RTK stands for 'real time kinematic [positioning]', and it's a system that works to make GPS data more accurate. So satellite data alone might help locate your robot lawn mower correct to within a few meters, but combine it with RTK positioning, and it can narrow that down to a few centimeters.
This is the technology that has enabled modern lawnbots to navigate without a boundary wire. It also means the lawnbot can mow in an ordered, logical fashion, rather than just traversing the lawn randomly.
However, the effectiveness of RTK positioning is reliant on specific factors – and if certain things are off, it might not work very well at all. Read on for more on how RTK positioning works in robot lawn mowers.
Why is satellite data not accurate enough?
We've been using the term GPS, but that is in fact just one of four systems that might be providing the satellite data that helps your lawnbot find its way around – so a more correct acronym would be GNSS (global navigation satellite system).
The satellites use two main signal types. The oldest type of satellite signal is the L1 signal, and this can't pass through any solid object. Newer L2 signals can penetrate through some objects – including things like thinner timber, plastic and light tree coverage – but every time it does so, the signal gets weaker.
The time it takes for the signal to get from the satellite down to earth is what creates the inaccuracies in positioning data. Even passing through our atmosphere causes delays; the signal passes through some areas more quickly than others.
How does RTK positioning help?
RTK essentially helps correct inaccuracies in satellite positioning data, to make it more precise. It does this in real time.
The system relies on a fixed, antenna-equipped base station with known coordinates (this is separate from the robot lawn mower's charging station, which is where the bot docks and recharges its battery). Both the RTK base station and lawnbot connect to satellites in the sky.
To do this they need a direct line of sight to each satellite – things like walls, roofs and even tree coverage can block the signal. What's more, both the base station and lawnbot both need to connect to the same satellites in order to function.
The RTK station needs to be placed so it has the widest unobstructed view of the sky as possible, in as many directions as possible. It should be high up, and away from overhanging buildings and trees.
Because the lawnbot needs to be able to connect to satellites when it's docked as well as when it's working, the charge station also needs to be in a position with a direct line of sight to the sky (you can generally get away with there being a wall behind, provided the other three directions are open and have good line of sight to the sky.)
The RTK base station also connects to the lawnbot, but this connection doesn't require a direct line of sight – it's usually a radio signal, which can pass through solid objects.
Are there any down-sides of the RTK system?
The big benefit of using RTK and GPSS data for navigation is that it means you don't need to go through the hassle of adding a boundary wire to your garden.
However, it's not a failsafe system. The whole thing hinges on there being plenty of spread out satellites that both the robot mower and the RTK station can connect to. Overhead obstructions like trees and buildings will impact on how effective it is, and having a good place to put your RTK station is vital.
Because they're so sensitive to environmental factors, lawnbots that rely on RTK are more prone to getting lost, or straying out of bounds, than those that use perimeter wires.
You might also find that the location data still isn't quite precise enough. An RTK-powered robot mower will will navigate your lawn in neat lines, but because there's still a few centimeters' margin of error, you might end up with unmown strips of grass between rows. You might also need to leave a larger buffer at potentially hazardous perimeters – for example, the edge of a pond. Lawnbots that use boundary wires will cover the area in a much more haphazard way, but they'll never go over a boundary, and are perhaps less likely to end up missing a spot.
These potential weak points mean almost all modern lawnbots will include another kind of navigation system as backup for when signal is poor. That might be something like radar or LiDAR (the method used in most of today's best robot vacuums), or a camera. There are of course limitations for each system, but technologies are getting ever more advanced, and as a result, so are the lawnbots they power.
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