How a robot knows where it is, down to the centimeter

The GPS in your phone is off by several meters. On the road that's good enough: the street is wide, and a voice tells you when to turn. A robot in an orchard can't get away with that. Rows of berry bushes are about three and a half meters apart, and a two-meter error puts a track on top of a bush. So our robot works out its position three ways at once: by satellite, by lidar and by camera.

How GPS works. Every navigation satellite continuously broadcasts the time from its own extremely precise clock. A receiver on the ground picks up the signal and works out from the delay how far away the satellite is. Knowing the distance to several satellites, it can fix its own position. It's a bit like working out where you are standing from how late the chimes of three different bell towers reach you. Several satellite navigation systems are in orbit today - GPS, GLONASS, Galileo and BeiDou - and our robot's receiver sees about forty satellites at a time.

Where the meters of error come from. The signal travels down through the atmosphere, and the upper layers delay it, sometimes more and sometimes less, depending on the weather and the time of day. The clock in the receiver isn't perfect either. Each small effect adds centimeters to the measured distance, and together they add up to meters.

How RTK removes the error. A fixed base station stands nearby, its position surveyed very precisely in advance. It watches the same satellites through the same atmosphere and sees almost exactly the same error as the robot. The gap between what the station calculates and where it actually stands is the correction. Every second the station sends that correction over the mobile network; the robot subtracts it from its own calculation and knows where it is to within a couple of centimeters. The method is called RTK.

Why the robot needs lidar and a camera. Satellites tell the robot where it is on the map, but not what is around it. Under tree canopy and next to buildings the signal weakens or bounces, and over a summer a bush can grow out into the row. Lidar is a laser rangefinder: many times a second it sweeps everything around it and builds a 3D picture of where the row is, where a post is, where a person is. The camera adds what lidar can't make out. Together they keep the robot centered between the rows and stop it short of an obstacle, while the satellites make sure it sticks to its route and, at the end of a row, turns into the right one next.

Satellite precision like this has been at work in fields for years. Guidance systems and tractor autosteer run machinery in straight parallel passes with no skips or overlaps; precision agriculture is built on it. The difference is that the tractor still needs a person in the cab.

What we do. VETER Robotics machines drive by satellite navigation with RTK corrections, while lidar and a camera guide them down the row and watch for obstacles. That is how our robot runs a route on a berry farm by itself: it drives a row about five hundred meters long, turns around and enters the next one. The image shows a real point cloud captured by our robot's lidar among the berry rows, along with a diagram of how the satellites, the base station, the lidar and the camera work together.