Navigation Systems

Although earlier in human history travelers might seek guidance from the stars using a mechanical compass, in the global era, the dominant form of a navigation system is provided by satellite. Global navigation satellite systems (GNSS) are satellite constellations that generate signals that can be received virtually anywhere, anytime, and in any weather anywhere in the world.

As of 2010, two satellite-based navigation systems are fully operable. The global positioning system (GPS), developed and operated by the U.S. Department of Defense, and the global orbiting navigation satellite system (GLONASS), developed and operated by the Russian—Republic. Two other GNSS—Galileo and Compass—are under development by the European Union and China, respectively. The GPS, GLONASS, and Galileo systems are intended to be independent, complementary, and interoperable. China's system will be regional in nature—that is, useful in the airspace above its footprint.

The GPS now includes 30 active satellites that are approximately uniformly dispersed around six circular orbits with five or more satellites each. The orbits are inclined at an angle of 55° relative to the equator and are separated from each other by multiples of 60°. The orbits are nongeostationary and approximately circular, with orbital periods of 10 hours and 58 minutes. Theoretically, three or more GPS satellites will always be visible from most points on the Earth's surface, and four or more GPS satellites can be used to determine an observer's position anywhere on the Earth's surface 24 hours per day.

In nontechnical terms, the determination of a user's position is accomplished as follows: The GPS receiver calculates the distance from the user to three satellites. This is called ranging. Next, the receiver finds the position of these satellites from the data provided by the navigation messages from the satellites. This information (the range and the position of the satellites) is used to calculate the user position (latitude, longitude, and altitude) on the surface of the Earth. The clock in the user's receiver and the clock on the satellite are not synchronized, and thereby, the clocks’ bias must be calculated. This introduces another unknown to determine, and it requires four satellites in total to find the user position and the clocks’ bias. The GPS's horizontal position accuracy is currently advertised by the Standard Positioning Service as 100 meters, the vertical-position accuracy as 156 meters, and time accuracy as 334 nanoseconds—all at the 95% probability level. The Standard Positioning Service also [Page 1232]guarantees the user-specified levels of coverage, availability, and reliability.

To increase the accuracy and integrity of the GNSS and thus enhance the users’ safety, regions have developed and are developing augmentation systems that are also space based. These include the United States’ WAAS (Wide Area Augmentation System), European Union's EGNOSS (European Geostationary Navigation Overlay System), Japan's MSAS (MTSAT Satellite Based Augmentation System), Canada's CWAAS (Canadian WAAS), China's SNAS (Satellite Navigation Augmentation System), and India's GAGAN (GPS & GEO Augmented Navigation).

Generally, these augmentation stations involve the user's GPS receiver receiving corrections determined from a network of reference stations distributed over a wide geographical area. Separate corrections are usually determined for specific error sources—such as satellite clock, ionospheric propagation delay, and ephemeris. The corrections are applied in the user's receiver or attached computer in computing the receiver's coordinates. The corrections are typically supplied in real time by way of a geostationary communications satellite or through a network of ground-based transmitters. Corrections may also be provided at a later date for post-processing collected data.

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