An overview of Global Positioning System (GPS)

This post describes the HISTORY of GPS, WHAT IS GPS?, GPS ELEMENTS,  HOW IT WORKS?, SOURCES OF GPS SIGNAL ERRORS,  APPLICATIONS

History

  • The GPS project was developed in 1973 to overcome the limitations of previous navigation systems.
  • GPS was created and realized by the U.S. Department of Defense and was originally run with 24 satellites.
  • It became fully operational in 1995. “Bradford Parkinson”, “Roger L. Easton”, and “Ivan A. Getting” are credited with inventing it.

What is GPS?

  • A space-based satellite navigation system provides location and time information in all weather.
  • Maintained by the United States government and is freely accessible by anyone with a GPS receiver.
  • Official name : “Navigational Satellite Timing and Ranging Global Positioning System” (NAVSTAR GPS)
  • Consists of 30+ GPS satellites in medium Earth orbit (2000km - 35,000 km).
  • Made up of two dozen satellites working in harmony are known as a satellite constellation
  • Mainly used for navigation, map-making and surveying.

GPS Elements

Three segments -

1.      Space segment.

2.      Control segment.

3.      User segment

 Fig: Space, Control and User Segment
Fig: Space, Control and User Segment

Space Segment

• GPS satellites fly in circular orbits at an altitude of 20,200 km and with a period of 12 hours.
• Powered by solar cells.
• The satellites continuously orient themselves to point their solar panels toward the sun and their antenna toward the earth.


Figure: Space Segment

• Orbital planes are centered on the Earth.

• Orbits are designed so that, at least, six satellites are always within line of sight from any location on the planet.

Control Segment

The CS consists of 3 entities:

            • Master Control System

            • Monitor Stations

            • Ground Antennas

Master Control System

        The master control station, located at Falcon Air Force Base in Colorado Springs

        Responsible for overall management of the remote monitoring and transmission sites

        Check-up is performed twice a day, by each of 6 stations, as the satellites complete their journeys around the earth.

        Can reposition satellites to maintain an optimal GPS constellation.

Monitor Stations

        Checks the exact altitude, position, speed, and overall health of the orbiting satellites.

         The control segment ensures that the GPS satellite orbits and clocks remain within acceptable limits.

         A station can track up to 11 satellites at a time.

         This "check-up" is performed twice a day, by each station.

Monitor Stations of GPS
Figure: Monitor Stations of GPS

Ground Antenna

        Ground antennas monitor and track the satellites from horizon to horizon.

        They also transmit correction information to individual satellites.

        Communicate with the GPS satellites for command and control purposes.

Ground Antenna
Figure: Ground Antenna 

User Segment

GPS receivers are generally composed of –

            1. an antenna

            2. receiver-processors and

            3. highly-stable clock

        They can also include a display for showing location and speed information to the user.

        A receiver is often described by its number of channels

        As of recent, receivers usually have between twelve and twenty channels.

User Segment of GPS

Figure: User Segment of GPS 

Working Principle of GPS 

 Geometric Principle:

You can find one’s location if you know its distance from other, already-known locations.

        Things which need to be determined:

        Current Locations of GPS Satellites.

        The Distance Between Receiver’s Position and the GPS Satellites.

Current Locations of GPS Satellites

        GPS satellites are orbiting the earth at an altitude of 11,000 miles.

        The orbits, and the locations of the satellites, are known in advance.

        GPS receivers store this orbit information for all of the GPS satellites in an ALMANAC*.

* The Almanac is a file which contains positional information for all of the GPS satellites

Distance Between Receiver and Satellites

To get the distance to each satellite:

   By measuring the amount of time taken by radio signal (the GPS signal) to travel from the satellite to the receiver.

       Radio waves travel at the speed of light, i.e. about 186,000 miles per second.

   The distance from the satellite to the receiver can be determined by the formula “distance = speed x time”.

    Distance measurements from two satellites limits our location to the intersection of two spheres, which is a circle.


     A third measurement narrows our location to just two points


      A fourth measurement determines which point is our true location


Accuracy of GPS system

The position calculated by a GPS receiver relies on three accurate
measurements:

            • Current time

            • Position of the satellite

            • Time delay for the signal

The GPS signal in space will provide a "worst case" accuracy of 7.8 meters at a 95% confidence level.

GPS time is accurate to about 14 nanoseconds. Higher accuracy is available today by using GPS in combination with augmentation systems. These enable real-time positioning to within a few centimeters.

Sources of Errors in GPS System

Different errors can cause a deviation of +/- 50 -100 meters from the actual GPS receiver position which are :

1. Satellite clock :

One nano second of inaccuracy in a satellite clock results in about 30 cm (1 foot) of error in measuring the distance to that satellite.

2. Receiver clock :

Any error in the receiver clock causes inaccuracy in distance measurement. However, it is not practical to equip receiver with very accurate atomic clocks.                

3. GPS Jamming :

• It limits the effectiveness of the GPS signal.

• GPS jammer is a low cost device to temporarily disable the reception of the civilian coarse acquisition (C/A) code.

4. Atmospheric errors:

Speed of GPS signal is affected by ionosphere &troposphere which cause a deviation of 0 to 30 m from the actual position of receiver.

5. Multi-path error :

• Bouncing of GPS signal due to a reflecting surface before reaching to receiver antenna.
• Which cause a deviation of 0 to 1 m. from the actual position of receiver

Applications

        Surveying

        Vehicle Tracking

        Military applications

       GPS integrated into fighters, tankers, helicopters, ships, submarines, tanks, jeeps, and soldiers' equipment.

       Target tracking

       Search and rescue.

 

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