation signals into frequency bands has also been done on the basis of the transmission of four carriers, one for each frequency band. Other solutions with three carriers only may be possible but still require further investigation:
- E5 and L5, covering the range 1164 MHz to 1215 MHz. Within this band, the use of 24 MHz of spectrum is being
considered with the final selection of the centre frequency depending on interoperability issues with E5/L5, co-existence
with other services such as DME, JTIDS/MTIDS, and on GALILEO autonomy requirements. The current studies
recommend centre frequency of 1202 or 1207 MHz. In E5/L5, an open signal for supporting the Open and Safety of Life
Service can be included;
- E6, 1260 to 1300 MHz. Within this band, the use of 30 MHz of spectrum is being considered, to accommodate the signals
for the Public Regulated Service and the Open (Commercial-encrypted, TCAR) Service.
- E2, 1559 to 1563Mhz. This band would accommodate a signal for the Public Regulated Service.
- E1, 1587 to 1591MHz. This band would accommodate a signal for the Open and Safety of Life Service.
Depending on the bandwidth considered on each of these bands, chip rates for the signals ranges from 2-4 Mcps for the E1 and E2 carriers, to 5-10 Mcps for the E6 carrier (chip rate limited due to the combined use for PRS and TCAR), and 10-20 Mcps for the E5 carrier. Available data rates on each carrier are up to 1000 bps. Since, for the data supporting the computation of position, only 50 bps are strictly required (e.g. GPS), there is sufficient capacity to accommodate Integrity, Search and Rescue
and Commercial data within each of the data channels supported by each carrier.
The navigation message is being defined as a number of frame types to be repeated or sent as needed for each type of data. In this way, when data channels are not fully occupied, the basic navigation data can be repeated more often which would reduce the time to first fix or to reacquire on the receivers.
Alternative frequency plans are also under evaluation addressing the sharing of bands with GPS and GLONASS. Inter-operability and performance are issues being considered when evaluating those alternative scenarios.
A selection of the optimum GALILEO signal structure and associated frequency plan is planned in 2001.
Lower L-BandC-BandUpper L-Band
ARNSARNSARNSRNSSRNSS*RNSS*RNSSRNSSGalileo E2Galileo E5Galileo E6Galileo C1
Galileo E3GlonassGlonassG2G1GPS L5GPS L2GPS L1
Figure 2: RNSS Frequency Spectrum 5030MHz1587MHz1188MHz1260MHz 1261MHz
DEFINITION OF SERVICES
The GALILEO Open Service provides positioning, navigation and timing signals that can be accessed free of direct charge. 1239MHzThis service is suitable for mass-market navigation applications, such as in-car navigation and applications of positioning with
mobile telephones. The Open Service also provides a precise timing service (UTC) when used with receivers in fixed locations. This timing service can be used for applications such as network synchronisation or scientific applications.
The performance for the Open Service is given in Table 1.
Table 1: Open Service performance characteristics
The Commercial Service provides added value with respect to the Open Service. The specific capabilities of GALILEO, which can be exploited for the Commercial Service, are mostly related with the design of the signal which supports:
- Dissemination of encrypted value-added data in the Open GALILEO signals.
- Very precise local differential applications (Sub-meter accuracy) using the open (option encrypted) signal overlaid with
the PRS signal on E6.
- Pilot signal for supporting integration of GALILEO positioning applications and wireless communications networks.
The performance of these services would be defined by the service providers based on the quality of the commercial data broadcast and by the performance provided by the local components.
Safety of Life Service
The performance of the Safety-of-Life service is compatible with the requirements of the Approach with Vertical Guidance (APV-II) as defined by ICAO SARPs . Through the Definition Phase, it has been verified that the performance needs of other modes of transport (land, rail, maritime) are covered adequately through those requirements. The service availability above 99.9% would make it usable for primary means only. Combination of this GALILEO service either with the current GPS as augmented by EGNOS corrections, or the future improved GPS and EGNOS integrity-only, would support CAT-I performance and offer the prospect of sole means availability. Other applications covered would be ship docking, train control, advance vehicle control, robotics (satellite signals combined with local components when required).
