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GDOP (geometric dilution of precision) or PDOP (position dilution of precision) describes the error caused by the relative position of the GPS satellites. Basically, the more signals a GPS receiver can “see” (spread apart versus close together), the more precise it can be.

The effect of geometry of the satellites on position error is called geometric dilution of precision (GDOP) and it is roughly interpreted as ratio of position error to the range error. Imagine that a square pyramid is formed by lines joining four satellites with the receiver at the tip of the pyramid.

There are 4 kinds of DOP: Geometric or Position (3D) dilution of precision (GDOP or PDOP – 3D); Horizontal dilution of precision (HDOP); Vertical dilution of precision (VDOP); and Time dilution of precision (TDOP). The maths behind a better spread of satellites means that you get less dilution and hence a better signal.

HDOP, VDOP, PDOP, and TDOP are respectively horizontal, vertical, positional (3D), and temporal dilution of precision. They follow mathematically from the positions of the usable satellites. GPS receivers allow the display of these positions ( skyplot) as well as the DOP values.

GDOP Components are: PDOP = Position Dilution of Precision (3-D), sometimes the Spherical DOP HDOP = Horizontal DOP (Latitude, Longitude) VDOP = Vertical Dilution of Precision (Height) TDOP = Time DOP (Time) While each of these GDOP terms can be individually computed, they are formed from covariances and so are not independent of each other.