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

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.

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.

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.

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.