GPS User Segment

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

The GPS User Segment consists on L-band radio receiver/processors and antennas which receive GPS signals, determine pseudoranges (and other observables), and solve the navigation equations in order to obtain their coordinates and provide a very accurate time.

GPS Receivers

A GPS Receiver is a device capable of determine 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 obtain the navigation message, the satellite’s signal shall be detected and tracked. The receiver’s elements that perform these tasks are the antenna, the front-end and the baseband signal processing (in charge of acquiring and tracking the signal).

Once the signal is acquired and tracked, the receiver application decodes the navigation message and estimates the user position. The Navigation Message includes:

• Ephemeris parameters, needed to compute the satellite’s coordinates
• Time parameters and Clock Corrections, to compute satellite clock offsets and time conversions
• Service Parameters with satellite health information
• Ionospheric parameters model needed for single frequency receivers
• Almanacs, needed for the acquisition of the signal by the receiver. It allows computing the position of all satellites but with a lower accuracy than the ephemeris

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

In order to decode the navigation message properly, GPS provides 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 the Global Positioning System (GPS) provides a critical fourth dimension: 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.

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:

• 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, flee 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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