The next category of tracking/location device to discuss is Bluetooth Trackers.
This, I suspect, may be a little controversial, because after a fair amount of research, and some product testing, my conclusion is that bluetooth trackers have little or no use as tracking/location devices for pets. Allow me to explain:
Bluetooth trackers are small, fairly cheap (they range from a few US$ to a few tens of US$), and have a battery life of many months. They do not measure position themselves, rather they communicate via low power bluetooth to nearby cellphones, and the cellphone provides the location information. And this is where the problem lies. The maximum range between the bluetooth tracker and your phone is in the region of 10m/30ft (typically, some have longer range.) Some, if not all of the devices extend this by having a "crowd finding" functionality, where other cellphones are used to report the position of your device (of course provided the user of those phone has allowed this via the app.) That would only work well in an area where multiple people have cell phones in close proximity to your lost pet, and multiple of those people had given permission via the same app that their phones be used to report location (apps from different manufacturers are not compatible.)
Bluetooth trackers typically have a "find me" function, where a user triggers an alarm, and the tracker starts to beep, useful for finding those lost keys. Some have an "out of range" alarm. If, for example, the tracker could no longer communicate with your phone, then the phone would sound an alarm to warn you that you had, for example, left your wallet behind.
Those are fine feature for lost keys, but not suitable for lost pets. Things that beep loudly on a pet's collar are not that great an idea. In fact, on the TrackR website, they state "We strongly discourage the use of TrackR on cars, pets or people" and later "the volume of TrackR’s ringer may cause your pet discomfort." (https://support.thetrackr.com/hc/en-us/articles/115005535046--Will-this-work-for-tracking-my-pet-)
In conclusion: Bluetooth trackers have their uses, such as tracking keys, wallets, computers, tablets, gadgets and inanimate things that are typically close to you. I do not recommend them for tracking and locating pets (though I am sure some will disagree with me)