Galileo User Segment

GALILEO
Title	Galileo User Segment
Author(s)	GMV
Level	Basic
Year of Publication	2011

The Galileo Global Component will also include a set of Test User Receivers. Their main function is to receive Galileo signals, determine pseudoranges (and other observables), and solve the navigation equations in order to obtain their coordinates and provide a very accurate time.

Basic elements of a generic GNSS Receiver are an antenna with pre-amplification, a L-band radio frequency section, a microprocessor, an intermediate-precision oscillator, a feeding source, some memory for data storage, and an interface with the user. The calculated position is referred to the antenna phase centre.

GALILEO Receivers

A GALILEO Receiver is a device capable of determining the user position, velocity and precise time (PVT) by processing the signal broadcasted by satellites.

Any navigation solution provided by a GNSS Receiver is based on the computation of its distance to a set of satellites, by means of extracting the propagation time of the incoming signals traveling through space at the speed of light, according to the satellite and receiver local clocks. Notice that satellites are always in motion, so previous to obtaining the navigation message, the satellite’s signal is detected and tracked. The receiver’s functional blocks that perform these tasks are the antenna, the front-end and the baseband signal processing (in charge of acquiring and tracking the signal).

The Galileo global navigation satellite system will employ many new methods and technologies to offer superior performance and reliability. Development of the advanced receivers required to make use of the system is continuing.

Once the signal is acquired and tracked, the receiver application decodes the navigation message. The navigation data contain all the parameters that enable the user to perform positioning service. They are stored on board each satellite with a validity duration and broadcast world-wide by all the satellites of the Galileo constellation. The 4 types of data needed to perform positioning:

• Ephemeris which are needed to indicate the position of the satellite to the user receiver
• Time and clock correction parameters which are needed to compute pseudo-range
• Service parameters which are needed to identify the set of navigation data, satellites, and indicators of the signal health
• Almanac which are needed to indicate the position of all the satellites in the constellation with a reduced accuracy
• Ionospheric parameters model needed for single frequency receivers

Three receiver development activities have been initiated within the Galileo programme, addressing the different needs of the system development process and covering the range of signals and services that will be offered.

Activities in receiver development are in the following areas:

• test user segment
• receivers for the signals transmitted by the first, experimental satellites
• receivers for the Galileo receiver chain

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The ephemeris and clocks parameters are usually updated every two hours, while the almanac is updated at least every six days.

The GALILEO Signal In Space is specified in the following documents:

• IS-GPS-200E: Interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for radio frequency link 1 (L1) and link 2 (L2)
• IS-GPS-705A: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for radio frequency link 5 (L5).

Receivers can be categorized by their type in different ways, and under different criteria. For instance, receivers can be stand-alone, or may benefit from corrections or measurements provided by augmentation system or by receivers in the vicinities (DGPS). Moreover receivers might be generic all purpose receivers or can be built specifically having the application in mind: navigation, accurate positioning or timing, surveying, etc. In addition to position and velocity, GPS receivers also provide time. An important amount of economic activities, such wireless telephone, electrical power grids or financial networks rely on precision timing for synchronization and operational efficiency. GPS enables the users to determine the time with a high precision without needing to use expensive atomic clocks.

Test user segment

The test user segment is being used for system validation and signal experimentation. Two parallel developments have been performed, with the aim of securing equipment availability and achieving the highest confidence in the results. The test user segment consists of:

• a test user receiver for the open, commercial and safety-of-life services
• a test user receiver for the public, regulated service
• search and rescue test beacon equipment
• test support tools, such as a simulator for the satellite constellation

The receivers are based on a highly flexible software-defined concept implementing 14 different receiver configurations. They are able to emulate different receiver classes and provide a variety of internal measurements when combined with an analysis sub-system running on an attached laptop computer.

Applications

GPS applications are all those applications that use GPS to collect position, velocity and time information to be used by the application. For instance, the position and velocity provided by GPS may be used for civil applications such as:

• Agriculture: GPS-based applications in precision farming are being used for farm planning, field mapping, soil sampling, tractor guidance, crop scouting, variable rate applications, and yield mapping.
• Aviation Applications: GPS provides position determination for all phases of flight from departure, en route, and arrival, to airport surface navigation.
• Rail Applications: Rail system use the GPS in combination with other sensors to maintain smooth flow of traffic, prevent collisions by precise knowledge of where a train is located, increase efficiency and capacity, etc.
• Road Applications: GPS may be used to provide in-vehicle navigation, fleet management, tolling applications, etc.
• Surveying and mapping: The main limitation of the traditional surveying techniques is the requirement for a line of sight between surveying points. Using the accurate position provided by GPS surveying and mapping results can be obtained faster and with a lower cost.

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