ST2276
Scientech Technologies Pvt. Ltd.
21
Experiment 5
Objective :
1.
Understanding the principle of Geometry of the Satellite.
2.
Understanding the importance of PDOP, HDOP, and VDOP.
Theory :
DOP (Dilution of Precision) :
The accuracy with which a position can be determined using GPS in navigation mode
depends, on the one hand, on the accuracy of the individual pseudo-range
measurements and on the other, on the geometrical configuration of the satellites
used. This is expressed in a quality, which in navigation literature is termed DOP
(Dilution of Precision).
There are several DOP designations in current use :
GDOP :
Geometrical DOP (position in 3-D space, incl. time deviation in the
solution).
PDOP :
Position DOP (position in 3-D).
HDOP :
Horizontal DOP (position on a plane).
VDOP :
Vertical DOP (height only).
The accuracy of any measurement is proportionately dependent on the DOP value.
This means that if the DOP value doubles the error in determining a position increases
by a factor of two.
Figure 15
Satellite Geometry and PDOP :
PDOP can be interpreted as a reciprocal value of the volume of a tetrahedron, formed
by the positions of the satellites and user, as shown in figure. The best geometrical
situation occurs when the volume is at a maximum and PDOP at minimum.
PDOP played an important part in the planning of measurement projects during the
early year of GPS, as the limited deployment of satellites frequently produced phases
when satellite constellations were geometrically very unfavorable. Satellite
deployment today is so good that PDOP and GDOP values rarely exceed 3.
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