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Annex 2 GPS overview
GPS is short for Global Positioning System. It utilizes 28 satellites flying around the
earth to measure the precise position and time of random point on earth. Generally, its
position precision is in 20m-1m and its time precision is in 60ns-5ns. It is developed by
U.S. Department of Defense for military and civil. The civil signal can be used without
charge. 28 satellites are running along six different tracks at a height of 20180km far
from the earth. They can make sure any location on the earth is monitored by at least
four satellites.
GPS’ positioning precision is related to the quality of the received signal. The positioning
precision means the result measured by satellite analog signal generator in the lab. The
satellites’ transmission power is not high and the signal is easy to be blocked or
reflected, which affect positioning precision. Therefore, the receiving sensitivity of the
receiver shall be high. The meter’s embedded GPS module has high receiving sensitivity,
so it can receive weak signal for positioning in city center or indoor. As the receiver’s
catching sensitivity is lower than the tracking sensitivity, so the user can firstly move
the receiving antenna to signal-stronger place (such as outside the window) when
using it indoor or in the weak-signal environment. After the receiver catches 4 satel-
lites’ signal and positioning can be made correctly, move the antenna to the place to be
positioned.
GPS positioning measurement of ST-25D mainly measures longitude, latitude, altitude,
movement speed, movement direction and straight-line distance between two points.
The unit of longitude and latitude is degree. The user can find the satellite pictures of
relevant longitude and latitude through inputting the following content on the internet.
http://maps.google.com/maps?t=k&hl=en&ie=UTF8&z=18&ll= 25.0309073, 121.5431877&
In the "&ll= 25.0309073, 121.5431877&", the 25.0309073 is latitude and 121.5431877 is
longitude. It is required to change by users themselves.
The altitude error is larger, and the movement speed and movement direction are
calculated by the distance between two points within fixed interval. When the move-
ment speed is lower or in static state, the two data errors are much larger. The move-
ment speed error is about ±3.6km/h. To reduce the error caused by satellite movement,
it is suggested for the user to measure the distance within shortest time as can as
possible.
