About GPS
From Pilots Almanac
Contents |
GPS Overview
The Global Positioning System (GPS) is a space-based radio-navigation system consisting of a constellation of satellites and a network of ground stations used for monitoring and control. Initially designed for military applications, civilian users have found numerous applications using GPS. A minimum of 24 GPS satellites orbit the earth at an altitude of approximately 11,000 miles providing users with accurate information on position, velocity, and time anywhere in the world and in all weather conditions.
GPS is operated and maintained by the Department of Defense (DoD). The National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee manages GPS, while the U.S. Coast Guard acts as the civil interface to the public for GPS matters. The Federal Aviation Administration is investigating and applying the use of GPS as it pertains to aviation.
On March 29, 1996, a Presidential Decision Directive (PDD) was signed by President Clinton that described GPS as an international information utility. The PDD included the following directives:
- The U.S. government will continue to operate, maintain and provide basic GPS signals worldwide, free of direct user fees.
- The U.S. will advocate the acceptance of GPS and it's augmentations as a standard for use by initiating international discussions and agreement with Japan and Europe.
History and Development
GPS, formally known as the Navstar Global Positioning System, was initiated in 1973 to reduce the proliferation of navigation aids. By creating a system that overcame the limitations of many existing navigation systems, GPS became attractive to a broad spectrum of users worldwide. GPS has been successful in virtually all navigation applications, and because its capabilities are accessible using small, inexpensive equipment, GPS is being utilized in a wide variety of applications across the globe.
How GPS Works
Satellite Navigation is based on a global network of satellites that transmit radio signals in medium earth orbit. Users of Satellite Navigation are most familiar with the 24 Global Positioning System (GPS) satellites*. The United States, who developed and operates GPS, and Russia, who developed a similar system known as GLONASS, have offered free use of their respective systems to the international community. The International Civil Aviation Organization (ICAO), as well as other international user groups, have accepted GPS and GLONASS as the core for an international civil satellite navigation capability known as the Global Navigation Satellite System (GNSS).
The basic GPS service provides users with approximately 100-meter accuracy, 95% of the time, anywhere on or near the surface of the earth. To accomplish this, each of the 24 satellites emits signals to receivers that determine their location by computing the difference between the time that a signal is sent and the time it is received.
GPS satellites carry atomic clocks that provide extremely accurate time. The time information is placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the distance, or range, from the receiver to the satellite. The receiver must account for propagation delays, or decreases in the signal's speed caused by the ionosphere and the troposphere. With information about the ranges to three satellites and the location of the satellite when the signal was sent, the receiver can compute its own three-dimensional position. An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. However, by taking a measurement from a fourth satellite, the receiver avoids the need for an atomic clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and time.
Aircraft Application
GPS receivers in aircraft have revolutionized aerial navigation. Whether certified for full IFR or VFR-only, these devices can calculate and display a straight-line solution between any two points on earth. In fact, most include flight plan capability that allow a pilot to plot multi-legged trips. These provide time/distance/bearing information for each leg, updated in real time.
GPS Augmentation
The United States is modernizing GPS through a program that will improve its civilian service in the next decade and beyond. For many critical safety-of-life applications, GPS service alone may not meet the requirements of
- Integrity - the ability of a system to provide timely warnings to users when the system should not be used for navigation or other purposes.
- Accuracy - the difference between a GPS-measured position at any given time and the actual or true position.
- Availability - the ability of a system to be used for navigation when and where ever it is needed by the users.
A number of agencies and private firms are implementing augmentation systems that will improve the services available to civilian GPS users. These augmentations cover a wide range of geographical areas, up to continental distances.
Augmentations of GPS to meet user requirements are most commonly accomplished using a technique known as "differential GPS" or DGPS. A DGPS reference station is used to continuously monitor GPS signals in a given area. Since the position of the reference station has been precisely surveyed, any errors in the satellite signals themselves can be calculated and corrections broadcast to users in the area. The user's DGPS receiver applies the correction message to improve the accuracy of its own position. The DGPS broadcast may also include integrity warnings for any satellite signals that should not be used. Additionally, the DGPS service can broadcast a GPS-like signal to improve the accuracy and availability of GPS services.
