Global Positioning System (GPS)

Learn how the Global Positioning System (GPS) works, including its components—space, control, and user segments—and how satellites, signals, and receivers collaborate to pinpoint precise locations. Explore GPS applications in navigation, cartography, mineral exploration, and more, along with insights into signal transmission, navigation messages, and system accuracy.

what is global positioning system

The Global Positioning System (GPS) is a radio navigation system involving satellites and computers that can determine the latitude and longitude of a receiver on the earth by computing the time difference between signals reaching the receiver from different satellites. GPS is utilized to support a wide range of military, commercial, and consumer applications Global Positioning System .

There are almost 30 GPS satellites, 27 of which are active and the rest are spare. They are situated in 6 orbits at a height of 10600 miles above the earth’s surface. The positions of the satellites are such that from any point on the earth, every four satellites will be above the horizon.

The GPS satellites contain a computer, an atomic clock, and a radio. Each satellite continuously broadcasts its changing position with time to the receiver on the earth. The receiver contains a computer, which triangulates its position by getting bearings from three of the four satellites. So the exact location of the receiver at a specific time instant can be determined in terms of latitude and longitude. If the receiver contains a display screen, the position is displayed over the map concerning time. The moving receiver can also determine the speed, direction of translation, and the estimated time to reach a specific destination Global Positioning System .

In earlier times, GPS was referred to as a ‘Navigation System with Timing and Ranging’ or ‘NAVSTAR’. But, GPS has applications beyond navigation and location determination. It is used for cartography, forestry, mineral exploration, wildlife habitation management, Global Positioning System and the movement of humans and materials over the world.

A. Functioning of GPS

The GPS satellites are orbiting the Earth continuously. The radio signals from the satellites are controlled and corrected by control stations. Signals are received by the GPS receiver on the earth. The GPS receiver needs only three satellites to plot a 2D map. By using four satellites, it can correctly draw a 3D map to locate the geographical position of the object over the earth’s surface.

The entire functioning is carried out by three main components. They are as follows:

1. Space segment
2. Control segment
3. User segment

1. Space Segment

The space segment consists of 30 GPS satellites inclined at 55° and orbiting around every 12 hours from a height of 10660 miles above the earth’s surface. Due to the Earth’s rotation on its axis, a satellite will take 24 hours for a complete rotation around the Earth. The higher altitude covers a large area over the earth’s surface. The positions of the GPS satellites are such that every four satellites cover a specific point (receiver) on the earth’s surface. Satellite signals can be received anywhere within a satellite’s effective range.

Global Positioning System The low radio signal emitted continuously from the satellites has a definite frequency for allowing the receiver to identify the signals. The signal moves at a speed equal to that of light. The time elapsed for reaching the signal from the satellite to the receiver can ultimately determine the distance of the receiver from the corresponding GPS satellites Global Positioning System .

2. Control Segment

The control segment consists of five unnamed monitor stations and one master station. The monitor stations continuously receive radio signals emitted by the GPS satellites and transmit them to the master station for their necessary correction on time and orbital location. The corrected information is then sent back to the GPS satellites through ground antennas.

3. User Segments

The user segment consists of the user and their GPS receivers and its number is numerous. The signals transmitted by the satellites take approximately 67 milliseconds to reach a receiver. Four different signals are generated in the receiver having the same structure as those received from the four satellites. By synchronizing the signals generated, the four signal time shifts “△t” are measured as a timing mark.

The time shift for all four satellite signals is used to determine the signal transit time. The signal transit time is used to determine the distance of the respective satellites. The receiver then calculates the latitude, longitude, height, and time of the user from the known range of the four satellites. The signal transmitted with different C/A codes by the satellite is used by CDMA processing for this purpose Global Positioning System .

B. GPS Signal Transmission

The GPS satellite transmits the time signal and data synchronized on broad atomic clocks at a frequency of 1575.4 MHz. The signal strength is received by the earth ranging from -158 dBW to 160 dBW. The satellite transmits a signal at a rate of 50 bits/second. By using navigation messages, the receiver determines the travel time for each of these four satellites to locate the exact position of them in orbit Global Positioning System .

The generation of satellite signals comprises of Coarse/Acquisition (C/A) code, PNR code, and carrier frequency code or coarse reception code for L₁ carrier. The data is modulated and transmitted as a satellite signal.

The frequency of 1575.42 MHz as carrier frequency is processed through Code Division Multiple Access (CDMA), and data is transmitted by Direct Sequence Spread Spectrum Modulation (DSSSM).

C. Principle of GPS

The flashing due to lightning during a stormy night can determine the distance between the flushing point and the position of the observer. The time interval between observing the flashing of light and hearing its sound multiplied by the traveling speed (330 m/s) of sound gives the required distance.

The GPS system functions following the same principle. To determine the exact position, the distance is measured by the transit time between the GPS satellite to the point of observation multiplied by the velocity of transmission, i.e. the velocity of light Global Positioning System .

D. GPS Navigation Message

The navigation message comprises the following data transmitted at 50 bits/second.

1. System time and clock correction values
2. Highly accurate orbital data
3. Approximate data for other satellites (almanac)

The navigation message is calculated as the position of the satellite and the transit time of information.

The data is transmitted in a logical format, known as frames, is 1500 bits long, and takes 30 seconds, to transmit. Each frame is again subdivided into 5 subframes having 300 bits long in size and taking 6 seconds for transmission. There are 25 such frames required to complete the total data sets or almanac in 12.5 seconds.

E. GPS Application

By using GPS, the following two values can be determined:

1. The exact position of the observer in terms of latitude, longitude, altitude, and coordinates with an accuracy of 20 m to 1 mm, and
2. The time in terms of world time, universal time accuracy up to 60 ns to 1 ns.

The additional parameters like speed, acceleration, local time, and measurements of approximate travel time for a location are to be determined by using GPS. The recent use of electronic car navigation systems opens a new area of locating objects on the earth’s surface. Actually, GPS can be used anywhere, where satellite signal and reception are possible Global Positioning System.

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