An artist's concept of a GPS Block 2R satellite orbiting Earth. Photo: Lockheed Martin

The Global Positioning Systems Wing is a joint service effort directed by the US Air Force and managed at the Space and Missile Systems Center (SMC), Air Force Space Command, Los Angeles Air Force Base, Calif. The Wing is the DoD acquisition office for developing and producing GPS satellites, ground systems, and military user equipment. The GPS constellation is operated and controlled by the 50th Space Wing's (Air Force Space Command) 2nd Space Operations Squadron, Schriever Air Force Base, Colorado.

GPS is a space-based radio-positioning system nominally consisting of a minimum of 24-satellite constellation that provides navigation and timing information to military and civilian users worldwide. GPS satellites, in one of six medium earth orbits, circle the earth every 12 hours emitting continuous navigation signals on two different L-band frequencies. In addition to the satellites, the system consists of a worldwide satellite control network and GPS receiver units that acquire the satellite's signals and translate them into precise position and timing information.

GPS provides the following:

• 24-hour, worldwide service
• Highly accurate, three-dimensional location information
• Precision velocity and timing services
• Accessibility to an unlimited number of global military, civilian, and commercial users

GPS Block IIR satellites began replacing the older GPS Block II/IIA satellites in 1997. The GPS Block IIR satellites boast dramatic improvement over the previous blocks with reprogrammable satellite processors to enable fixes and upgrades in flight.

GPS Block IIR-M satellite improvements include a new military signal (M-Code) (M-Code) on both the L1 and L2 channels, and a more robust civil signal (L2C) on the L2 channel. Presently the GPS constellation consists of 16 GPS Block IIA satellites built by Boeing, 12 GPS Block IIR and 4 GPS Block IIR-M satellites built by Lockheed Martin with 4 GPS Block IIR-M satellites remaining. The next GPS Block IIR-18(M) launch is planned for December 2007.

GPS Block IIF satellites are the next generation of GPS space vehicles. The GPS Block IIF satellites, being built by Boeing, will provide all the capabilities of the previous blocks with additional benefits to include an extended design life of 12 years, faster processors with more memory, and a new civil signal on a third frequency (L5). The first GPS Block IIF satellite is scheduled to launch in 2008.

GPS Block III will be the next block of GPS satellites. GPS IIIA will transmit a new civilian signal (L1C), which is designed to be highly interoperable with the European Galileo satellite navigation system signal and intended to be fully compatible and interoperable with those signals planned for broadcast on Japan's Quazi-Zenith Satellite System (QZSS). For military users, GPS IIIA satellites will provide further increases in the anti-jam capability of the M-Code signals. The GPS III satellites will be developed in three increments with each increment including more capabilities based on technical maturity. GPS IIIA is projected to be available for launch in 2013.

GPS continues to perform as the world's premier space-based positioning, navigation, and timing service. Endeavors such as mapping, aerial refueling, rendezvous operations, geodetic surveying, and search and rescue operations have all benefited greatly from GPS's accuracy. GPS capabilities are integrated into nearly all facets of US military operations. GPS receivers are incorporated into nearly every type of system used by the DoD: aircraft, spacecraft, ground vehicles and ships. In addition, GPS-guided munitions have showcased their increased accuracy in recent conflicts with unprecedented precision, thus improving military capability while decreasing the number of weapons required to achieve military objectives.