The two dozen satellites are split into six groups, or orbital planes, with four primary spacecraft in each. An artist's concept of a Lockheed Martin Space Systems-built GPS Block 2R satellite orbiting Earth like the one to be launched this week. The receiver measures the time delay for the signal to reach itself from the satellite, which ultimately shows the distance to the orbiting satellite. Military users can then determine their exact location to within feet, speed within a fraction of a mile per hour and time to within a millionth of a second whether it be on the ground, in the air or at sea. GPS was used extensively in the featureless desert battlefield of the Gulf War.
Civilian receivers are also popular for hikers, boaters, pilots and mapmakers.
On May 1, U.S. President Bill Clinton announced the intential degrading of GPS signals to weaken accuracy for civilians would be stopped, improving location information tenfold. There are currently 27 GPS satellites working today, and GPS 2R-4 will become the 28th. The extra satellites beyond the minimum 24 needed bolster the constellation and serve as orbiting spares. As its name suggests, GPS 2R-4 is the fourth of 21 the advanced series of Block 2R satellites. Manufactured by Lockheed Martin Space Systems, the latest generation of spacecraft were built with improvements over the earlier Block 2 and 2A including the ability to operate longer when out of contact with ground controllers, better accuracy by communicating with other GPS satellites, larger fuel supplies and additional radiation protection.