Dilution of Precision

Dilution of Precision or DOP describes the geometric quality of a GPS satellite configuration in the sky. The smaller the DOP number, the better the geometry. Factors that affect the DOP are, besides the satellite orbits, the presence of obstructions which make it impossible to use satellites in certain sectors of the local sky. Especially in urban measurements, this may be limiting.

We speak of HDOP, VDOP, PDOP and TDOP respectively, for Horizontal, Vertical, Position (3-D) and Time Dilution of Precision. These quantities follow mathematically from the positions of the useable satellites on the local sky. GPS receivers allow the display of these positions ("skyplot") as well as the DOP values.