Geometric Dilution of Precision (GDOP)

Geometric Dilution of Precision (GDOP) is a number that tells how the geometry of the tracked satellites multiplies the ranging error into position error. It is the ratio of position-and-time error to measurement error: GDOP times the user-equivalent range error gives the expected fix error. A low value, near 1 to 2, means satellites well spread across the sky and a tight fix; above about 6 the satellites are clustered and the fix degrades. GDOP splits into PDOP for position, HDOP for horizontal, VDOP for vertical, and TDOP for clock. A bridge receiver reports HDOP so the officer can judge fix quality; a high value among high superstructure or in a steep harbor warns against trusting the position.