What is Dilution of Precision?

Dilution of Precision (DOP) is a term used to specify the error in positional fix provided by a GNSS receiver due to the geometry of the navigational satellites from which signals are received. It is also called geometric dilution of precision (GDOP), where geometry refers to the number of satellites that are visible (for line-of-sight propagation), their altitude and the bearing towards them. DOP is only a value of probability for the geometrical effect on GPS accuracy and is roughly interpreted as the ratio of position error to the range error. The complete calculation of DOP can be complex and is computed from the geometric relationships between the receiver position and the positions of the satellites being used for navigation.

Figure 1 Example of satellites with poor geometry for high DOP values

A GNSS receiver usually requires only four satellite signals to provide a complete positional fix in three dimensions. The accuracy of this fix depends to some degree on the exact positions of the four satellites relative to the receiver. If the four signals acquired come from satellites spread throughout the sky relative to the receiver, then the fix will be highly accurate. But if all four satellites are close to each other within a single quadrant, then the fix will be less accurate. If two or more satellites are aligned and to appear to occupy the same space, then it may be impossible to obtain any fix. Alternatively, the fix obtained may be out to the tune by 150 or even 200 meters. When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when they are far apart, the geometry is said to be strong and the DOP value is low. Hence, a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a GNSS receiver unit's position.

Figure 2 Example of satellites with good geometry for low DOP values

The various interpretations of DOP values are as follows:

DOP can be expressed as several separate measurements with different definitions:

Position (3D) DOP (PDOP): This value describes how many satellites are spread evenly throughout the sky. The more the satellites directly above and the less on the horizon, the lower the PDOP value is.

Horizontal DOP (HDOP): This describes the effect of DOP on the horizontal position value. The more good visible satellites low in the sky, the better the HDOP and the horizontal position (Latitude and Longitude) are.

Vertical DOP (VDOP): This refers to the effect of DOP on the vertical position value. The more good visible low satellites in the sky, the better the VDOP and the vertical position (Altitude) are.

Time DOP (TDOP): This value describes the difference in time values of the satellites and receiver’s internal clocks. The more in sync their clocks are the lesser the TDOP value.

While each of these DOP terms can be individually computed, they are formed from covariances and so are not independent of each other. For example, a high TDOP (time dilution of precision) will cause receiver clock errors which will eventually result in increased position errors. The above DOP components are used because the accuracy of the GPS system varies as satellites move, thus varying the geometry with time.

DOP values have much importance in satellite navigation, geomatics engineering and other location systems that employ several geographical spaced sites. In electronic warfare DOP values of electronic-counter-counter-measures are important when computing the location of enemy emitters (radar jammers and radio communications devices).