If your crook is accommodating enough to carry a cell phone you can track, you don't have to be concerned about hardware, but you do have to secure a search warrant with the cellular service provider and be dependent on its network for the information. Dedicated GPS recorders and transmitters are a surer bet here.
GPS surveillance devices fall into two major categories: active and passive. The active devices transmit their locations (as well as speed and direction of travel) in real time, so you know where the device is at any given moment.
The chief limiter of these gadgets is power. It requires quite a bit of current to transmit continuously, and few of them have batteries that will last more than a day or two. A subset of these preserves battery power by transmitting a text message at pre-programmed intervals, giving the location coordinates from the GPS receiver.
Passive devices merely record their travels in internal memory. When you retrieve the device, you dump the memory to see when and where it's been. These are considerably cheaper than the active devices, and their batteries can run for a week or more before going dry.
New research at The University of Memphis gives rise to new technology that largely defeats the battery/power problem. AutoWitness is a penny-sized device intended for concealment inside theft-prone items like computers. A motion-detecting accelerometer, gyroscope, and vibration sensors on the AutoWitness determine when the object is being moved, and whether the movement is characteristic of everyday activities or something new, indicating theft. The sensors also pick up on direction of movement, serving as a kind of dead reckoning positioning system.
The movements are logged internally until a pre-programmed interval has passed (so as to avoid detection) and the AutoWitness senses that there is a sufficiently strong cellular network or other RF signal suitable for transmitting its data. It works best when it is surrounded by a dedicated network of base stations intended to detect the devices and receive their transmissions. However, location detection is about 90 percent accurate when the only localization data is from cell towers. When it comes time to find the specific location of the item carrying the AutoWitness, a dedicated handheld receiver can interrogate and locate the device.
AutoWitness has the potential for some interesting moving surveillance applications. One of the most attractive aspects of this technology (which is truly remarkable and far too complex for me to address here) is that the finished product is expected to cost only $10-$20 per unit.
Sometimes your eyes are not enough. If you need to know what's inside a car, package, or other container, you either need to search it by hand or use some tool to make that unnecessary. The latest and greatest tech in this area is backscatter (as opposed to transmitted) X-ray.
Traditional X-ray images are transmitted. The X-rays pass through the imaged object, which absorbs some of the energy. The rest falls onto a sensor or X-ray film, which displays a shadow in areas of less density and light areas to represent greater density.