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Navstar Global Positioning System

GPS Logo The Navstar Global Positioning System (GPS) is a constellation of orbiting satellites that provides navigation data to military and civilian users all over the world. The system is operated and controlled by members of the 50th Space Wing located at Falcon Air Force Base, Colo.

GPS is a space-based global radionavigation system which is operated by the U.S. Air Force. Two GPS services are provided. The Precise Positioning Service (PPS) provides full system accuracy primarily to U.S. and allied military users. The Standard Positioning Service (SPS) provides an accurate positioning capability for civil users throughout the world. The GPS has three major segments: space, control, and user.

The GPS Space Segment is composed of 24 satellites in six orbital planes. The satellites operate in circular 20,200 km (10,900 nm) orbits at an inclination angle of 55 degrees and with approximately a 12-hour period. A minimum of four satellites must be visible to a user in order to compute a three-dimensional position solution.

The GPS Control Segment has five monitor stations and three ground antennas with uplink capabilities. The monitor stations track all satellites in view. The information from the monitor stations is processed at the Master Control Station (MCS) to determine satellite clock and orbit states and to update the navigation message of each satellite. This updated information is transmitted to the satellites via the ground antennas.

The GPS User Segment consists of a variety of receiver types to provide positioning, velocity, and precise timing to the user.

The GPS is a space-based radio-positioning and time transfer system. The GPS has three major segments: Space Segment, Control Segment, and User Segment. As a universal positioning system, GPS provides several characteristics not found in other existing equipment which will enhance the conduct of mission operations: Extremely accurate (3-dimensional) position, velocity and time (PVT) determination; a worldwide common grid easily converted to other local datums; passive, all weather operation; real-time and continuous information; and survivability in a hostile environment.

GPS provides 24-hour navigation services which include:

  • Extremely accurate three-dimensional location information (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.
  • Support to an unlimited number of users and areas.
  • Support to civilian users at a slightly less accurate level.

GPS satellites orbit the earth every 12 hours emitting continuous navigation signals. With the proper equipment, users can receive these signals to calculate time, location and velocity. The signals are so accurate, time can be figured to within a millionth of a second, velocity within a fraction of a mile per hour and location to within a few feet. Receivers have been developed for use in aircraft, ships and land vehicles as well as for hand carrying.


Today, there are 24 fully operational GPS satellites. GPS reached full operational capability in April 1995. The NUDET Detection System (NDS) was declared operational in July 1995. Installations of user equipment are on schedule to meet the congressional mandate to equip all DoD aircraft with GPS receivers by the year 2000.

GPS promises to significantly enhance many of the functions being provided by current positioning and navigational equipment and will result in greater accuracy at lower cost. Such functions as mapping, aerial refueling and rendezvous, geodetic surveys, and search and rescue operations will benefit from GPS capabilities.

Such capabilities were put to the test during the U.S. involvement in Operations Desert Shield and Storm. Allied troops 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.

The Delta II expendable launch vehicle is used to launch GPS satellites from Cape Canaveral Air Station, Fla., into nearly 11,000-mile circular orbits. While orbiting the earth, the systems transmit signals on two different L-band frequencies. Their design life is 7.5 years.

The GPS Master Control Station (MCS), operated by the 50th Space Wing's 2nd Space Operations Squadron at Falcon Air Force Base, Colo., is responsible for monitoring and controlling the GPS satellite constellation. The GPS-dedicated ground system consists of five monitor stations and four ground antennas located around the world. The monitor stations use GPS receivers to passively track the navigation signals on all satellites. Information from the monitor stations is then processed at the MCS and used to update the satellites' navigation messages.

The master control station crew sends updated navigation information to GPS satellites through ground antennas using an S-band signal. The ground antennas are also used to transmit commands to satellites and to receive satellites' state-of-health data (telemetry).

Air Force Materiel Command's Space and Missile Systems Center (SMC) at Los Angeles Air Force Base, Calif., acts as the executive agent for the Department of Defense in acquiring GPS satellites and user equipment.

GPS is undergoing a Modernization Program that is a joint effort between the Department of Defense and the Department of Transportation. Working with the Department of Defense, the civilian agencies of the federal government plan to add two more civilian signals to future GPS satellites in the 2010 to 2015 timeframe. The future GPS will have a total of three civilian GPS signals. Two are protected for safety-of-life applications, such as aviation, and the other will be available for non-critical civilian uses.

When all three civilian GPS signals are broadcast from a sufficient number of satellites, the accuracy of GPS will approach the accuracy now only possible using Differential GPS. In addition, the improved service will be worldwide, not only where the Differential GPS service exists. Additional civilian GPS signals will enable receivers to reduce ionospheric errors with signal-processing techniques. With more than one signal, GPS also is less susceptible to unintentional interference.

