Navigation Requirements

• Reliable Positioning System with Quality of Service
• Positioning Service with Integrity information about the displayed Position
• High coverage in Urban Environments
• Good Accuracy for both Horizontal and Vertical Positions
• “Wearable” GNSS receiver
• Connectable GNSS Receiver to other on person equipment
The following table summarises the main User Needs for the different Sector and Market segments. The most important parameters are related to the Integrity Concept and are highlighted in yellow.

Available Navigation Technologies

There are currently two satellite navigation networks in the world, one developed and managed by United States (GPS) and the other Russian (GLONASS). The GLONASS signal has been continuously deteriorating in recent years although more recently there have been moves to augment the technology. The GPS system was subject to Selective Availability, which meant that the service could be removed or degraded for strategic purposes. This had a significant impact on its accuracy. Since Selective Availability (SA) has been discontinued, an accuracy of 10-20m (usually better) is achievable.
Both systems have been designed to determine the position of military targets, vehicles or units, with great accuracy. These networks are used for civil purposes but show several serious drawbacks, i.e. among others:
• no guarantee of service or liability cover is provided by their operators with all the implications that might imply in the event, for example, of air accidents;
• reliability which is uncertain (lack of integrity information): e.g. users are not informed immediately of errors that occur, and transmission is sometimes unreliable, particularly in towns and regions situated in the high latitudes of northern Europe;
• applications requiring better performances (below 10m) have to rely on local augmentation (differential) or hybridization (inertial sensors) that are either expensive or local by nature.
The following sections consider a range of navigation technologies including both satellite and ground based techniques (and some employing both). The sattelite technologies are GPS, Differential GPS, EGNOS, Gallileo.

GPS – Global Positioning System

GPS was the first positioning satellite system, commencing operation at the beginning of the 1990’s. It has been under the control of the American DoD (Department of Defence). It offers two types of services, one of precision (narrow only to the military field) and one free (SPS). The SPS performance is shown in the following table.

SPS Horizontal Accuracy. 100m (95%)
SPS Vertical Accuracy 156m (95%)
Speed Accuracy 0,2 m/s (95%)
Service Availability 99.85%
Service Reliability 99.97%

A constellation of 24 satellites wheel circularly around to the Earth. At least four satellites are always visible from every point of view (Within cities this is not always true due to an effect known as Urban Canyons). A GPS receiver receives transmitted signals from the visible satellites in its position and calculates, with information delivered from the satellite, its position in LATITUDE, LONGITUDE and ALTITUDE.

AgGPS – Augmented GPS

AgGPS is an evolution of the GPS. The AgGPS receiver is equipped of a system RAIM (Receiver Autonomous Integrity Monitoring) through which it calculates the performance (accuracy) of GPS system. Four satellites are necessary in order to calculate the position of every user at a particular moment. If five satellites are visible at one moment, the receiver calculates the 5 possible solutions using all the possible combinations of four satellites and discards that solution that goes away from the other solutions. In such a way the satellites with a low contribution to the determination of the position can be easily discarded from the receiver (therefore in an INDEPENDENT way that is without support of earth stations members), but the technique always demands that full geometry is available in order to find multiple errors.
Fundamentally, AgGPS adds the function of INTEGRITY which is the ability of a positioning system to supply, in suitable time, a warning message to the user when the system should not be used for Navigation.
Positioning accuracy is currently about 5 metres over the horizontal plane since the removal of Selective Availability.

DGPS – Differential GPS

DGPS is based on the same idea of the other augmentation systems.
A correction signal is generated at a base station Reference Site, knowing the precise position of that site. This differential correction signal is broadcast in the VHF or UHF band or via satellite to stations receiving the original GPS signal.
The performances are generally better than GPS alone. Within confined areas that might include a dedicated reference station, accuracy can be of the order of centimetres.

EGNOS (European Geostationary Navigation Overlay Service) System Test Bed

EGNOS is a regional satellite-based augmentation system for both GPS and GLONASS. The task of EGNOS is to augment the GPS and GLONASS signals with additional information, which will enhance the accuracy, availability and integrity. The signals of both GPS and GLONASS satellites are tracked by a set of ground stations and processed at central processing facilities. Differential correction data and integrity information for GPS and GLONASS are transmitted via up-link stations to three Geostationary satellites and re-broadcasted to the users.
At the moment, an EGNOS System Test Bed (ESTB) is available. The ESTB, which became operational in January 2000, is a real-prototype of EGNOS, providing the first continuous GPS augmentation service within Europe. The ESTB architecture is made up of a space segment comprising nominally two transponders onboard the Inmarsat-III Atlantic Ocean and the Indian Ocean satellites, a ground segment comprising a number of reference stations (RIMS) spread across Europe and beyond, a processing centre and the Inmarsat up-link stations. Communication links interconnect all stations.
By using GPS and ESTB Signal-In-Space users within Europe can nowadays determine their positions with an error of less than 3m horizontally and 5m vertically, for 95% of the time.
The ESTB is also providing an integrity service, represented by the vertical and horizontal protection levels computed by the user with the ESTB information data, which are to bound with a probability of 2*10-7/150 sec the Alert limits associated with a particular operation.

GALILEO

GALILEO will be the first satellite network to provide navigation and positioning system for civilian goals under the direct control of European commission. Its applications and benefits are applicable in many areas such security and transports. It will be possible to determine own position through little satellite receivers with the accurance of few meters. The GALILEO system is composed of 30 satellites on 3 circular orbits at an altitude of 24000 km to cover all the terrestrial surface. This satellites will be supported by an international network of terrestrial stations.
It could be used without limitations for civil customers and will guarantee services that will not be limited to the simple determination of the position; indeed it will be structured in such way to supply:
• High precise position accuracy for mobile vehicles shorter than 4m
• Guaranteed coverage at high altitude (over the 75°N)
• GPS interoperability
• EGNOS integration
• Integrity Information Control with signal within max 6sec
• Communication system for Acknowledge messages
• SAR-SAT (Search And Rescue) Built

GALILEO Operation

The Galileo system will be different from the GPS because the European Union in collaboration with the european space agency have developed a system that reach high level of accurance, reliability and safety.l This satellite network will provide a different satellite signal that will provide a certification on the delivered position.
The satellite network will offer different types of service:
• Open Access Service (OAS)
The GALILEO Open Service provides positioning, velocity and timing information that can be accessed free of direct charge. This service is suitable for mass-market applications, such as in-car navigation and hybridisation with mobile telephones. The timing service is synchronised with 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 objectives in terms of position accuracy and availability will be competitive with respect to existing GNSS and further planned evolutions. In addition, the Open Service will also be interoperable with other GNSS, in order to facilitate the provision of combined services.

The Open Service signals are separated in frequency to permit the correction of errors induced by ionospheric effects by differentiation of the ranging measurements made at each frequency. Each navigation frequency will include two ranging code signals (in-phase and quadrature). Data are added to one of the ranging codes while the other “pilot” ranging code is data-less for more precise and robust navigation measurements.