How do PDOP, HDOP, and VDOP reflect satellite geometry, and what range indicates good geometry?

• A. A low PDOP (<4) indicates good geometry, while HDOP and VDOP vary with horizon.
• B. They quantify how satellite geometry affects position error; lower values indicate better geometry; typical good geometry is PDOP < 4, HDOP and VDOP vary with horizon.
• C. They measure clock bias in satellites.
• D. The product of PDOP, HDOP, and VDOP must be less than 1 for good geometry.

Answer: (D)

PDOP, HDOP, and VDOP show how the geometry of satellites in the sky influences potential position error. They’re dilution of precision values: PDOP combines the effects of geometry in all three dimensions, HDOP relates to the horizontal plane, and VDOP relates to vertical accuracy. The smaller these numbers, the tighter the satellite geometry and the more accurate the position can be expected to be.

A practical takeaway is that lower values mean better geometry. A commonly used guideline is that a PDOP below about 4 indicates good overall geometry, with HDOP and VDOP reflecting how spread-out the satellites are in the horizontal and vertical directions. These values can vary with horizon coverage, so their exact numbers depend on the satellite configuration at the time.

They do not measure clock biases in satellites. Clock biases are separate error sources that are absorbed into the receiver’s solution as part of estimating position and receiver clock error. The idea that the product of the three Dop values must be less than 1 is not a standard criterion; each value stands on its own to describe geometry, and there isn’t a universal product threshold used to judge goodness of geometry.