Satellite navigation systems have become so widespread that they are almost taken for granted. However, the fact that they are so useful is leading to an expansion of available systems. Several countries are working on systems so product selection may become more complicated. To start, one must have a solid understanding of how satellite navigation systems work, as well as the terminology that is commonly used when referring to the technology.
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What is a satellite navigation system and how do they work?
A satellite navigation system (also known as a sat nav system) is a system of satellites, usually managed by one company or country that provides geo-spatial positioning, which is a technical term for a specific location on or above the Earth in 3 dimensions. A sat nav system receiver can be used to locate latitude, longitude, altitude, velocity and time information. Commercial systems are accurate to within a few meters. High-end systems are accurate to within centimeters. The satellites broadcast a signal that contains orbital data and the exact time the signal is transmitted. The orbital data is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock (most accurate time and frequency standards known) to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite.
The receiver measures signals from several satellites at the same time so that it can use triangulation to determine its location. Triangulation is the process of determining the location of a point by measuring the angles to it from two known points. The precise satellite locations are included in the transmission and the time-of-flight of the signal is used to calculate the distance to each satellite. The receiver then does some math and calculates its location on the Earth. The more satellites the receiver can track, the more accurate the location calculation.
The receiver calculates 4 parameters; latitude, longitude, altitude and time. As a result, the receiver generally needs to see at least 4 satellites to calculate the 4 unknowns. It can give estimates for the values with fewer satellites, but the potential error increases.
The basic triangulation math is not that complicated, but the fact that the known points, the satellites, are moving very fast and the fact that the Earth is a curved surface adds quite a bit of complexity. In addition, the Earth is not a perfect sphere and is not uniformly shaped or curved. This adds some error depending on how far off the average curvature a specific location is. For this reason, local augmentation systems are used. The receiver can also use regional data sets that better describe the local geography and ultimately give a more accurate position.