Global Positioning System – An overview


GPS –Definition:

Global Positioning System is based on Satellite Navigation. A network framed with 24 satellites is used here (actually 27satellites, but 3 satellites are left as standby). This satellite network revolves the earth two times a day. It travels in a fixed orbit. It provides continuous 3 dimensional positioning (latitude, longitude and Height) for the whole day, through the entire world.

These solar powered satellites are launched by the U.S Government for the purpose of defense. But now, it is used in many applications throughout the world, to find the correct location anywhere in the earth.

Components of GPS

GPS mainly consists of two components:

  • A Transmitter (Satellite)
  • A Receiver (GPS device)

Transmitter:

The satellites are the transmitters which gives us the details about the Position, and distance of particular point in the earth. These transmitters are controlled by, one master control station and 5 monitoring stations from earth.

It continuously broadcast navigation signals and send navigation messages in 3 formats. They are:

  1. The details of Transmitting satellite- Pseudorandom code
  2. Orbital details of satellites – Ephemeris
  3. Position and condition of satellite- Almanac

Receiver:

The receivers are the handy GPS devices, which the user can design or purchase. It receives the signal from the satellites and corrects the error and displays the position of the required area. It comprises of:


  • A receiving antenna
  • Controlling unit
  • Display unit
  • Recording Unit
  • Power supply Unit
  • All along with a Microprocessor

Working:


In the revolving orbit of 24 satellites, GPS figures out any 3 (exactly 4) satellites and use it for positioning purpose. When we switch on the GPS receiver, it immediately sends a signal to the satellite network and gets linked with any three satellites.

It calculates the time taken by the signal to reach the satellite. The signal travels with the velocity of a light. We know that Distance = Time x Velocity. So, by multiplying the velocity with the time taken, it calculates the distance.

Then, the position of the GPS receiver is calculated by the principle of trilateration. ie., by taking the time point of intersection of three satellites.

When the signal travels, there may be a time mismatch between the satellites time (which it sends as a packed data – the navigation message) and the receiver time. The errors of synchronization of the satellites atomic clock and receiver’s quartz clock are corrected by this principle of trilateration. So, we get the correct position of the receiver.

Satellite Links to a GPS receiver

Error sources:

  • Non-sync of transmitter and receiver clock
  • Noise disturbances
  • Multi path error
  • Delay due to crossing Ionosphere layer and Troposphere layer.

Applications of GPS:

  • Land – In vehicles (Car, Bikes, Buses etc.)
  • Water – In Ships
  • Space – In all air moving vehicles (Aeroplanes, Jets etc)
  • In Military, scientific applications, Surveying, Navigations,

Advancement and Future scopes in GPS:

  • Correction made with DGPS.
  • GPS technology is embedded in many advanced Mobile phones.
  • GPS communications is being happening with Bluetooth.
  • Many Communication systems are implied of GPS.
  • Most GPS receivers are enabled with map display.

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