What does DOP measure and how do HDOP/VDOP relate?

• A. DOP quantifies the effect of satellite geometry on position error; HDOP and VDOP reflect horizontal and vertical components.
• B. DOP measures the signal strength of GNSS satellites.
• C. HDOP and VDOP are time dilation measurements.
• D. DOP is a constant that does not vary with geometry.

Answer: (A)

DOP, or Dilution of Precision, measures how the geometry of the satellites relative to the receiver affects how measurement errors translate into position errors. The idea is that even if each satellite measurement has the same noise, where the satellites sit in the sky determines how those errors combine to affect the estimated position. A low DOP means the satellites are well spread out and the potential position error is smaller; a high DOP means they are clustered or poorly distributed and the potential error is larger.

HDOP and VDOP are the specific components of this effect in the horizontal and vertical directions. HDOP tells you how much the horizontal position (latitude and longitude) could be off due to geometry, while VDOP tells you about the altitude error. The overall PDOP (not asked here but related) combines horizontal and vertical effects to describe the total 3D position error potential. DOP values change with satellite geometry and do not reflect signal strength or time dilation, nor are they constants.