What is RTK GPS?
Real-Time Kinematics (RTK) navigation is a technique used to reduce and remove common positioning errors. Unlike code-based positioning, RTK GPS or RTK GNSS uses carrier-based ranging and provides real-time corrections with the help of a single reference station. This enhances the precision of position data received from satellite-based positioning systems, and results in the centimeter-level accuracy required for many applications. You can find more information about GNSS here.
When using code-based positioning, a receiver establishes its position by determining the range of at least four satellites. These ranges, along with the known position of the satellite let the receiver establish its position within a few meters. On the other side of things, carrier-based ranging calculates a position by determining the number of carrier cycles between the satellite and the rover station, and multiplies this number by the carrier wavelength. Carrier-based ranging allows more precise measurements.
RTK GPS/GNSS Components:
In short, RTK GPS/GNSS comprises three components: a local base or reference station, one or more rovers (also called mobile), and a communication channel.
A local base station with a known geospatial location monitors satellite signals, sending information to rover user(s) through a communication channel which allows the base to broadcast correction data in real-time. The diagram below shows a base sending corrections to a rover using UHF radio.
By combining its measurements with those of the base station, the rover corrects common errors such as ionosphere errors and satellite clock errors. Once these are removed, the phase measurements can be used to determine an exact position within 2-5 centimeters.
GNSS Network Corrections
A GNSS Network Corrections service can also provide corrections in real time to a rover with no need for a local base.
The corrections are transmitted to the rover over the internet via a cellular connection. The NTRIP (Network Transport of RTCM via Internet Protocol) lets rovers access the data from the reference station and delivers centimeter-level accuracy.
That reference station also has a known geospatial location. For example, throughout the United States, its territories, and a few foreign countries, a network of Continuously Operating Reference Stations (CORS) offer Global Navigation Satellite System (GNSS) data. The data available includes carrier phase and code range measurements in support of three-dimensional positioning, meteorology, space weather, and geophysical applications.
The CORS Network RTK process is as follows:
- Collection and cleaning of carrier phase and pseudorange data at all CORS stations;
- Network processing using a priori information of station coordinates and GNSS orbits, with the goal of producing very precise estimates of ionospheric delays;
- Generation of ionospheric corrections for mobile users by interpolation of the CORS network ionospheric delays; and
- Precise user GNSS positioning/navigation by applying the CORS ionospheric corrections.
RTK GPS/GNSS provides highly accurate positioning information and should be used to establish several unknown points within the vicinity of the known base location. The closer the base, the better! For optimal results, it is recommended that the distance between the rover and reference station be limited to 20 to 30 kilometers, after which RTK GPS/GNSS position accuracy begins to decrease.
Many manufacturers claim 40 to 50 km as the maximum distance. The more satellites in common viewed by both the base and the rover, the better! Additionally, as the distance increases, atmospheric conditions may be significantly different between the base and rover.
To verify where the closest reference station to your project is, the CORS website provides reference station information, including the name, position, status, sample rate, availability, GNSS satellite constellation in use, and the agency responsible for the station. The screenshot below from the CORS website, for example, shows information on the NCMU reference station in Northern Carolina:
Global Correction Service
A Global Correction Service employs a large ground network of real-time reference receivers. Reference stations are located all (or almost all) over the world. Their corrections can also be captured via cell connection and often, depending on the provider, via L-Band. This type of service offers a great alternative when cell reception is poor or non-existent. There’s no need to worry about losing radio signal connectivity. The service provides correction in real time. The only catch is the convergence time. So, before you start collecting centimeter level accuracy data, one needs to wait for the positioning solution to convergence to the satisfactory or required level of accuracy. Convergence time may vary from 10 to 40 minutes.