So if I turn my phone off and get onto a bus or train with a tracking tag, other passengers will get an alert?

Also, the wording indicates that the tag needs to be marked as lost. But could that be used as plausible deniability -- that someone had stolen it -- by a person engaged in illicit tracking?

Interesting research. Could have done with some motivation - why would you want to do this exactly? And it's a shame they couldn't get it to work with Google's network (in a non-awful way anyway).

Both let you transmit arbitrary data, but the custom setup here is a lot of overhead. Hubble gives you an SDK and lets you get back to building your device.

The clients can receive synchronisation data every minute and listen for a year on a coin cell. It’s broadcast, so a single beacon node can service hundreds of clients simultaneously.

BLE also can manage data connections over a kilometer and a half with reasonable (not great) antennas.

It’s not terribly fast, but modern radio protocols are opening up the possibilities. Lora and BLE are bringing the environment alive with communication.