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The Global Positioning System (GPS) is used to create an electronic reference frame around the earth.
It consists of 24 satellites that orbit the earth in just under 12 hours. Each satellite transmits a unique signal and follows a strict orbital path. Every GPS antenna stores these orbits inside its memory, so it knows where each satellite is at any given time.
The GPS antenna is able to accurately calculate its current location on the earth’s surface as soon as it knows the exact distance to a minimum of 4 different GPS satellites. Distance is calculated by simply timing how long each satellite’s radio signal takes to reach the antenna.
GPS coordinate systems explained
GPS coordinates define a single point on an imaginary mathematical model of the earth, or datum. There are a number of different datums in use around the world. Importantly, each datum will give rise to different coordinates for the same physical location. So it is important that we reference the same datum when describing coordinates.
The Global Position System (GPS) uses the WGS-84 (World Geodetic System 1984) datum.
A coordinate can be written with varying levels of decimalisation.
Degrees Minutes Seconds
Eg 38° 33’42.43” N, 121° 26’11.70” W
When navigation of the sea was first pioneered, the earth was divided into 180 imaginary lines of latitude parallel to the equator and 360 imaginary lines of longitude running from Pole to Pole. Each degree was then subdivided into 60 (minutes), and again into 60 (seconds). Positions are described as being North / South of the equator and West / East of the Prime Meridian, which passes through Greenwich, London.
Degrees Decimal Minutes
Eg 3833.7072 N, 12126.1950 W
GPS “raw data” use this format.
GPS antennae produce standard GPS NMEA sentences containing the coordinate data. This data is then used by the GPS enabled device for navigation or other location based services.
The standard for these sentences was developed by the National Marine Electronics Association (NMEA), originally for marine applications.
Eg. 38.561787, -121.436584
Decimal Degrees is perhaps the easiest way to understand GSP co-ordinates. The Minutes and Seconds are simply represented by a decimal fraction. The North / South and East / West markers are replaced by +ve and –ve number notation.