General Information
GPS is used to support land, sea, and airborne navigation, surveying, geophysical exploration, mapping and geodesy, vehicle location systems, aerial refueling and rendezvous, search and rescue operations, and a wide variety of additional applications. Civilian users range from commercial airlines, trucking fleets, and law enforcement agencies to farmers, fishermen and hikers. New applications are continually emerging.
These capabilities were put to the test during Operation Desert Shield and Desert Storm. Coalition forces relied heavily on GPS to navigate the featureless Saudi Arabian desert. Forward air controllers, pilots, tank drivers and even cooks used the system so successfully that several U.S. defense officials cited GPS as a key to the Desert Storm victory. Recently, during Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom, GPS proved invaluable to coalition forces navigating in difficult conditions. These operations included the use of immensely successful GPS guided munitions, such as JDAM (Joint Direct Attack Munition), allowing pinpoint accuracy with minimum collateral damage.
GPS provides the following 24-hour navigation services: extremely accurate three-dimensional location (latitude, longitude and altitude), velocity and precise time; a worldwide common grid that is easily converted to any local grid; passive all-weather operations; continuous real-time information; unlimited support to worldwide users; and civilian user support at a slightly less accurate level. The GPS signals are so accurate that time can be calculated to within a millionth of a second, velocity within a fraction of a mile per hour and location to within feet.
GPS consists of three segments: space, control and user:
The Space Segment, consists of 24 operational satellites in six circular orbits 20,200 km (10,900 nm) above the Earth at an inclination angle of 55 degrees with a 12 hour period. The satellites are spaced in orbit so that at any time a minimum of 6 satellites will be in view to users anywhere in the world. The satellites continuously broadcast position and time data to users throughout the world. The satellites transmit signals on two different L-band frequencies.
The Control Segment consists of a master control station operated by the 50th Space Wing's 2nd Space Operations Squadron at Schriever Air Force Base, Colorado, with five monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite through the ground antennas, using an S-band signal. The ground antennas also transmit and receive satellite control and monitoring signals. The current Block IIF contract includes development of the Control Segment. In December 2003, a new Boeing GPS Center (BGC) was dedicated in Colorado Springs, Colorado. The center was created to develop, integrate, test and sustain the control segment hardware and software for the latest GPS IIF satellites. This facility will also have the capability to support evolutionary software development for GPS IIF and other versions of the GPS satellite.
The User Segment consists of the signal receivers/processors, antennas and control/display units that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise latitude, longitude, altitude, velocity and precise time at any time, in any weather. The system can accommodate an unlimited number of users without revealing their positions.
The GPS concept of operation is based upon satellite ranging. Users figure their position on the Earth by measuring their distance from the group of satellites in space. The satellites act as precise reference points.
Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position, velocity and time.
GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.
GPS provides two levels of service -- a Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense. SPS signal accuracy is intentionally degraded to protect U.S. national security interests. This process, called Selective Availability (SA), controls the availability of the system's full capabilities. The SPS accuracy specifications, given below, include the effects of SA.
SPS provides accuracies of (for position, the accuracy with respect to geographic, or geodetic coordinates of the Earth) within:
- 100 meters (2 drms) horizontal
- 156 meters (2 Sigma) vertical
- 300 meters (99.99% probability) horizontal
- 340 nanoseconds time (95% probability) time
SPS coverage is continuous and worldwide, with a position dilution of precision (PDOP) of 6 or less.
The Delta II expendable launch vehicle is currently used to launch GPS satellites from Cape Canaveral Air Force Station, Fla. In the future, Block IIF/III spacecraft will use the Air Force's Evolved Expendable Launch Vehicle (EELV).
Under management of a Joint Program Office at the U.S. Air Force's Space and Missile Systems Center, Los Angeles Air Force Base, Calif., Boeing Reusable Space Systems, Seal Beach, Calif., designed, built, and tested 11 developmental Navstar GPS satellites, developed and qualified a second-generation production prototype, and built 28 production Navstar GPS satellites under a $1.35 billion contract awarded in 1983. The original Air Force contract for GPS was awarded in 1974, resulting in the first Block I Navigational Development Satellite (NDS) satellite being launched in 1978 by an Atlas F rocket.
Today, the majority of the constellation consists of Boeing Block IIA satellites. The GPS Block IIR satellites, built by Lockheed Martin, are currently being launched on Boeing Delta II rockets.
Boeing is currently under Air Force contract to build at least twelve GPS IIF satellites. The GPS IIF satellites, with a design life of 12.7 years, will have improved anti-jam capability and substantially increased accuracy from earlier satellite versions. According to Air Force plans, the first GPS IIF launch is scheduled for late 2009.