GPS devices are an incredibly convenient tool to have if you are lost and need to find your way. However, because they can be used for tracking they also open up the door for abuse. If you believe that someone is following you by using a GPS device then there are a few ways that you can locate and disable it. Although it may require a few tools it is not a terribly technical process.
Locate the GPS device
Do a physical and visual inspection of your car. Look for a small rectangular box about the size of a deck of cards. It will probably be black or grey. Search the bumpers and the undercarriage of your vehicle as well as the interior. GPS devices have become smaller and easier to install, which means that it is unlikely that it is hard-wired into the dashboard of your car like the older systems were.
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Get a GPS bug detector. You can buy these from most online spy and gadget stores (see resources). As of 2010 they generally cost about $200. This device hones in on and detects the signal that is being sent from a GPS tracking device.
Turn the GPS detection device on and move it over you car. It should pick up any signal within a 3 to 5 meter radius. If it detects a signal it will begin beeping quickly and a light will start to flash.
Do two or three scans at different times as there are some tracking devices that only transmit at certain time intervals. Since the GPS detector can only find devices when they are transmitting there is a chance that it will miss these semi-passive systems.
Once you have found it, you can easily disable the GPS tracker by simply removing the battery.
Jam the GPS Signal
Purchase a GPS jamming device. These devices will block the signal that GPS broadcasts on. As of 2010 the price of these devices are around $100 and up. You can find them online at several stores that specialize in signal jamming devices (see resources below).
Install the jamming device in your car by removing the cigarette lighter and plugging the power socket into it.
Turn the device on and it will automatically begin emitting a signal that will prevent the GPS device from broadcasting its signal.