Concept · RTK Fundamentals

What is PDOP and why does it affect my accuracy?

PDOP tells you how well your satellites are spread across the sky. A low PDOP means good geometry and reliable RTK accuracy. A high PDOP means satellites are clustered together — and your position could be off by metres even at Fix.

What PDOP means

PDOP stands for Position Dilution of Precision. It is a dimensionless number that describes how much your satellite geometry amplifies ranging errors into position errors. A lower PDOP is better.

Think of it this way. Your receiver calculates its position by measuring the distance to multiple satellites simultaneously. If the satellites are spread evenly across the sky — some ahead, some behind, some to the sides, some high overhead — the geometry is strong and small ranging errors produce small position errors.

If all visible satellites are clustered in one part of the sky — say, all to the south — the geometry is weak. The same ranging errors now produce much larger position errors because the receiver cannot triangulate well. PDOP quantifies exactly this effect.

PDOP scale — what values are acceptable

PDOP is a unitless number. Lower is better. Here is how to interpret what you see on your receiver or field software:

How satellite geometry affects PDOP

The interactive diagram below shows how different satellite distributions affect the PDOP value. Click each scenario to see how geometry translates to PDOP.

PDOP, HDOP, VDOP — the full family

PDOP is the most commonly referenced DOP value, but your receiver typically reports several DOP types. Each measures a different aspect of the satellite geometry.

The mathematical relationship between them is: PDOP² = HDOP² + VDOP². In practice, VDOP is always larger than HDOP because satellites can only be seen above the horizon — there are no satellites below you to improve vertical geometry. This is why RTK height accuracy is always worse than horizontal accuracy.

How to improve PDOP in the field

PDOP is primarily driven by the number and distribution of visible satellites. Here is what you can do:

• Use multi-constellation tracking. Enabling GPS, GLONASS, Galileo and BeiDou simultaneously gives your receiver 30–50+ satellites to choose from instead of 8–12. More satellites across more positions in the sky always improves PDOP.
• Lower the elevation mask carefully. The elevation mask excludes satellites below a set angle (typically 15°). Low-elevation satellites are noisier but they also occupy unique geometry positions that can dramatically improve PDOP. In open sky, a 10° mask often gives better PDOP than 15°. Never go below 5° — multipath increases sharply.
• Move away from obstructions. Trees, buildings and terrain features block specific parts of the sky, clustering your visible satellites into the remaining open area. Even moving 5–10 metres can open up a new sky quadrant and halve your PDOP.
• Wait for better geometry. Satellite positions change continuously. A poor PDOP window typically lasts 10–30 minutes before geometry improves naturally. Check a satellite prediction app (such as GNSS View) to plan your session around good geometry periods.
• Avoid working near large reflective surfaces. Metal roofs, water surfaces and glass buildings reflect satellite signals, causing multipath. Multipath increases effective PDOP by adding false satellite measurements that degrade geometry quality.

PDOP and RTK Fix — what your receiver does

Most professional RTK receivers apply an automatic PDOP mask — they refuse to output a Fix solution when PDOP exceeds a threshold. The default threshold varies by manufacturer:

The PDOP mask is a safety feature, not a limitation. If your field software rejects a measurement because PDOP is too high, it is protecting you from collecting data that looks like Fix but is not reliable. Trust the mask.