Distinctive cat goes missing in Jamaica Plain

GPS trackers still need something to power them, and I don't think anyone wants to change the batteries on their cat, especially inside their cat. Plus, the total unit size is still too big to be implantable.

There are a variety of GPS dog collars out there. That only helps if your dog is genuinely lost, rather than stolen, as they're not exactly subtle devices.

The microchips used for pets are basically just a tiny RFID tag. They're read by a handheld reader that blasts them with radio waves and listen for an echo, essentially; the shape of that echo encodes an ID. They're about the size of a large grain of rice and require no built-in power source.

A GPS tracker has a much, much harder job: It has to have an antenna of the right size to listen to the weak radio signals from the satellites (at least 3 cm across according to Wikipedia); the antenna is actively driven by a resonater; there needs to be a little computer to there; and it needs to then broadcast that information somewhere. (4G cellular connection, maybe?) All of that's a lot of power and antenna, not something you'd want to implant. You can put it on a collar, but it's bulky and needs recharging.

Some wildlife radio collars work differently: You just have something that emits a signal, and you walk around with a directional antenna until you can triangulate on it. That requires special equipment for locating, and still needs recharging.

And then there's Apple's clever little tracker tags, which use Bluetooth to ping nearby phones with their IDs. This offloads a huge amount of power, processing, and antenna gain to the phones. But even this needs powering. I guess you could have a wireless charging mat under the pet's bed...