How robot lawn mowers navigate: wire, RTK, vision and lidar

Boundary wire: the classic method

A boundary wire is a thin insulated cable pegged on or buried just under the lawn round its edge. The charging station sends a weak electrical signal round the loop. Sensors in the mower detect the signal and tell it whether it is inside or outside, so it turns back when it reaches the wire. Beds, trees and ponds inside the lawn are cut out by running the wire out to them, round them and back alongside itself.

Many wired robots also use a guide wire, a single wire from the station across the lawn, which the mower follows home or out to a far zone. Some brands add GPS assistance to track which parts of the lawn have been covered, but the wire still sets the boundary.

Wire works almost anywhere: under trees, next to tall houses and in narrow passages. Its weaknesses are the installation work and breaks, since a spade, edging tool or burrowing animal can cut it and stop the mower.

RTK: satellite positioning to a few centimetres

Ordinary GPS on a phone is accurate to a few metres, which is not enough to keep a mower out of a flower bed. RTK, short for real time kinematic, fixes that. A fixed base antenna in the garden, or a network service reached over the internet, compares the satellite signals it receives with its known position and sends corrections to the mower. With those corrections the mower can place itself to within a few centimetres.

With RTK you draw the boundary and no-go zones in an app, usually by driving the mower round the edge once. There is no wire to lay. The catch is that the mower and the base need a reasonably clear view of the sky. Dense tree cover, tall walls, overhanging roofs and narrow side passages between houses can weaken the signal so the mower slows, waits or stops in that area.

• Works best: open lawns with a wide view of the sky.
• Struggles: under dense canopy, close to tall buildings and in narrow passages.
• Typical fix: move the antenna higher or more central, or choose a model that adds cameras or lidar.

Vision: cameras and AI

Vision robots use one or more cameras and image recognition to tell grass from paths, beds and obstacles. Some, such as the Bosch VisiMow in our wire-free guide, rely on vision alone and need no wire or antenna. Others use cameras alongside RTK so they can keep working when the satellite signal drops and avoid objects such as toys and garden hoses.

Vision needs light and contrast. Low light, strong shadows and edges where grass runs straight into a similar coloured border can confuse it. A clear edge, such as a path, lawn edging or a bed with bare soil, helps a vision robot hold its line. Cameras also need keeping clean, since mud and clippings reduce what they can see.

Lidar and 3D sensors

Lidar measures distance by sending out laser pulses and timing their return, building a three dimensional picture of the garden: fences, walls, trees and the house. A lidar robot can recognise where it is from those landmarks, so it does not need a clear sky. The Ecovacs Goat O1000 LiDAR Pro in our guides, for example, uses dual lidar with AI vision and needs no RTK antenna according to its listing.

Related 3D sensors, such as time of flight cameras, are also used for obstacle avoidance. Lidar suits shaded, tree lined gardens where RTK struggles. It can find wide, featureless open spaces harder, since there are fewer landmarks to measure against, so many makers pair it with other methods.

Which method suits which garden?

Most gardens can be mowed by any of these methods, but some features favour one over another. Use the table as a quick guide, then check the specification of the model you like.

Compare the best robot lawn mowers →

Random or systematic mowing

How a robot moves within the boundary depends on how well it knows its position. Most wired robots mow in a random pattern, turning at a fresh angle each time they meet the wire. Over a week this covers the whole lawn evenly, though it looks haphazard while it happens.

Mapping robots that use RTK, vision or lidar usually mow in straight parallel lines, then move on to the next zone. That finishes the lawn faster and can leave a light striped look. Either way, a robot that cuts little and often keeps grass within the RHS advice of removing no more than a third of the height at each cut.

If you are deciding between types, read our wire-free vs boundary wire comparison, and for setup see how to set up a wire-free robot mower.

Note:Whatever the navigation method, switch the robot off and use the stop button before lifting it, clearing debris or cleaning sensors and cameras. Wear gloves near the blades and follow the manual.

Frequently asked questions

How does a robot lawn mower know where to cut? It either follows a boundary wire signal or uses a map built from RTK satellite positioning, cameras or lidar. Within that boundary it mows randomly or in planned lines.

Do robot mowers use GPS? Many wire-free models use RTK, an enhanced form of satellite positioning accurate to a few centimetres. Some wired models also use GPS to track coverage, while the wire sets the boundary.

Is a lidar robot mower better than RTK? Not always. Lidar copes better in shaded gardens with trees and walls, while RTK is very accurate on open lawns. Some models combine both with cameras.

Can a robot mower see hedgehogs? Some models list 3D obstacle avoidance aimed at small animals, but no system is guaranteed. Mowing in daylight rather than at night is the simplest way to protect wildlife.