Site Survey using RTK

The method of choice for a centimetre-accuracy site survey is RTK (Real Time Kinematic). RTK involves the use of 2 GPS receivers (stationary Base Station and a Rover) communicating together via a radio link. The base station must be located such that it will have a clear view of the sky and a continuous line-of-sight to the rover. The precision of the Rover position relative to the Base station is dependant on baseline-length so it is desirable to keep the baseline as short as possible (<10 km).

RTK performs Real Time Phase Differential and computes the 3D vector (D X, DY, DZ) between the rover and base antennas. Base Station coordinates and both antenna heights need to be entered to compute ground coordinates at the Rover.

How does RTK work?

Data from the Base Station (either raw GPS data or RTK corrections; there are different RTK approaches) is sent in real time via radio to the Rover. With a sufficient number of common satellites visible at both GPS antennas, a “FIXED” solution of centimetre-level precision can be calculated.

If there are insufficient common satellites, a “FLOAT” solution of lower precision (a few decimetres) is calculated. You can try waiting for a “FIXED” solution or re-initializing the system (consult your manual) but these may not be successful as most often the FLOAT solution is due to poor satellite visibility at the Rover.

RTK systems are available in dual-frequency and single-frequency versions. Dual-frequency systems deliver greater precision, faster and over longer baselines than single-frequency systems. Your choice in equipment will ultimately depend on your budget, expected baseline lengths and precision needs. See manufacturer’s specifications.

Here are typical horizontal precisions (RMS) you can expect for various baseline lengths under ideal conditions: