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The Global Positioning System (GPS)

Introduction to GPS Tracking

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

Global Positioning System (GPS) comprises of three parts:

  • 24 satellites that orbit the Earth
  • Ground control stations which monitor the satellites
  • GPS receivers can be attached to persons or animals, or mounted on an object, such as a vehicle

The satellites are synchronised to emit encoded navigational information (exact positioning and exact time). Any vehicle equipped with a GPS receiver will intercept these transmissions. Using a simple mathematical formula derived from triangulation, the receiver is able to calculate its own longitude, latitude, velocity and even altitude. For companies implementing GPS applications, this information, most often, would be transmitted to a central dispatch or control location.

Originally designated the NAVSTAR (Navigation System with Timing And Ranging) Global Positioning System, GPS was developed by the US Department of Defense to provide all-weather round-the-clock navigation capabilities for military ground, sea, and air forces.

Since its implementation, GPS has also become an integral asset in numerous civilian applications and industries around the globe, including recreational uses (e.g. boating, aircraft, hiking), corporate vehicle fleet tracking, and surveying. GPS employs 24 spacecraft in 20,200 km circular orbits inclined at 55 degrees.

These spacecraft are placed in 6 orbit planes with four operational satellites in each plane. All launches have been successful except for one launch failure in 1981. The full 24-satellite constellation was completed on March 9, 1994. GPS receivers use triangulation of the GPS satellites' navigational signals to determine their location.

The satellites provide two different signals that provide different accuracies. Coarse-acquisition (C/A) code is intended for civilian use, and is deliberately degraded. The accuracy using a typical civilian GPS receiver with C/A code is typically about 100 metres. The military's Precision (P) code is not corrupted, and provides positional accuracy to within approximately 20 metres.

Numerous on-line tutorials on how GPS works and it's applications are available, including those at the University of Colorado. GPS satellites are controlled at the GPS Master Control Station (MCS) located at Falcon Air Force Base outside Colorado Springs, Colorado. The ground segment also includes four active-tracking ground antennas and five passive-tracking monitor stations.

Basic Concept of GPS-based Applications

Fundamental concept in GPS applications is that you can determine fairly accurately (ordinarily within 10-30 metres) the location of any device that has a GPS transceiver mounted inside the device. This determination of this location is facilitated by a series of satellites maintained by US defense establishment. Having determined the location of an object one can track it's movement as it moves. Therefore, you can implement a number of business and personal applications based on this location like fleet management, health services in the public sector and number delivery services.

How GPS Tracking Works

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location.

Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude).

Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

GPS Applications

One of the fast-growing GPS applications is vehicle tracking. GPS-equipped fleet vehicles, public transportation systems, delivery trucks, and courier services use receivers to monitor their locations at all times.

Public safety services, police, fire, and emergency medical services, are using GPS receivers to determine the nearest service vehicle to an emergency, enabling the quickest response in critical situations.

Recently, automobile manufacturers are installing moving-map displays guided by GPS receivers. For example, several Florida rental car companies are demonstrating GPS-equipped vehicles that give directions to drivers on display screens and through synthesised voice instructions.

Triangulation: Collecting of signals from three or more satellites in carefully monitored orbit from which the receiver computes it's own spatial relationship to each satellite to determine it's position.

Automatic Vehicle Location (AVL) is a technologically advanced method of remote vehicle tracking and monitoring using GPS.

Each vehicle is equipped with a module that receives signals from a series of satellites, and calculates it's current geographical location, speed, and heading. This information can be stored for later retrieval or, frequently, transmitted to a central dispatch/control location where it is displayed on a high-resolution geographical map.

For additional information please click on the links below:

GPS - The Global Positioning System
Digital Wireless Communications
Embedded Computer Control
Systems Software
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