GPS User Segment

GPS
Title	GPS User Segment
Edited by	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.

The GNSS Market Report, Issue 3, provided by European GNSS Agency, estimated that the number of GNSS enabled devices in 2012 were about two billion units, from which the 100% were GPS capable. The GNSS installed based in 2019 has increased to 6.4 billion units, in which all of them are at least GPS enabled.

GPS Receivers

A GPS Receiver is a device capable of processing the signal of the GPS satellites and 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).

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.

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

• IS-GPS-200: 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-705: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for radio frequency link 5 (L5).
• IS-GPS-800: interface between the space segment of the Global Positioning System and the navigation user segment of the GPS for signal L1 Civil (L1C) transmitted in the frequency band of L1.

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.

Applications

GPS applications are all those applications that use GPS to collect position, velocity and time information to be used by the application. As stated by the US Government, 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: GPS provides position determination for all phases of flight from departure, en route, and arrival, to airport surface navigation.
• Environment: GPS provides accurate and timely information to support better decision making related to the Earth's environment
• Marine: GPS provides the fastest and most accurate method for mariners to navigate, measure speed, and determine location. This enables increased levels of safety and efficiency for mariners worldwide.
• Public Safety and Disaster Relief: A critical component of any successful rescue operation is time. Knowing the precise location of landmarks, streets, buildings, emergency service resources, and disaster relief sites reduces that time -- and saves lives. This information is critical to disaster relief teams and public safety personnel in order to protect life and reduce property loss. The Global Positioning System (GPS) serves as a facilitating technology in addressing these needs.
• Rail: 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.
• Recreation: GPS has eliminated many of the hazards associated with common recreational activities by providing a capability to determine a precise location. GPS receivers have also broadened the scope and enjoyment of outdoor activities by simplifying many of the traditional problems, such as staying on the “correct trail” or returning to the best fishing spot.
• Roads and Highways: 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.
• Space: GPS is revolutionizing and revitalizing the way nations operate in space, from guidance systems for crewed vehicles to the management, tracking, and control of communication satellite constellations, to monitoring the Earth from space.
• 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.
• Timing: GPS enables users to determine the time to within 100 billionths of a second, without the cost of owning and operating atomic clocks. Precise time is crucial to a variety of economic activities around the world. Communication systems, electrical power grids, and financial networks all rely on precision timing for synchronization and operational efficiency.

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