Module 2 : Global Positioning System
  Lecture 11 : Satellite geometry and Accuracy measures
Geometrical interpretation of Dilution of Precision DOP
As mentioned earlier DOP is an indicator of three dimensional positioning accuracy as consequence of relative position of GPS satelllites with respect to a GPS receiver. In order to attach a geometrical interpretation to this, we use a term called geometriacl dilution of precision or GDOP. Figure 11.2 and 11.3 explain a simple interpretation for this. A close bunching or poorly spaced satellites gives poor GDOP value whereas well distributed satellites yield good GDOP. For an ideal situation i.e one satellite overhead with the others spaced at equal horizontal angles about the observer, will give the best GDOP.

Figure 11.2 Spatial geometry for GDOP values (Dana, 2000)

Figure 11.3 Spatial geometry for GDOP values (Dana, 2000)

 

In literature, different types of DOP designations are is use. For example, GDOP, PDOP, etc. These are give below:
GDOP : Geometrical Dilution of Precision, (measure of accuracy in 3-D position and time)
PDOP : Position Dilution of Precision (measure of accuracy in 3-D position), also called spherical DOP
HDOP : Horizontal Dilution of Precision (measure of accuracy in 2-D position, for example Latitude and Longitude)
VDOP : Vertical Dilution of Precision (measure of accuracy in 1-D position, Height)
TDOP : Time Dilution of Precision (measure of accuracy in Time)

PDOP is also interpreted as inversely proportional to the volume, V, of a tetrahedron that is formed from the satellite and user positions ( PDOP α 1/V). These DOP values are related to positioning errors and UERE by the following expressions where σr is UERE, σH, σV, σP and σT are standard deviations in horizontal, vertical, position (both horizontal and vertical), and time respectively.
 
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