She's on a leash 50 metres long
Our buoy lives on a mooring in open water. Ropes chafe, shackles work loose, and boats clip things. So she keeps watch on herself, and this is how that works.
Step one: know where you are (cheaply)
There's a GPS inside, but running GPS all day would flatten the battery in no time. So she takes little sips. While she's already awake recording waves, the GPS listens in the background, gets a fix, and goes back to sleep with everything else. One position per cycle, almost free.
Step two: the circle
When we moor her, we press one button in the dashboard: "this is home." From then on, every check-in, the server measures the distance from her latest fix to that home point. Inside 50 metres? All good, say nothing. That circle is her whole world. Rope swing, tide, GPS wobble... it all fits inside comfortably.
Step three: the alarm
Cross the circle and everything changes. The server flags her ADRIFT and pushes an alert to our phones. And here's my favourite part: the next time she checks in, we tell her to stop being a wave buoy and start being a tracker. She skips wave recording entirely and reports her position every 60 seconds, battery rules suspended, because there is no point saving power on a buoy you're about to lose. On the map you see a breadcrumb trail of where she's heading.
Then, if the positions settle back inside the circle (say it was a giant wave, or a fisherman moved her and thought better of it), she stands down by herself. Alert cleared, back to measuring waves. No one has to remember to switch anything off.
Does it work? Ask the false alarm
Best test we ever ran was an accident. During the August repairs she was sitting in the garage, and her maintenance timer quietly expired one Saturday evening. The system looked at her position, saw "3,475 metres from home," and lit up exactly like it should. Adrift alarm, tracker mode, the lot. She was on a workbench. Embarrassing, but honestly the most convincing drill we could have asked for: the alarm chain works end to end.
On deployment day we did the proper version. Boat out, buoy clipped on, one tap on "this is home" while we floated next to her. The old home point was 25 metres off (from the previous season's mooring), so re-centring it meant the circle now sits exactly where she actually swings.
The quiet part
Most of the anchor watch is nothing happening. Every 15 minutes, one distance check, one shrug, carry on. It's been shrugging for weeks now. That's the whole idea: the alarm you never hear from is the one doing its job.