APRS: Automatic Packet/Position Reporting System
Some Ham Operators choose to "beacon" their current location, from their vehicle, bicycle, or even their backpack or purse, using the APRS System, which uses Amateur Radio Frequencies.
The APRS Home Page, by Bob Bruniga WB4APR
An excellent APRS slide-deck at WPARC.US
A Gentle Introduction to APRS (by N6NBN, 2013)
In North America, the most commonly-used frequency is 144.390 MHz Simplex.
An APRS beacon collects GPS information from the GPS Satellite Network, and broadcasts the Ham's Position, Speed, Bearing, and Altitude, to an "iGate" gateway (listening station), which in turn forwards the data over the Internet to the APRS Server System, where it can be used in many ways.
There are many "works out of the box" solutions, including Byonics.com; I use 2 of their "MT-AIO" units, which can be carried easily in a back-pack, or installed in a vehicle; they will run off of 8 AA batteries, or from external 12-volts DC.
I have one unit in the back window of my vehicle, connected to the car's electrical system, so it operates whenever the car's ignition is on.
The other is in my "Go Bag" backpack.
The two units are programmed with different "SSID" suffixes; you can use your same Ham Callsign on several different devices, and they show up separately on APRS Tracking Web Sites.
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But, you ask: How can I "see" where these "beaconing" Hams are at the moment?
This site works really well: APRS.fi
Here's another: FindU.com
Here's a third site: OpenAPRS.net