There are five enhancement systems for GPS

       There are a number of augmentations to GPS that enable GPS to meet user-specific requirements for positioning, navigation and timing (PNT). An augmentation is a system that improves the accuracy, integrity, reliability, and usability of positioning, navigation, and timing in a way that was not originally possible with GPS itself. It includes, but is not limited to, the following five items.

◆ Nationwide Differential GPS System (NDGPS): NDGPS is a ground-based augmentation system operated and maintained by the Federal Railroad Administration, the U.S. Coast Guard, and the Federal Highway Administration that provides more accurate and complete GPS for users on the ground and in the water.Modernization efforts include the ongoing development of a high-accuracy NDGPS system (HA-NDGPS ), which is used to enhance performance to bring accuracy to 10 to 15 centimeters over the entire coverage area.NDGPS is built to international standards, and more than fifty countries around the world have adopted similar standards.

◆ Wide Area Augmentation System (WAAS): WAAS is a satellite-based augmentation system operated by the U.S. Federal Aviation Administration (FAA) that provides navigation for all phases of vehicle navigation. Today, this capability is widely used in other areas, as this GPS-like signal can be processed by simple receivers and does not require additional equipment. Using the International Civil Aviation Organization (ICAO) standards, the FAA continues to work with other nations to provide a complete service to all users in any region. Other ICAO-standard space-based augmentation systems include the European Geostationary Navigation Overlay System (EGNOS) in Europe, the GPS and Earth Navigation Augmentation System (GAGAN) in India, and the Multi-functional Transmission Satellite (MTSAT) Satellite Augmentation System (MSAS) in Japan. All of these international applications are based on GPS, and the FAA will improve the WAAS to take advantage of future GPS life-safety signals and provide better service, and will also promote the implementation of these new capabilities globally.

◆ Continuously Operating Reference Stations (CORS): The U.S. CORS network, managed by the National Oceanic and Atmospheric Administration (NOAA), is responsible for storing and distributing GPS data, primarily through post-processing, for precise positioning and atmospheric modeling applications. CORS is being modernized and updated to support real-time users.

◆ Global Differential GPS (GDGPS): GDGPS is a high-precision GPS augmentation system developed by the Jet Propulsion Laboratory (JPL) to support the real-time positioning, timing, and orbit determination needs required by NASA's science missions.NASA's future plans include the use of the Tracking and Data Relay System (TDRSS) to distribute via satellite a real-time differential correction information. This system is called the TDRSS Augmentation Service Satellite (TASS).

◆ International GNSS Service (IGS): IGS is a network of 350 GPS monitoring stations provided by 200 organizations from 80 countries. Its mission is to provide the highest quality data and products in accordance with Global Navigation Satellite System (GNSS) standards to support Earth science research, interdisciplinary applications and education, and to promote other socially beneficial uses. Approximately 100 IGS monitoring stations can broadcast their tracking data within one hour of collection.

There are other augmentation systems around the world, both governmental and commercial. These systems use differential, static or real-time techniques.