Robot lawn mowers of yore started off using a boundary wire to determine where it could and could not go, and they were problematic. The boundary wire had to make a complete, unbroken circle. And if anything happened to the wire, you had problems. These days, robot mowers work via GPS, but it's GPS with a little bit extra in an RTK beacon. Scott Porteous, head of robotics for Husqvarna broke it down for me.
Adam Doud/CNET
GPS navigation
GPS today is accurate down to a few feet. That's fine when you're navigating in your car, or looking for Tupperware in the woods, but it's not good for things like property lines. The Real-Time Kinematic beacon also communicates with GPS satellites to provide a fixed location on Earth that the satellites know about. In essence, the RTK beacon tells the satellites, "I'm always here." Meanwhile, the mower drives around, and the beacon tells the mower that the satellites think it's 5 feet to the north, so the mower should take the location that GPS is reading and adjust it accordingly. This allows robot mowers to get accuracy down to the centimeter.
GPS coverage
Another thing to consider is your general GPS coverage. If you live out in the suburbs, there's a good chance you'll be able to find a spot in your yard to set up a base station and RTK beacon. If you live in the city, that might be challenging. Also, wherever you set up your mower, you'll probably need to be within Wi-Fi range for the mower to operate properly. A mower like the Yarbo actually gets an ethernet cable that runs to a router. If you decide to go with the Yarbo, that's something you'll need to do.
Adam Doud/CNET
Cutting width
Robot mowers have a lot of difficulty with narrow paths. The reason for this is due to those blades and its overall construction. For example, the Segway mower's chassis is 20.5 inches with a cutting width of 9 inches. The Sunseeker, one of the smaller bots, has a 14-inch width accompanied by a cutting width of 8 inches. This is not conducive to cutting narrow paths. Even if the body of the mower fits there, it can't get the right angle to cut all the grass. Mostly, they just wiggle back and forth, which doesn't do a heck of a lot.
Camera systems
Most mowers have camera systems that allow them to avoid obstacles and navigate. Eufy relies on that camera system, so much so that it can't operate at night at all. Other mowers have the ability to turn off nighttime mowing because obstacle detection is a lot worse for obvious reasons.
Adam Doud/CNET
Scheduling
All of the mowers have scheduling abilities, so the mower only runs when you want it to. Some of the mowers even take weather into account -- if it's raining when it's time to mow, it delays the mowing. It's important to remember that consistent mowing times are crucial to the proper operation of the mowers so the grass won't get too long for it to handle.
Human oversight
Finally, it's important to remember that robot mowers are exactly that -- robots. A human can look at a patch of grass and know exactly how to move through it. Robots, not so much. It can be frustrating to watch a mower make its way through a lawn and just skip past a piece of grass.
If you don't obsessively watch the mower (like I had to in order to review them), you might come back after it runs a cycle and see patches that it skipped, which can also be frustrating. As a human, you might have to do some tidying up after a robot mower, but it'll be significantly less work than doing the whole thing yourself.
Adam Doud/CNET
My backyard is a wonderful minefield of obstacles and grades that embodies just about any condition you might have in a yard. I have a trampoline that none of the mowers seemed to be able to navigate (the supporting pipes on the ground couldn't be climbed over). My tree is large and has roots sticking out of the ground. I have dogs who were curious about their new robot friends creeping back and forth across the lawn. I also have a fair amount of landscaping and some narrow paths that none of the mowers did a good job of cutting.
My front yard is much more straightforward except that neither of my neighbors have a boundary between their property line and mine, so I had to rely on GPS and the RTK beacon to test. I found them to be accurate enough. I programmed one of the mowers to also cut my elderly neighbor's yard (not the colorful one, the other one), which he appreciated. This is why you don't mess with the tech reviewer.
Adam Doud/CNET
During my testing time, temperatures varied wildly from 40 degrees Fahrenheit to up over 90 degrees -- welcome to the midwest. We also had a few rainstorms, including one torrential downpour that didn't kill any of the bots, so that's a plus.
After every mower did its thing, I waited several days for the grass to grow back to test the next one, however I didn't wait too long because -- again -- maintenance, not mowing. I put these mowers through their paces for the past six weeks, but I anticipate some long-term testing will be necessary. We plan to keep this list updated with more mowers as they're released, or even mowers we didn't get to this time around, so be sure to check back.