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 military operations in Iraq and Afghanistan where coalition forces rely heavily on GPS to navigate the featureless desert. Forward air controllers, pilots, tank drivers and even cooks use the system so successfully that several U.S. defense officials cited GPS as a key to the many victories. During recent operations in Afghanistan and Iraq, 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 billionths 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 six 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 sustainment of the Operational Control Segment (OCS), which gained full operational acceptance with the Air Force in April 2011. To support OCS Boeing maintains the Boeing Mission Operations Support Center (BMOSC) 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 also has the capability to support evolutionary software development for GPS IIF and other versions of the GPS satellite. The flexible design of the OCS system provides a foundation for new capabilities by enabling it to accommodate technology improvements and new missions as required.
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, under any weather condition. The system can accommodate an unlimited number of users without revealing their positions.
The GPS concept of operation is based upon satellite ranging. Users calculate 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.
The U.S. Government is committed to providing GPS to the civilian community at the performance levels specified in the SPS Performance Standard. For example, the GPS signal in space will provide a "worst case" pseudorange accuracy of 7.8 meters at a 95% confidence level. The actual accuracy end users can expect depends on factors outside the government's control, including atmospheric effects and receiver quality. Real-world data show that some high-quality GPS SPS receivers currently attain better than 3 meter horizontal position accuracy. Higher accuracy is available today by using GPS in combination with various augmentation systems. These enable real-time positioning to within a few centimeters, and post-processed positioning to within millimeters. SPS users can obtain a time transfer accuracy to UTC (USNO) within 340 nanoseconds.
Evolved Expendable Launch Vehicles (EELV) are currently used to launch GPS satellites from Cape Canaveral Air Force Station, Fla.
Under management by the U.S. Air Force's Space and Missile Systems Center (SMC/GP), Los Angeles Air Force Base, Calif., Boeing Space & Intelligence Systems (S&IS), 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, Boeing is building 12 GPS IIF satellites for the Air Force at its Satellite Development Center in El Segundo, Calif. The GPS IIF satellites incorporate several key technology enhancements that deliver the following improvements to the constellation: Greater navigational accuracy through improvements in atomic clock technology; a more secure and jam-resistant military signal to ensure the GPS service to and improve safety of those in harm's way; and a protected, more precise, and interference-free civilian L5 signal for to assist in commercial aviation and search and rescue operations.
The innovative Pulse Line leverages a "one Boeing" approach to adapt commercial aircraft manufacturing operations to satellite production, enabling Boeing to deliver 12 GPS IIF satellites faster, more efficiently and with higher quality.
Boeing has served as a prime contractor for GPS satellites for more than three decades, with many satellites far exceeding their design life spans. GPS IIF's next-generation capabilities strengthen the GPS constellation, and Boeing satellites will form the core of the system for many years to come. The GPS IIF satellites, with a design life of 12 years, have improved anti-jam capability and substantially increased accuracy from earlier satellite versions. The first GPS IIF satellite launched on May 27, 2010, from Cape Canaveral Air Force Station. The second GPS IIF spacecraft is scheduled to launch in 2011.