Global Navigation Satellite System or GNSS influences every industry: navigation to farming, construction, and archaeology. In this article, we’ll discover how GNSS positioning and the related Real-Time Kinematic (RTK) technique work.
How GNSS Positioning Works
Before we move on with RTK, it’s essential to understand the general idea behind the positioning methods. The whole GNSS system is based on measuring how long it takes for a signal to travel from a satellite to the receiver.
You can determine your position on the planet, knowing the precise orbits of the space vehicles (the ephemeris) and travel times from at least four satellites—to first form the area and then determine the exact location by finding the point where all the spheres cross. To get a closer look into positioning, check out our Positioning With GNSS video.
When you determine your position, you may wonder how accurate it is. Most of the GNSS or GPS receivers, like the one you can find in your smartphone or on most robotic platforms, determine your position with 2-4 meters accuracy. It’s enough for you to find a shop you were looking for on Google Maps. However, the estimation is way too rough for, say, land surveying. So what’s the source of such an error, and how to eliminate it?
What Is RTK and How Does It Work
Real-Time Kinematic (RTK) is the technique that eliminates errors as much as possible to give you accurate results and enhanced positional data down to centimeter resolution.
When the signal from satellites travels towards the receiver, it goes through 20,200 km of ionosphere and atmosphere down to Earth. The ionospheric effect significantly slows the signal and also can disturb it on the way. In addition, many factors, such as clouds, or obstacles, can affect the travel time and increase the position error.
To deal with these issues, RTK comes to the rescue. For RTK, you need two GNSS receivers: one is static and called a “base station,” the other is moving and is called a “rover.” While both receivers observe the same satellites simultaneously, the static base station is placed at a point with known coordinates (a benchmark or a point measured beforehand). Taking into account the known coordinates and receiving satellite signals, the base transmits corrections to the moving rover. This way, the rover can get sub-centimeter accurate positioning. The idea is simple, but the devil’s in the math here. Find out more about the Real-Time Kinematic in our guide How RTK works.
You don’t necessarily need two units for RTK all the time. You can also perform RTK with only one rover. In this case, you’ll need local services that share base corrections over the Internet and are called NTRIP. Acting as a base station, the NTRIP service transmits the corrections to your rover. For more information about working with NTRIP, check out our article What Is NTRIP.
If you need to survey in an area without Internet coverage required for NTRIP corrections, there’s another solution for you. PPK or Post-Processed Kinematic technique works pretty much like RTK, but without a real-time connection between the base and the rover. Instead, both units record raw data during the survey, and then these logs are processed together in PPK software. To learn more about PPK, watch our tutorial How PPK Works.
How to Choose a Receiver
RTK receivers may differ by the number of frequency bands they work with. There are single-band and multi-band receivers for RTK. Both receivers are capable of centimeter-level accuracy. The difference is that more factors can influence the stable fix solution in the single-band receiver, such as a baseline between a base and a rover, the sky view, and the surrounding environment. So the choice of the receiver mainly depends on your working conditions.
The multi-band units work with several frequency bands. The more signals that are available, the less time is needed to obtain a fix solution. Reach RS2 multi-band receivers require ~5 seconds to get a fix solution. Multi-band receivers can also keep the reliable fix solution even with a partly blocked sky view and work at a longer baseline. To learn more about the difference, have a look at our Single-Band vs. Multi-Band article.
Reach Receivers for RTK
Reach RS+ and Reach RS2 are an excellent fit for both RTK and PPK scenarios. Both receivers provide centimeter accuracy and precision. All receivers come with the ReachView 3 app and support data import in industry-standard formats. You can easily export the data from Reach receivers to popular GIS or CAD services for further processing.