Dilution of precision (GPS)

Dilution of Precision (DOP) to German as reducing the accuracy in satellite navigation systems is a measure of the scatter of the measured values. Since DOP depends on the relative position of the satellites to each other and to the observer, it is also called geometric dilution of precision ( GDOP ).

Survey

For satellite navigation systems for position determination of the distances are determined to several satellites by measuring signal transit time. The accuracy of the position determination is dependent on the position of the satellites to each other and to the observed location, as well as the accuracy of the measurement of time or run time. Cheap angle between the satellite and the location of the receiver are 90 ° angle. For very small angles or angles around 180 °, ie when the satellites are low and only slightly above the horizon, the distance circles or spheres intersect at shallow angles and reduce the measurement accuracy. The reciprocal of the area or the volume that span the unit vectors in the direction of the satellite, determined the size of the DOP.

DOP is a measure of how well this combination is suitable for the measurement for a given satellite constellation of a particular location. The value of one describes the best geometrical arrangement of the satellites, a value of about 6 is still good. DOP values ​​can be no more analysis is greater than 10.

Santander loop section

Similar to the resection in geodesy one talks of dangerous places ( dangerous intersection balls ) between the relevant measurement points.

In the figure, two receivers receive at B and B ' signals from the transmitters S1 and S2. At B ' causes the angle of the direction vectors of 135 ° in the plane of a DOP in the plane of the second spatial uncertainty (red circle) is slightly larger than the local error by the signal propagation time measurement. At B, the angle deviates only little from 180 ° DOP is greater than 10, depending on the distance of the transmitter is the red error ellipse extends over a multiple of the surface resulting from the inaccuracy of signal.

DOP values

The following are the DOP values ​​:

DOP values ​​are provided by GPS receivers often in the NMEA sentence GSA in boxes 15, 16 and 17 available.

Accuracy

DOP itself is not a measure of the accuracy of the position determination. DOP instead influences the size of the scattering of the measured values ​​. A large DOP increases the spread of the measurements. In the graph two experimentally determined positions S1 and S2 are shown. A small DOP in S1 caused less dispersion than in position S2. Due to systematic errors ( for example, variations in the satellite ephemeris ) S1 and S2 are outside their error limits. Also due to systematic errors ( for example, by variations in the Ionosphärenkorrektur ) is the true position outside the measurement variations. The shift to the true position is called offset. The offset and therefore the absolute GPS accuracy is of the order of 10 m.