What PDOP means
PDOP stands for Position Dilution of Precision. It is a number that describes how the geometry of the satellites affects the quality of your position calculation. It does not measure correction quality, mobile signal or receiver hardware. It measures one thing only: whether the satellites are spread across the sky in a useful pattern.
When satellites are distributed widely around the horizon and overhead, the receiver can determine your position confidently. When they are clustered in one part of the sky, small errors in the satellite measurements have a much larger effect on the calculated position.
< 2
Excellent satellite geometry
2–4
Good for normal RTK work
4–6
Use extra care and verify
> 6
Poor geometry — wait if possible
Why satellite geometry matters
Imagine trying to locate yourself by measuring distance to several landmarks. If all landmarks are in the same direction, a small measuring error can move your estimated position a long way. If the landmarks surround you from different directions, the same error has much less effect.
GNSS works the same way. Your receiver needs signals from several satellites, but the number of satellites alone is not enough. Ten satellites grouped low in the southern sky can give a worse PDOP than eight satellites distributed evenly across the whole sky.
Low PDOP
High PDOP
Satellite pattern
Satellites are spread widely across the sky and at different elevations.
Satellites are clustered in one direction or blocked by the environment.
Position confidence
Small measurement errors have a limited effect on the final position.
Small measurement errors can produce a much larger position error.
RTK behaviour
Fix initialisation is usually faster and more reliable.
Fix can take longer, drop to Float or fail to initialise.
PDOP ranges in practice
| PDOP | Meaning | What to do |
|---|---|---|
| Below 2 | Excellent geometry | Ideal for survey, stakeout and control measurements. |
| 2–4 | Good geometry | Suitable for normal RTK work. |
| 4–6 | Marginal geometry | Check your result on a known point and avoid critical measurements if you can wait. |
| Above 6 | Poor geometry | Wait for the satellite pattern to improve, or move to a clearer location. |
PDOP is not an accuracy value
A PDOP of 2 does not mean your position is accurate to 2 centimetres or 2 metres. It is a quality indicator for geometry. RTK accuracy still depends on corrections, baseline length, multipath, receiver quality and whether you have a genuine Fixed solution.
What causes high PDOP?
Buildings, trees and terrain
Obstructions remove satellites from parts of the sky. A narrow street, forest edge or steep valley can leave your receiver with satellites only in one direction.
Time of day
Satellite positions change continuously. A poor geometry window at 10:00 can become excellent 30 minutes later without you changing anything.
Limited constellations
GPS-only tracking gives the receiver fewer geometry options. Enable GLONASS, Galileo and BeiDou when your receiver and correction stream support them.
An aggressive elevation mask
A high elevation mask excludes low satellites. This can reduce multipath, but it can also leave too few satellites for a strong geometry pattern.
How to improve PDOP in the field
Move to a clearer sky view
Step away from walls, machinery, tree canopies and building overhangs. Even a few metres can reveal satellites that were previously blocked.
Enable every available constellation
Use GPS, GLONASS, Galileo and BeiDou where possible. More constellations give the receiver more satellites to form a good geometry pattern.
Wait for the geometry window
If PDOP remains high in open sky, wait 15–30 minutes and check again. The satellite configuration changes naturally throughout the day.
Review your elevation mask
In open sky, an elevation mask around 10–15° is a practical starting point. Raising it too far can unnecessarily remove useful satellites.
Verify critical work on known control
When PDOP is marginal, check a known point before collecting high-consequence measurements. A Fixed status alone is not a substitute for verification.
PDOP, HDOP and VDOP
PDOP is the combined three-dimensional geometry value. You may also see two related values in your receiver status screen:
HDOP — Horizontal Dilution of Precision
Describes the geometry for latitude and longitude. It matters most for mapping, stakeout and horizontal control.
VDOP — Vertical Dilution of Precision
Describes the geometry for height. It is usually higher than HDOP because vertical GNSS measurements are naturally less well constrained.
PDOP — Position Dilution of Precision
Combines horizontal and vertical geometry into one overall indicator. It is the value most field applications display by default.
A practical rule: do not chase a perfect number
For most RTK work, a PDOP below 4 with a stable Fixed solution, good corrections and an open sky view is entirely practical. Use PDOP as a warning signal and a planning tool — not as the only quality check.
