Global Positioning System (GPS)

The Global Positioning System (GPS) is a collection of satellites owned by the U.S. Government that provides highly accurate, worldwide positioning and navigation information, 24 hours a day. It is made up of twenty-four GPS satellites which orbit 12,000 miles above the earth. With a GPS receiver, users can calculate their location anywhere in the world. Position and navigation information is essential for a huge range of professional and personal activities, such as surveying and mapping, aviation and marine navigation, vehicle tracking systems and mobile computer and cellular platforms.

The fundamental basics of GPS technology are precise time and positional information. Utilizing atomic clocks (accuracy to within one second every 70000 years) and location data, every satellite continuously transmits the time and position. These signals are picked up by the GPS receivers, listening to three or more satellites simultaneously, and determine the user's position on earth.

GPS has many potential applications for civilian and commercial purposes. To meet these needs, GPS offers two levels of service, a commercial Standard Positioning Service (SPS) signal for world-wide civilian use and Precise Positioning Service (PPS) signal for military use. The SPS is a positioning system and timing service which is available to all GPS users on a continuous, worldwide basis with no direct charge.

The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz for civilian use and L2 = 1227.6 MHz for military use. Due to the spread spectrum characteristics of the signals, the system provides a large margin of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behavior, system time and status messages. In addition, an almanac is also provided which gives the approximate data for each active satellite. This allows the user to find all the satellites once the first has been expired.