How Satellite Navigation Works

Global navigation satellites continuously transmit time and distance information as they orbit the earth in a precise formation. Navigation satellite receivers use this information to calculate an exact location through triangulation. Every point on Earth is identified by two sets of numbers called coordinates. These coordinates represent the exact point where a horizontal line, known as latitude, crosses a vertical line, known as longitude. The receiver locks on to at least three satellites and uses the information received to determine the coordinates of the device.

By comparing the time the signals were transmitted from the satellites and the time they were recorded, the receiver calculates how far away each satellite is. The distance of the receiver from three or more satellites reveals its position on the surface of the planet. With these distance measurements, the receiver might also calculate speed, bearing, trip time, distance to destination, altitude and more.

The satellite navigation device may display its position as longitude/latitude, Universal Transverse Mercator (UTM), Military Grid (MG) or simply as a point on an electronic map. Many Thales Navigation receivers provide comprehensive mapping data, making satellite navigation an easy tool to enhance your recreational and industrial activities.

Line of Sight

Satellite navigation receivers operate by line of sight with global positioning satellites. This means that at least three satellites must be in "view" of a receiver in order to calculate longitude and latitude. A fourth satellite must also be within line of sight to calculate altitude. On average, eight satellites are continuously within line of sight of every position on Earth; the more satellites in view, the more accurate the positioning.

Though the radio signals of navigation satellites will pass through clouds, glass, plastic and other lightweight materials, satellite navigation receivers will not work underground or in other enclosed spaces.

Precision

On average, a satellite navigation receiver is accurate to within 15 meters. Thales Navigation employs several technologies to increase the accuracy of their professional and Magellan®-branded receivers. An accuracy of 3 meters or better is achieved using correction signals from satellite navigation augmentation systems. In the U.S., an accuracy of 3 meters is achieved using signal corrections from a network of ground stations and fixed position satellites known as WAAS (Wide Area Augmentation System). Throughout Europe a similar system provides the same accuracy; EGNOS (European Geostationary Navigation Overlay System). In Asia, satellite navigation signal correction is provided by MSAS (Multifunctional Transport Satellite-based Augmentation System). Other ways to increase the accuracy of satellite navigation include the use of DGPS (Differential Global Positioning System); ground relay stations, set at known positions, that transmit corrected satellite navigation signals. Various methods and applications of DGPS can increase satellite navigation accuracy from a few meters to within a few millimeters. Using DGPS requires a differential beacon receiver and antennae in addition to a satellite navigation device. Accuracy can also be increased using an RTK (Real-Time Kinematic) satellite navigation system. This is a receiver capable of transmitting a phase-corrected signal from a known position to one or more rover receivers.

A number of positioning errors can occur, limiting accuracy to within 15 to 25 meters. These errors are monitored and compensated for in a number of ways:

• Orbiting errors - Occasionally a satellite's reported position does not match its actual trajectory. In the U.S., the Department of Defense continuously monitors each satellite, making orbital corrections with onboard booster rockets.
• Poor geometry - If all the satellites within line of site of a receiver are clustered closely together, or lined up relative to the position of the receiver, the geometric calculations necessary for triangulating a position become difficult and less reliable. The use of differential correction signals from satellite-based augmentation systems or DGPS can compensate for both orbital errors and poor geometry.
• Multi-path signals - Signals may be reflected off tall buildings or other obstructions before reaching the receiver, increasing the distance a signal travels, reducing accuracy.

Most good consumer GPS receivers make a number of complex mathematic calculations to effectively compensate for other potential errors in positioning:

• Atmospheric delay - Satellite navigation signals slow as they pass through the Earth's atmosphere. Most good consumer GPS receivers calculate the average delay in nanoseconds to compensate.
• Clock errors - The clock built into a receiver is not as accurate as the atomic clock on a navigation satellite, which is accurate to one second every million years. Most good consumer GPS receivers compensates for time differentials by comparing the time signals of several satellites and adjusting its calculations and its clock to match.