As we run, drive, bike, and fly, we leave behind telltale marks of our movements on Earth—if you know where to look. Physical tracks, thermal signatures, and chemical traces can reveal where we’ve been. But another type of trail we leave comes from the radio signals emitted by the cars, planes, trains, and boats we use. On airplanes, technology called ADS-B (Automatic Dependent Surveillance–Broadcast) provides real-time location, identification, speed, and orientation data. For ships at sea, that function is performed by the AIS (Automatic Identification System). Operating at 161.975 and…
