GNSS RTK

RTK is one of the precise geolocation systems compatible with SonarVision. It is a very accurate “GPS” receiver that provides positioning within less than one meter and connects to the application via Bluetooth.

Here is a demo recorded by one of our users:

What is GNSS?

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

GPS receivers (meaning GNSS receivers) are generally able to pick up signals from all these constellations. However, depending on their antenna, electronic chip, algorithms, and the surrounding environment, their accuracy can greatly vary.

For example, with an iPhone 13 in an open area like a park, accuracy ranges between 3 and 10 meters. In contrast, in a narrow street surrounded by 10-story buildings, accuracy is more often between 10 and 50 meters. The GPS antenna only has a direct line of sight of satellites in a thin strip of sky between the buildings, and the signals from other satellites arrive after bouncing off façades: this is known as the canyon effect.

The iPhone 13 has a basic antenna that only detects one satellite frequency — the L1 frequency. However, there are “multi-band” antennas capable of detecting several frequencies from the same satellite at once. With an appropriate chip and algorithms, it becomes possible to know whether a signal bounced off a wall before reaching the antenna and therefore ignore it. These receivers are commonly called “dual-band” and are found in many consumer devices such as the iPhone 14 Pro and above, or some smartwatches. In terms of accuracy, this significantly reduces the errors caused by the canyon effect and generally keeps accuracy below 5 meters, even in cities.

So is it possible to reach meter-level or even centimeter-level accuracy with a GPS system — something extremely valuable for pedestrian navigation? Yes it is, thanks to the invention of RTK with corrections.

What is RTK?

RTK means Real-Time Kinematics. It is an advanced technique that can theoretically measure position with 1–2 cm accuracy. In practice, however, the satellite signals are distorted by the ionosphere, a very high layer of the atmosphere, which disturbs the measurements made by the receiver and limits the precision of a standalone RTK to around 5 meters…

Thankfully, atmospheric physics has our backs: between two points located within 10 km, these disturbances are almost identical, and remain very similar up to 50 km. The clever designers of RTK systems therefore had the idea of using a first fixed RTK receiver – known as the “base” – placed at a well-known GPS coordinate to correct the measurements of another mobile RTK receiver – known as the “rover”. If the base measures a positional error of 5 meters to the north, is broadcasts a correction to the rover over the internet, which can correct its own position 5 meters to the south. Thanks to these corrections, the rovers final position can be known down to about 1 centimeter.

This technology was initially heavily used in precision agriculture, allowing farmers to minimize the overlap when sowing seeds. Originally, 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 like u-blox have been offering much cheaper RTK chips, making it possible to build more affordable receivers costing only a few hundred euros. French farmers adopted this and created a free, open correction network called “Centipede”.

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

Centipede network availability

The Centipede network covers most of mainland France, some overseas territories, and a few other regions in the world. The SonarVision app automatically connects to the nearest active RTK base.

You can view the official map to see all existing RTK bases worldwide.