Tracking devices, also called 6-degree-of-freedom (6-DOF) devices, are defined as technological tools that are used to observe a specific person's movements by the use of a transmitter to relay radio signals back to a receiver which then enables one to pinpoint the exact location of that particular person.

Global Positioning Satellite, homing devices and cell phones are part of the tracking device technology, and they have all very useful purposes.

GPS, for instance, enables car drivers to have a knowledgeable virtual ?companion? that can give out directions and help you be located by the proper authorities if and when you've gotten into an accident.

As a corollary, this technology can also be implanted on very important persons just in case they go missing or worse, kidnapped, and need to be found pronto. In fact, even non-celebrities are afforded this option.

In Mexico, for instance, senior staff of a company is required to have these chips as kidnapping is on the rise on that country.

On a more practical note, rental car companies like Payless have incorporated this type of technology on their cars so that they could tell when renters are crossing into forbidden states.

The guilty parties can be assured of an extra $1 charge for every mile driven on non-permissible states. The costs of these devices and their maintenance can vary greatly.

So we all know what tracking devices are and what they could do. But what exactly makes them tick? Such devices are based on electromagnetic, acoustic, mechanical or optical technology.

Mechanical tracking devices can measure orientation and position with the use of a direct mechanical connection between a reference point and the target. The advantage of this type of gadget is that they are accurate and their lag is short. The main drawback is that the operator's motion is limited to the mechanical arm.

Optical tracking devices use the information contained in the projection of the LED's to identify the position and orientation of the head.

There come in two kinds: in the first one, cameras are placed on top of HMD's while a set of infrared LED's are placed above the head at specific locations in a given environment.

The other type has cameras mounted on a fixed frame with fewer LED's located at known position on the HMD. These types of tracking devices both have high update rates and short lags, although they usually suffer from line of sight problem.

Electromagnetic tracking devices work by measuring the magnetic fields? strength generated by sending current through three tiny wire coils, placed perpendicular to each other.

These are then implanted in a small unit attached to the entity the system needs to track. These types of devices can track multiple objects simultaneously although they may experience interference with mechanisms that produce magnetic fields of their own.

Acoustic tracking devices employ ultrasonic sound waves that measure the position and orientation of the entity being tracked. This can be done through time-of-flight tracking and phase-coherence tracking.

The first type works by measuring the time needed to take for the sound sent out by transmitters placed on the target to reach sensors located at specific points.

The second type works by measuring the difference in phase between the sound waves produced by a transmitter on the target and those given off by a transmitter at some reference point.