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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. Maritime and rail transportation will also see improvements in safety and efficiency through the use of GPS augmentation systems. State and local governments are using GPS augmentations in such diverse areas as inventory management, road-condition mapping, land use management, geographic information systems, and traffic databases. Of particular importance are GPS uses by emergency response services such as police, fire, and rescue units.
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