Global Positioning System (GPS): Description, Accuracy, and How it Works

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Introduction
Global Positioning System (GPS) is a global radio-navigation system that was developed by the
United States Department of Defense to offer location, navigation and timing information for the
military operations. The testing of systems by using limited number of satellites was officially
launched in 1978 with the system being fully declared functional in 1975.The system became
available for the civilians to use in the 1980s and since then it has seen growing civilian
application for mapping and navigation [1].
GPS description
The GPS comprises of three basic elements: user segment, space segment, and control segment.
The space segment comprises of the constellation of up to 24 active NAVSTAR satellite in six
orbital tracks [2]. The satellites are not in the geosynchronous orbit and are usually in constant
motion relative to the user on the ground.
The control segment
The control segment is made up of many ground-based stations which usually serves as an
uplink to the satellites, and that makes adjustments to the satellite orbits and clock when there is
a need.
User segment

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The user segment is made up of the GPS receiver which is consist of the multi-channel receiver,
antenna and the processing unit [3]. The user segment comprises receivers which decode the
signals received from the satellites.
Functions of Antenna:
Exposure of the electromagnetic waves received from the satellites
Change the wave energy into Electric Current
Hands the Signals over to the Receiver Electronics
Amplifies the signal strength
GPS Accuracy Factor
The accuracy of the basic GPS system is roughly 15m. The accuracy can be affected by a
number of factors such as atmospheric delays, Availability features, orbit errors and satellite
clock, signal strength. satellite geometry in relation to the user and multipath signals.
During the initial implementation of the GPS, the Department of Defense used a feature known
as selective availability (SA) to degrade the accuracy and subsequent quality of the GPS signals
to non-DoD users. By having selective availability enabled, the accuracy of position fixes could
be as poor as 100m without the application of differential correction method [4]. In the Modern
times, there is no Selective Availability limitation in accuracy in regions with stated Executive
Branch intention to not return to the use of the Selective Availability signal degradation.
Some of the signals are blocked by obstructions as the satellites keep moving in their own orbits
due to that the geometry of the available satellite signals relative to the user will constantly be
changing. The moment the satellites having available signals are clustered closely together in the