There are many different Spy GPS systems out on the market, but they are all designed to do one thing: letting you know where another person is or was. The reason why keeping tabs on someone is important vary from one individual to the next, but common reasons that you might want to track another person could be: concerns over corporate secrets, martial/relationship infidelity, trying to determine the honesty level of another individual, or any number of other issues. Regardless of why the other party is being tracked, it is important to note that there is a very big difference between the words is and was.

If you need to know where someone is, then you need a real time spy GPS system. These systems cost most, and generally require monthly fees on top of the purchase price, but there is no better way to track down where someone is right now than by using a real time spy GPS system. There are modules that can be placed in, on, or under vehicles, inside of briefcases or handbags, or even very small spy GPS systems that can be placed discretely in any number of other places. These units send out their location every so often, usually in pre-set intervals of 10 or 15 seconds, and thus allow you to know where the person being tracked is. 

A passive spy GPS system tends to be slightly less expensive than a real time spy GPS system, but it does require more patience because it records GPS data to its internal memory or a data card of some kind. This data can only be retrieved when the passive GPS device is returned, which means that real-time tracking is not possible, but it is still possible to determine where the person being discretely monitored went and how long they were there for. Of course, if the other party discovers the passive spy GPS module, they might just throw it into the nearest garbage can or river. This could result in the total loss of data, whereas someone using a real-time spy GPS tracking system would have information that is never more than a few seconds old.

Most quality spy GPS systems can be integrated with mapping software, and can also be used to create a history profile. For example, if Bob is being tracked either in real-time or passively, the data gathered can generally be overlaid using GPS-based mapping software to show where Bob went and how long he was there for. This may prove helpful if Bob went to the strip club again, but if he is visiting a secret mistress in one of the many tall buildings in Manhattan, then tracking her down might still prove to be very difficult.