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Global Positioning System (GPS)

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, the GPS system 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.

The GPS system 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.

The first eleven spacecraft (GPS Block 1) were used to demonstrate the feasibility of the GPS system. The orbit inclination used for these satellites was 63 degrees, differing from the 55 degrees used for the operational system. The Block 2 spacecraft began the operational system. The Block 2A spacecraft (A = Advanced) were a slight improvement over the Block 2.

The Global Positioning System (GPS) was designed as a dual-use system with the primary purpose of enhancing the effectiveness of U.S. and allied military forces. GPS is rapidly becoming an integral component of the emerging Global Information Infrastructure, with applications ranging from mapping and surveying to international air traffic management and global change research. The growing demand from military, civil, commercial, and scientific users has generated a U.S. commercial GPS navigation systems equipment and service industry that leads the world. Augmentations to enhance basic GPS services could further expand these civil and commercial markets.

GPS systems 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 meters. The military's Precision (P) code is not corrupted, and provides positional accuracy to within approximately 20 meters. Numerous on-line tutorials on how GPS works and its applications are available, including those at the University of Texas and Rentec International. GPS systems 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.

GPS receiver technology has developed by leaps and bounds over the last few years. GPS receivers were initially the size of a suitcase with the antenna the size of a kid's blow up swimming pool. Over time, the system has been developed into a civilian friendly program, and GPS receiver technology has miniaturized as well. Automobile GPS receivers are the size of a deck of cards. The gps receiver used in hand held devices is not much larger than a small cell phone. Many newer cell telephones have a GPS receiver integral in their hand set. As manufacturers develop the GPS receiver, they will have to work through display, power use and dexterity limitations. An individual will need a screen with a size that can be viewed from any angle and at a reasonable distance. The GPS receiver is generally always on while in use, so managing power will continue to be an on going problem. The ability to push the small buttons will limit just how small a GPS receiver can be. As touch screens develop and other input systems are introduced, we will see the GPS receiver continue to change in appearance and use.

Author: John B. Whitsell
Making Tracks GPS
http://www.makingtracksgps.com

Information referenced from NASA and USCG data.