I am looking for a tracking device that actively transmits and uses GPS or mobile networks for tracking, as well as transmitting the information to an application.
The more miniscule or lightweight, the better, unless it puts battery life below a reasonable use threshold (say, a few days).
Spatial resolution is good but not essential.
Ideally the price of the service should not be too high, and the price of the device needs to be reasonable enough to buy a few of them (i.e. under 200 USD).