What is RTK? — How centimetre positioning works
Your phone's GPS is always off by a few metres. But with RTK (Real-Time Kinematic), the very same satellites give your position to a few centimetres. Surveying, precision agriculture, drones, construction machinery, self-driving — every field that needs position “to the centimetre” rests on this technology. Here's how it works, without the jargon.
Why ordinary GPS is off by metres
GNSS (the umbrella term for GPS and others) measures distance from the “time it takes” for a satellite's radio signal to arrive, then computes your position from several satellites. But on the way down from space, that signal is slightly delayed as it passes through the ionosphere and the atmosphere (troposphere), and the satellite's orbit and clock carry tiny errors too. A single receiver cannot remove these, so the position is off by several metres.
How RTK works — the base station “cancels” the error
This is where the base station comes in. A base station is a fixed GNSS receiver whose coordinates are precisely known. Because your receiver (the rover) and the base station receive signals from the same sky at the same moment, they experience almost the same errors. The base station measures “how much error is on the signal right now” and sends that correction. The rover subtracts it, cancelling the error and reaching cm accuracy (differential positioning).
RTK goes further and counts the “number of wave crests” (carrier phase) of the signal. GNSS wavelengths are short — about 19 cm — so counting these crests precisely yields very high accuracy. The shorter the distance between base and rover (the baseline), the more alike their errors, and the better the accuracy.
How the correction reaches you — NTRIP
A base station's correction data (a format called RTCM) is usually delivered over the internet by a mechanism called NTRIP. Connecting to a “mountpoint” on an NTRIP caster lets you receive that station's corrections in real time. Thousands of base stations are shared by volunteers worldwide; you can check their location and status on KUON's base-station ranking or the map.
Fix and Float — “solved” or “not yet solved”
The “number of crests” of the carrier phase is inherently an integer. When it is solved correctly, the state is called Fix, and cm accuracy is achieved. When it is not yet solved, the state is Float, with accuracy only at the decimetre level. An open sky, a short baseline and giving it time all make a Fix easier.
Even without a base station — QZSS CLAS
Japan's quasi-zenith satellites QZSS (Michibiki) broadcast augmentation called CLAS on the L6 signal. With a compatible receiver, you can achieve cm-level positioning from that augmentation alone — without connecting to a ground base station. Its strength is enabling high accuracy even where base-station infrastructure does not reach.
Where is it used?
Surveying (boundaries, terrain), precision agriculture (auto-steering tractors, row-level management), drone surveying, automatic control of construction machinery, point clouds and 3D mapping, and the coming world of robots and self-driving — every domain where “a machine needs to know exactly where it is”. As RTK has become affordable, these capabilities are reaching individuals too.