Potential interference to the GPS L1 civil signal is a serious problem, in part because of increasing use of and reliance on GPS. Nevertheless, it is possible to mitigate the worst vulnerabilities. A major element of GPS vulnerability lies in the very low power that makes it vulnerable to jamming. GPS also is vulnerable to spoofing, broadcast signals with deliberately misleading information, and to unintentional interference. The latter can be due to natural causes (for example, solar flares and ionospheric scintillation), but also to human sources (for example, TV broadcasts, Mobile Satellite Services, Ultra Wide-Band systems, military jamming/spoofing tests, and military communications systems). A peculiar but valid class of vulnerability is the degree of unrealistic expectations that can be produced in enthusiastic but unwary GPS users. If there is inadequate integrity monitoring, the ready willingness to accept a GPS-driven electronic display, for example, can magnify the effectiveness of jamming on the user. Loss of GPS is a threat not only to civil transportation users, but also to banking, communications, data processing and internet enterprises that rely increasingly on the GPS timing signal.

While disruptive technology is increasingly available, effective, and easy to hide, counter-measures, or mitigations, do provide reasonable expectation that the worst effects of GPS disruption can be mitigated by utilizing a combination of strengthening the GPS system and by integrating GPS with independent systems. An important part of ensuring sufficient robustness in GPS-based systems is to provide these systems with adequate integrity monitoring.

GPS repeatedly performed outstanding service in the Persian Gulf War. American military vehicles, ships, and airplanes were all fitted with GPS equipment, and portable receivers played complementary roles. In addition, many GPS receivers were fitted to the guidance and control equipment in tactical missiles and precision-guided ammunition. This situation caused the Pentagon to worry that, in future wars, if the enemy takes effective electronic countermeasures, an overreliance on GPS will bring about results that are just the opposite of what was desired. In mid-1993, the United States Department of Defense directed the Defense Science Board (DSB) to establish a task force specifically devoted to the study of the susceptibility of the Global Positioning System (GPS) to jamming and methods of countering jamming. Its purposes were to solve problems having to do with the survivability of GPS when encountering enemy jamming and to find ways to improve its antijamming capabilities. The office of the Assistant Secretary of Defense for C3I directed that the task force established by the DSB put forward an optimal technological plan to improve the anti-jamming capabilities of GPS and especially emphasized that this improvement should be applied to GPS in tactical weapon systems.

Users will lose the tremendous gains of using GPS to control smart weapons if the opposition is allowed to jam the system. Only dramatic upgrades will avoid this threat. The enemy can easily use off-the-shelf troposcatter hardware to jam GPS operations. By shifting digital troposcatter systems like the U.S. AMTD4 down 8 percent in frequency to the GPS range, a user can produce 10 kW with data rates up to 8.5 Mbps. A small 200-watt drum-sized package could be assembled from modified ham gear at a cost of $3,000, will allow a large distributed field deployment.

GPS users can combine a variety of demonstrated techniques for a robust battlefield capability, including adaptive nulling antenna arrays, integration and fielding of the numerous receiver upgrades available to provide matched dynamic tracking and increased dynamic range, and airborne UAV-carrying pseudo-satellites (pseudolites which provide high-power and close-in GPS navigational data), which will overpower jammers. Developing and using out-of-band pseudolites with GPS down converters for weapons, developing cheap radiation homing weapons for GPS and military communications frequencies, and developing a joint jamming evaluation group will also counter the effects of jamming. The appropriate combination of these to obtain 30 dB of processing gain will decrease the effectiveness of a 60-mile jammer to 1 miles and allow an inertial system to provide the desired weapons accuracy.

The relationship between the jammer size and the GPS receiver antijam capability determines the target protected area. The current military receivers have a jammer tolerance of 54 dB above the received signal levels from the satellites. Using advanced receiver improvements and nulling antennas in the future, it is possible to raise the tolerance level to a combined level of 98 dB. This 44-dB increase in protection will reduce the protected target area against a 1-kW jammer from a 190-km diameter circle to a 1.2-km diameter circle. It is possible to even raise the jamming margin up to 120 dB, reducing the protected circle to a 184-meter diameter. At the 120-dB level, the enemy must use psuedolites and homing weapons to offset this capability. A HARM missile tuned to the antijammer frequency can negate this jamming effort.

As users improve GPS systems, the opposition can increase its effective radiated power up to approximately 100 megawatts before it becomes too costly for the jammer to continue. Thus, antijam weapons provide an effective capability.

South Korea's defense ministry said on 01 april 2016 that the Democratic People's Republic of Korea (DPRK) has been sending disruptive signals to jam global positioning system (GPS) in the country. Defense Ministry spokesman Moon Sang-Gyun told a regular press briefing that the DPRK's jamming operations are expected to continue for the time being as Pyongyang seemed to aim to raise tensions on the Korean peninsula by showing off its capability of disrupting GPS signals in South Korea.

The DPRK began 31 March 2016 to send the GPS-disrupting signals to South Korea from several regions north of the military demarcation line (MDL) dividing the two Koreas, including the western port city of Haeju and Mount Kumgang in the east coast. Moon said there has been no damage reported in South Korea from the DPRK's jamming operations, but he noted that if any damage happens from South Korean ships and airplanes, Seoul will make Pyongyang "pay a due price".

Jamming signals could cause malfunction of mobile phones and disruption of GPS in planes and vessels, which depend on the positioning system for navigation. Pyongyang reportedly had GPS-disrupting devices. The country had allegedly tested its devices from a month earlier before launching an attack. The spokesman said the DPRK's jamming devices could reach more than 100 km and affect Seoul and its suburban areas.




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