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Satellite Navigation
What is Satellite Navigation?
Wikipedia’s definition of ‘satellite navigation (SatNav)’ is:
‘The method by which electronic receivers determine their location (longitude, latitude, and altitude) to within a few metres using time signals transmitted along a line-of-sight by radio from satellites.’
As of 2009, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational Global Navigation Satellite System (GNSS). The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2013.
The technology has been around since the 1960’s and was originally designed for military applications.
Most commonly these days a Sat Nav device can be found in cars, boats, planes and anything else that moves, including people as mobile phones have featured a GPS for a number of years now.
How does it work?
There are two aspects of how satellite navigation is achieved:
1. The receiver: The Sat Nav device itself.
2. The transmitter: The GPS components.
The receiver (Satelite Navigation Device)
The receiver is an electronic gadget that can receive satellite signals and show it’s location (to within a few meters) on a visual map display.
The SatNav device needs to have the following data to enable it to display it’s location correctly:
i. Clear line of sight to receive signals from one or more satellites located around 12,000 miles above Earth.
ii. Pre-loaded or downloadable (via 3G or GPRS) maps and Positions of Interest (POI’s)
iii. A constant power source.
Some examples of a Sat Nav device are: Tom Tom, Garmin, NavMan etc..
The transmitter (GPS)
The Global Positioning System is the term given to the collective components that provide an accurate calculation of a devices location on Earth.
The GPS consists of:
i. 24 to 32 Satellites which are located in precise orbits 12,000 miles above the Earth. (They orbit the Earth twice a day and travel at approximately 7000 miles per hour)
ii. Control or monitoring stations. A number of ‘bolted to the ground’ stations that control all the satellites’ orbits and on-board atomic clocks.
The whole System is controlled by US Military but is also open for civil use. There were once two methods by which a position was calculated:
1. The SPS (Standard Positioning System) – For civil use and is a close approximation of a receiver’s position.
2. The PPS (Precise Positioning System) - For military use and is an EXACT calculation of the receiver’s position.
The reason there were two methods is that the military don’t want people using an exact position for criminal/terrorist purposes. (Although it is now believed that these differences are now almost obsolete, the military does hold the right to ‘jam’ the SPS signal over a battlefield if needed)
There is now also a European version launched in Dec 2005 called Galileo.
The receiver locates its position by triangulation; having line-of-sight with at least 3 satellites (longitude, latitude, velocity) OR if it’s a more expensive GPS Receiver then a fourth satellite is required (altitude).