8. Background
information
8.1 GPS explained
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.
8.2 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.
8.2.1 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 360 imaginary lines of latitude running from Pole to Pole, and 180
imaginary lines of longitude parallel to the equator. 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.
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