Chapter1 MDP-621/641/640
General Information
Operation manual
1.5 About GPS
GPS is a navigation system using 24 satellites (21 plus 3 in service) orbiting 20,183 km high from the
earth every 11 hours 58 minutes.
Positioning by GPS
Your position is determined by calculating the distance from two satellites (in 2-dimensional
positioning) or three satellites (in 3-dimensional positioning) to your position. The distance is
determined by the time taken for a message to be sent from the satellites to the receiver. In
2-dimensional positioning, your position (latitude and longitude; height is preset) is determined at the
intersection point of three spheres formed by three satellites. In 3-dimensional positioning, your
position (latitude, longitude and height) is determined at the intersection of four spheres formed by four
satellites.
The 3 circles drawn referenced to
each satellite position
The GPS receiver position (Point of
position fix) is the point where 3 spheres
intersect.
Figure 1. 5 Constellation of GPS satellites
NOTE
The GPS system is based on the geodetic system called WGS-84. In conventional world maps
one coordinate system differs from others with region, and this causes the position fix made on
the map and GPS measurement to differ in the order of a few hundred meters.
Time required for position fixing
In the following circumstances, your GPS receiver takes more time to make a stable position fixing:
(1) When you turn the GPS receiver on for the first time.
(2) The stored orbital data is not suitable for the available satellite, or purged due to lengthy storage.
(3) When you use it after moving a long distance
When the GPS receiver is first turned on, it starts to store the orbital data sent from the satellite. It
takes about 15 minutes before the first fix is available. After this, the receiver can fix your position
within a minute by using the previously stored data.
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