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GPS technology and its applications
Major components of Global Positioning System (GPS)
The system of GPS consists of three segments: (Fig.1) the space segment comprises the satellites: the control segment deals with the management of the operations of satellites and the user segment covers the activities related to GPS users. However, the system description, its components and mode of operations of different segments depend on the type of radio navigation system in use such as Navstar GPS, Glonass (Global Navigational Satellite System) etc.
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Space Segment:
The space segment of GPS consists of 24 satellites fielded in nearly circular orbits with a radius of 26,560 km, period of nearly 12 hours and stationary ground tracks. The satellites are arranged in sis orbital planes inclined at 550 relative to the equatorial plane, with four satellites distributed in each orbit. With this constellation, almost all users with a clear view of the sky have a minimum of four satellites in view. Each satellite receives and stores information from the control segment; maintain very accurate time through on board precise atomic clocks.
Figure 1: The major components of GPS
- Control Segment:
The control segment of GPS consists of five tracking stations distributed around the earth of which one, located in Colorado Springs, is a Master Control Station. The control segment tracks all satellites, ensures they are operating properly and computes their position in space. The computed positions of the satellites are used to predict where the satellite will be later in time. These parameters are uploaded from the control segment to the satellites and referred to as broadcast ephemeredes.
- User Segment:
The GPS user segment consists of the GPS receivers and the user community. Almost all GPS tracking equipment have the same basic components: an antenna, an RF (Radio Frequency) sections a microprocessor, a control and display unit (CDU), recording device and a power supply. Usually all component, with the exception of the antenna, are grouped together and referred to as a receiver. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and time. GPS receivers are used for navigation, positioning, time dissemination, and other research.
- GPS Signals:
Each GPS satellite continuously broadcasts ranging signals containing wealth of information. The information contained in GPS signals includes the carrier frequencies (L1 & L2), codes (coarse acquisition [C/A] & Precise [P]) and the navigational message. These allow users to measure their pseudo ranges and to estimate their positions in passive, listen only mode.
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