Robot lawn mowers have been with us for a while now, and while the original models relied on boundary wires to make sure they didn’t stray into borders, newer lawnbots are using more sophisticated navigation methods – those that don’t require wires.
Modern robot lawn mowers use satellite positioning data, combined with RTK (real-time kinematic) positioning to improve accuracy, to find their way around your yard. Since this method of navigation relies on a decent satellite connection, most lawnbots will also come with a some sort of secondary sensor that’s designed to enable them to continue mowing in areas of poor signal. That might be radar or LiDAR (which is used in most of today’s best robot vacuums), or perhaps a camera.
Read on for a simple explanation of exactly how lawnbots navigate without a boundary wire. Or, for a more general overview, head to our guide to how robot lawn mowers work.
How do lawnbots use satellite positioning?
Glossary
Base station: the RTK receiver
Charge station: where the bot docks to charge
GNSS: global navigation satellite system
GPS: Global Positioning System, one of four GNS systems
RTK: real-time kinematic [positioning] – makes GNSS data more accurate
Lawnbots that don’t use boundary wires tend to use satellites for navigation. You’re probably more used to seeing the term GPS, but that’s just one of four global navigation satellite systems (GNSS) that could be helping your lawnbot to stay on course. GPS is owned by the US, and the other systems are Galileo from the EU, GLONASS from Russia and BeiDou from China.
Satellite signals experience delays as they make their way down to the earth’s surface, which means GNSS positioning isn’t all that precise. It can locate the lawnbot to within a few meters, but that isn’t accurate enough for a task such as mowing – it can be the difference between mowing a lawn and mowing your well-tended geraniums in the borders .
To improve the accuracy, the GNSS data works alongside RTK positioning. This helps correct the data, accounting for those signal delays, and means the lawnbot can be located to within a few centimeters. Adding RTK means the robot mower can traverse the lawn in a logical, uniform fashion, rather than just driving around randomly within an assigned area (as it would, if there’s a boundary wire in place).
The RTK system uses a fixed base station with an antenna. This connects both to the robot lawn mower and to the satellites in the sky (for clarity, it’s different from a dock / charge station, where the lawnbot returns to juice up). Read our how RTK works in robot lawn mowers, which cover this is more detail.
What affects how well a lawnbot can navigate?
The GNSS plus RTK system relies on direct line of sight to the satellites in the sky. What’s more, both the lawnbot and base station need to be able to connect to the same satellites.
While some satellite signal can penetrate though thin barriers – think light tree coverage or thin timber – in general, any walls, hedges, and structures will block the signal and impact on how well the robot lawn mower can find its way around.
That means the placement of the RTK base station is very important. And because the robot needs to be able to connect to satellites when it’s in its charge station, the positioning of that is important, too.
Where should I put my RTK base station?
In order to work, the RTK base station needs to have a direct line of sight to as many satellites as possible. As such, it should be positioned so it has the widest view of the sky as possible, in as many directions as possible. This makes positions up high – on the roof of your house or your shed, and above the top of most trees – ideal. Once installed, unless you need to reposition it because it isn’t working properly, the RTK station wants to stay in the same position.
The RTK station also transmits a signal to the lawnbot itself. This is generally a radio signal, which means there does not need to be a direct line of sight between the lawnbot and the RTK station. The radio signal can pass through walls, trees and so on. While you can place the RTK station can be next to the base station, it doesn’t need to be.
If you’re unfamiliar with lawnbots, the video above from Mammotion provides an idea of what an RTK station (and charge station) look like. It’s based on the Mammotion Luba 2, but most setups will be similar.
Where should I put my charging station?
The robot needs to have a decent GNSS signal not only when it’s covering your lawn, but also when it’s docked. This means the charging station also needs to have direct line of sight to the satellites in the sky. It can’t be under a cover – or at most, it could be under thin plastic or timber.
There should also be clear space to the left and right of the charge station. Most brands recommend leaving around 2m either side.
Ideally, the charge station should be out in the open; but, understandably, most people are unlikely to want their mower’s charge station sitting in the middle of their lawn. Generally, it’s okay to have the charge station against a wall, provided it still has a direct line of sight to the satellites in all three other directions.
What else can affect satellite connections?
Note that satellites don’t remain in the same place in the sky. The four GNS systems operate on different orbiting periods, and none will align precisely to the earth day. This means navigation might be better at some times of day than others, based on how many satellites are accessible to the lawnbot and RTK receiver at that time.
Different countries will also have different satellite coverage, and some will be more comprehensive than others.
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