What is GPS (Global Positioning System)
The Global Positioning System (GPS) is known as a radio navigation system concerning satellites and computers which will compute the latitude and longitude of a receiver on the world by computing the time difference for signals arriving from several satellites to the receiver.
GPS is employed to support a broad range of military commercial and consumer applications.
There are almost 33 GPS satellites out of which 27 satellites are active and rest are spare, situated in 6 orbits at a height of 12540 miles 20180 km above the Earth’s surface.
The positions of the satellites are such from any point on the surface of the earth, every four satellites are going to be above the horizon.
The GPS satellites contain a computer, an timepiece and a radio. Each satellite continuously broadcasts its changing position with time to the receiver on the world .
The receiver contains a computer, which triangulates its own position by getting bearings from three of the four satellites.
So the exact location of the receiver at a particular time instant will be determined in terms of latitude and longitude.
If the receiver contains a video display, the position is displayed over the map with regard to time.
The moving receiver also can determine the speed, direction of translation and therefore the estimated time to succeed in a particular destination.
In earlier times, GPS was referred as ‘Navigation System with Timing and Ranging’ or ‘NAVSTAR’. But, GPS has applications beyond navigation and site determination.
It is used for cartography, forestry, mineral exploration, wildlife habitation management and therefore the movement of humans and materials over the globe over.
Functioning of GPS (Global Positioning System)
The GPS satellites are orbiting the world continuously, The radio signals from the satellites are controlled and corrected by control stations.
Signals are received by the GPS receiver on the planet. The GPS receiver imperatives 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 thing over the surface of the earth surface.
The entire functioning is meted out by three main segments (Fig. A.2). they’re as Space Segment, Control Segment and User Segments
1. Space Segment
The space segment contains of 30 GPS satellites inclined at 55° and orbiting around in every 12 hours from a height of 10660 miles over the Earth’s surface.
Due to earth’s rotation on its own axis, a satellite will take 24 hours for an entire rotation round the earth.The upper altitude covers an oversized area over the surface of the earth.
The positions of the GPS satellites are specified every four satellites cover a particular point (receiver) on the Earth’s surface.
2. Control Segment
The control segment consists of 5 unnamed monitor stations and one master station. The monitor stations continually receive radio signals emitted by the GPS satellites.
And transmitted to the master station, to its necessary correction on time and orbital position. The corrected information is then sent back to the GPS satellites by ground antennas.
3. User Segments
The user segment formation of the user and their GPS receivers and it’s number is numerous.
The signals transmitted by the satellites take approximately 67 milliseconds to succeed in a receiver.
Four different signals are generated within the receiver having a similar structure as those received from the four satellites.
Through synchronizing the signals generated, the 4 signal time changes “∆t” are measured as a timing mark.
The time shift for all four satellite signals are wont to determine the signal transit time. The signal transit time is employed for determine the space of the respective satellites.
After the receiver calculates the latitude, longitude, height and time of the user from the defined range of the four satellites.
The signal despatched with various C/A codes by the satellite is employed by CDMA processing for this purpose.
GPS Signal Transmission
The GPS satellite transnlits the signal and data synchronised on broad atomic clocks at a frequency of 1575.4 MHz.
The signal strength is recived by the planet starting from 158 dBW to 160 dBW. The satellite transmits signal at a rate of 50 bits/second.
By using navigation messages, the reciver determines the time period for every of those four satellites to locate the precise position of them within the orbit (Fig. A.3).
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 information is modulated and transmitted as 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).
Principle of Global Positioning System (GPS)
The flashing because of lightning during a stormy night can determine the gap between the flushing point and also the position of the observer.
The interval between observing the flashing of sunshine and hearing of its sound multiplied by the travelling speed (330 m/sec) of sound gives the specified distance.
The GPS system functions following an equivalent principle.
To see the precise position, the space is measured by the transit time between the GPS satellite to the purpose of observation multiplied by the speed of transmission, i.e. the speed of sunshine.
GPS Navigation Message
The navigation message comprising of the subsequent 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 the position of the satellite and transit time of data.
The data is transmitted in logical format, referred to as frames, is 1500 bits long and takes 30 seconds, to transmit.
Each frame is again sub-divided into 5 subframes having 300 bits long in size and takes 6 seconds for transmission.
Actually, there are 25 such frames required to finish the whole data sets or almanac in 12.5 seconds.
Application Of Global Positioning System (GPS)
By using GPS, the subsequent two values may be determined:
l . The precise 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, Greenwich Mean Time accuracy up to 60 ns to 1 ns.
The additional parameters like speed, acceleration, time, measurements of approximate period of time for a location are to be determined by using GPS.
Recent use in electronic car navigation system opens a replacement area of locating objects on the Earth’s surface.
Actually, Global Positioning System (GPS) will be used anywhere, where satellite signal and reception is feasible .