Another form of GPS augmentation uses a transmitter on the ground to transmit GPS-like signals. These transmitters act like pseudo-GPS satellite or "pseudolites." In situations where signals from GPS satellites are blocked or obscured, such as in urban canyons or deep valleys, pseudolites can provide an additional source of GPS signals to improve the availability of GPS service. Pseudolites may be placed in fixed or mobile locations or mobile and transmit on frequencies close to or far from GPS itself depending on local conditions.
Aviation augmentation systems system will support all phases of flight and enable improvements to the safety and efficiency of global air transportation.
See WAAS.
GPS Fact Sheet from the FAA
The technology behind GPS is relatively simple. A constellation of 24 satellites orbiting 11,000 miles above the earth emits signals to receivers on earth. By measuring the travel time of a signal transmitted from each satellite, a receiver can calculate its distance from that satellite. Satellite positions are used by a receiver as precise reference points to determine the location of the receiver. When receiving the signals from at least four satellites, a GPS receiver can determine latitude, longitude, altitude, and time.
In 1989 the Department of Defense (DoD) launched the first production series of GPS satellites. This effort was the initial step in revolutionizing the way we find our location on earth. The 24 satellite system was declared operational by the DoD on December 8, 1993. A similar declaration was made by the Federal Aviation Administration (FAA) in February 1994 regarding the civil operational status of GPS.
The overall objective of the FAA is to support the operational use of satellite navigation for all civil aviation needs including departure operations, terminal, oceanic, en route, non-precision and precision approaches, auto-landing, and surface navigation. Some of the incremental steps to achieve this goal have already occurred, many are currently underway, and more are planned for the near future.
The international community can expect many benefits from satellite navigation (GPS and its augmentations) including:
- Increased landing capacity down to Category III precision approach service to all runways and airports for all aircraft types
- Improved safety with reduced separation minimums resulting in increased system capacity and capabilities
- Increased flexibility to implement accurate area navigation by using efficient, optimized, user-preferred flight paths
- Improved ground and cockpit situational awareness to reduce runway incursions
- Significant reductions in aircraft operating costs
- Accurate position reporting to enable uniform high-quality worldwide air traffic management
- Consolidation of navigation functions into a single satellite-based system thereby enabling the potential phase-out of older navigation aids at substantial savings.
Additionally, the FAA is striving to enhance existing relations with other civil aviation authorities and appropriate organizations to create a seamless, worldwide satellite-based navigation system. Such a system will permit the use of a single piece of equipment to support aviation navigation on a global basis. Furthermore, it will provide the foundation for future communications, navigation, and surveillance systems and increase safety for carriers and passengers internationally.
In furtherance of international acceptance of GPS, on March 29, 1996, the President of the United States officially reiterated the U.S. commitment to continue the broadcast of GPS signals on a worldwide basis free of charges for the foreseeable future. The President also made a promise to discontinue the use of selective availability (SA) by the year 2006.
In January 1999, as a direct result of the benefits brought about by GPS, Vice President Gore announced that the U.S. would begin a GPS modernization to further extend the systems capability. This modernization would include two additional GPS signals to enhance the civilian and commercial service.
On May 1, 2000 the President announced that effective immediately, the use of SA would be discontinued. This will result in accuracies up to ten times more accurate than previously possible. All users worldwide will benefit from these commitments.
Although GPS and its augmentations can provide major benefits for navigation, especially for the civil aviation community, there are many steps that must be taken to implement this technology. Standards and procedures development, facility preparation, training, international cooperation, and many other activities play a critical role in bringing these systems into operational use. Based on the capability of GPS, the FAA has charted a course for the future and is working to build a new navigation capability--one that will guide our Nation's Airspace System into the 21st century.
External Links
National Space-Based Position, Navigation and Timing Executive Committee