A single frequency Safety-of-Life service, with a similar level of performance as the dual frequency service and usable as degraded mode in case of local interference on one GALILEO frequency would be possible if GALILEO transmits a detailed map of the ionosphere, as it is being done by EGNOS. This aspect is currently under technical and economical examination. Alternative techniques to enable accurate single frequency operation are being evaluated.
The coverage area of the GALILEO integrity service is global, and to this extent, the system architecture is being optimised for
this requirement. Options for implementation of integrity will be analysed taking into account regional certification and liability constraints.
Table 2: Service Performance for Safety of Life Service with the
Satellite Navigation Signals only and without any other augmentations/elements
Public Regulated Service
The Public Regulated Service is provided on dedicated frequencies to provide the capability for greater continuity of service placed under EU Governments control for:
- Public applications devoted to European and/or National Security, such as police, civil protection, law enforcement, civil protection such as some emergency services, GMES and other governmental activities,
- Some regulated or critical energy, transports and telecommunications applications,
- Economic and industrial activities that are deemed of strategic interest for Europe.
The Public Regulated Service is robust, so as to be resistant to interference, jamming and other accidental or malicious aggressions.
Table 3: Service Performance for Public Regulated Service with
the Satellite Navigation Signals only
Navigation Services to be provided by Local Components
Local components providing differential corrections for single frequency users would reach positioning accuracy better than 1 meter. Those stations could report,also, integrity with a time to alarm of 1 second. It is expected that local service providers
will adapt the signal format to accommodate additional data.
The exploitation of the TCAR technique with local components allows users to determine their position with errors below 10 centimetres.
The pilot signal, provided with the open signals, enhances the performance of wireless telecommunications networks (GSM/UMTS)-assisted position determination applications in difficult environments (e.g. urban canyon and indoor applications).
Table 4: Performance for Services combining Satellite and Local Component signals
Local stations broadcasting satellite-like signals (pseudolites) are used for increasing the availability of GALILEO service in a
defined local area.
Search and Rescue Service
The GALILEO Search and Rescue service shall be co-ordinated with the existing COSPAS-SARSAT service and be compatible with both GMDSS and Trans European Transport Network guidelines. GALILEO will allow to improve the time to detection and the accuracy of location of distress beacons with respect to current search-and-rescue system performance. The position determination of the distressed beacons is carried out by COSPAS-SARSAT on the basis of the signals and data provided by the GALILEO Search and Rescue Service. Performances of position determination will be in the range of 5 km for the current beacons, to less than 10 meters for advanced beacons equipped with GALILEO receivers.
Table 5: GALILEO service performance for Search and Rescue Service
Navigation Related Communication Service
The baseline for this service is the combined use of GALILEO with existing wireless, terrestrial (e.g. GSM/UMTS) or satellite networks.
Table 6: GALILEO service performance for NRS
The possibility of providing an on-board communication payload within GALILEO satellites is under consideration.
This service is appropriate for regulated applications requiring global and high availability and reliable position reporting. The
service allows the quasi-instantaneous transmission of short messages from users to a service centre and vice versa.
The GALILEO Definition Phase executed in 2000 has lead to a sound definition of GALILEO System Architecture and Services. This paper has summarised the GALILEO System Architecture consisting of a Global component, Regional components, Local components, User Segment and Service Centres. It has also summarised the main system capabilities in terms of signals and data, encryption and service denial, and has provided a mapping with services. The different GALILEO services have been introduced, namely the Open service, the Commercial Service, the Safety-of-life service, the Public Regulated service, the services to be provided by Local components, the Search-and-Rescue service, the navigation-related communication service and the EGNOS-related services.
During 2001, the GALILEO Phase B2 is being executed, leading to a finalisation of the definition of services and the associated preliminary system design. GALILEO is planned to provide its full operational capability by 2008.
The authors wish to acknowledge the many valuable contributions made by industry during the GALILEO Definition Phase, as well as the co-operative efforts of EC and ESA experts and their support teams which have lead to the current status of definition of the GALILEO System and Services.
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