What Is RTK GPS and How Does It Work for Construction?

How RTK GPS Works

A standard GPS receiver computes position by measuring the time signals take to arrive from multiple satellites. Atmospheric conditions, satellite orbit errors, and signal noise limit standalone GPS accuracy to roughly 1-3 meters. RTK eliminates most of these errors by using a base station — a second GPS receiver on a precisely known point — that measures the same errors in real time and transmits corrections to the rover. The rover applies these corrections and achieves centimeter-level accuracy.

The base station and rover observe the same satellites simultaneously. The base knows exactly where it is; by comparing its known position to what the satellites are saying, it computes the errors and sends those corrections to the rover. The rover applies them and computes an accurate position. This is called differential positioning — the accuracy improvement comes from the difference between what the satellites say and what's actually true at the base point.

RTK Accuracy in Practice

Trimble, Topcon, and Leica RTK systems achieve 8mm horizontal and 15mm vertical under good conditions. 'Good conditions' means: fixed solution (not float), 5+ satellites with good geometry, base within 10-15km, no severe multipath. Under poor conditions (heavy forest, urban canyons, long baseline), expect 15-30mm. For most construction stakeout — structure layout, utility location, rough grading — 8-15mm is more than adequate. For precise control surveys or tight-tolerance machine control calibration, work in good conditions and run redundant checks.

RTK Fixed vs Float vs Autonomous

Fixed: Centimeter accuracy. The receiver has resolved carrier phase ambiguities — this is the mode you want for construction layout. Your controller shows 'Fixed' in the solution status.

Float: 0.1-0.5m accuracy. Corrections are being received but ambiguities haven't been resolved. Never use Float for construction stakeout — it looks normal but positions are off.

Autonomous: 1-3m accuracy. No corrections. No RTK. For emergencies only — not for layout work.

Network RTK vs Base Station RTK

Base station RTK requires setting up your own base on a known point. Network RTK (NTRIP/VRS) connects to a correction network via cellular — no base setup required. Network RTK is faster to deploy and works anywhere with cell coverage. Base station RTK works in areas without cellular coverage and doesn't require a subscription to a correction service. Most construction GPS fleets use network RTK in urban areas and base station in rural or remote projects.

Frequently Asked Questions

What accuracy does RTK GPS achieve?

Typically 8mm horizontal and 15mm vertical under good conditions with a fixed solution. This is centimeter-level accuracy sufficient for construction stakeout, grading, and most survey control applications.

What is the difference between RTK fixed and float?

Fixed solution is centimeter-accurate (8-15mm). Float solution is 10-50x less accurate (0.1-0.5m). Always confirm Fixed before staking any points — Float looks normal but positions may be significantly off.

How far can an RTK base station be from the rover?

Practical range for UHF radio-based RTK is 10-15km with line of sight. Network RTK (NTRIP) provides effectively unlimited range within cellular coverage areas.

Does RTK GPS work indoors?

No — GPS requires line of sight to satellites and cannot work indoors or in tunnels. For indoor layout, use total stations or laser instruments instead.