RTK is one of the high-precision geolocation systems compatible with SonarVision. It is a highly accurate “GPS” receiver that allows for geolocation with an accuracy of less than 10 centimeters under ideal conditions.

There are an increasing number of compatible and affordable RTK receivers available that can be connected to SonarVision via Bluetooth:

• Getting and Using an Ardusimple RTK

• Sparkfun’s “RTK Surveyor” series (article coming soon).

• A custom-built receiver made by a volunteer.

Here’s a demonstration created by a SonarVision user:

Centipede Network Availability

The Centipede network covers most of metropolitan France, certain overseas territories, and a few other regions around the world. The SonarVision app automatically connects to the nearest active RTK base station.

You can visit the official map to view existing stations around the world.

More Information

Here are a few paragraphs to help you better understand this system.

What is GNSS?

First of all, GNSS stands for “Global Navigation Satellite System,” commonly referred to as GPS in everyday language. GPS is actually the U.S. satellite constellation, the Global Positioning System, orbiting the Earth alongside the European (Galileo), Chinese (Beidou), and Russian (GLONASS) constellations.

GPS (i.e., GNSS) receivers are generally capable of receiving signals from all of these constellations. However, depending on their antenna, microchip, the algorithms used, and the environment, their accuracy can vary greatly.

For example, with an iPhone 13, in an open area such as a park, accuracy ranges from 3 to 10 meters. In contrast, in a narrow alley lined with 10-story buildings, accuracy is typically between 10 and 50 meters. This is because the GPS antenna can only “see” satellites in a straight line through a narrow strip of sky between the two buildings, and signals from other satellites arrive after bouncing once or more off the building facades: this is known as the “canyon effect.”

The iPhone 13 is equipped with a basic antenna that detects only one frequency transmitted by the satellites—the L1 frequency. However, there are “multi-band” antennas capable of simultaneously detecting multiple frequencies from the same satellite. With a specialized chip and algorithms, it is then possible to determine whether a signal has bounced off a wall before reaching the antenna and, therefore, to ignore it. These receivers are commonly referred to as “dual-band” and are found in many consumer devices, such as the iPhone 14 Pro and later models, as well as certain smartwatches. In terms of accuracy, this greatly reduces the errors associated with the canyon effect and, in practice, allows for accuracy of less than 5 meters, even in urban areas.

So is it possible to achieve meter-level or even centimeter-level accuracy with a GPS system—accuracy that’s so valuable for pedestrian navigation? Well, yes—ever since the invention of the RTK system with corrections.

What is RTK?

RTK stands for Real-Time Kinematics. It’s an advanced technique that allows you to measure your position with a theoretical accuracy of 1 to 2 centimeters. In practice, however, the signals from the satellites are deflected by the ionosphere—a very high layer of the atmosphere—which disrupts the measurements taken by the receiver and limits the accuracy of a standalone RTK system to around 5 meters…

However, physics has a nice surprise in store for us: between two points 10 kilometers apart, these disturbances are virtually identical and remain very similar up to 50 kilometers. The ingenious designers of the RTK therefore came up with the idea of using a first fixed RTK, placed at a well-known GPS coordinate and called the “base,” to correct the measurements taken by another mobile RTK—mounted, for example, on a tractor—and called the “rover.” If the base detects that it is 5 meters too far north, it transmits a correction via the internet to the rover, which can then correct its position by 5 meters to the south. The result: positional accuracy of about 1 centimeter.

This technology was initially widely used in precision agriculture, allowing farmers, for example, to drive a tractor year after year along the same furrows without overly compacting the soil. Initially, these antennas cost between €15,000 and €25,000, and a subscription to a correction network could cost up to €1,000 per year.

In recent years, companies such as u-blox have been marketing much cheaper RTK chips, making it possible to build more affordable receivers for just a few hundred euros. French farmers have embraced this technology and created a free and open network called “Centipede,” which provides corrections at no cost.

All that remains is to develop guidance applications capable of using them!