Scott Duncan
Cops use a constant data stream that needs to be broadcast (just like Wifi) on their vehicles. This has a Media Access Controller (MAC) address. They ALSO have GPS.
GPS also has a MAC address for every unit. Match 2 MACS on ONE vehicle, and BOOM! you have a target you can track... through that same GPS :D
It took 20 people with tablets to index every cop in Toronto; All of them "illegals". They will gladly work for 1 BTC a day, clicking an icon when they see a cop. :D