A GPS tracking unit is a device that uses the Global Positioning System to determine the precise location of a vehicle, person, or other asset to which it is attached and to record the position of the asset at regular intervals. The recorded location data can be stored within the tracking unit, or it may be transmitted to a central location data base, or internet-connected computer, using a cellular (GPRS or SMS), radio, or satellite modem embedded in the unit. This allows the asset's location to be displayed against a map backdrop either in real time or when analysing the track later, using GPS tracking software.

Abuses in USA
In the US, the use of GPS trackers by government authorities, such as police, is limited by the 4th Amendment of the United States Constitution, so, police, for example, usually require a search warrant in most circumstances. While police have placed GPS trackers in vehicles without warrant, this usage was questioned in court in early 2009.[11]

Use by a private citizens is regulated in some states, such as California, where California Penal Code Section 637.7 states: (a) No person or entity in this state shall use an electronic tracking device to determine the location or movement of a person. (b) This section shall not apply when the registered owner, lesser, or lessee of a vehicle has consented to the use of the electronic tracking device with respect to that vehicle. (c) This section shall not apply to the lawful use of an electronic tracking device by a law enforcement agency. (d) As used in this section, "electronic tracking device" means any device attached to a vehicle or other movable thing that reveals its location or movement by transmission of electronic signals. (e) A violation of this section is a misdemeanor. ( f ) A violation of this section by a person, business, firm, company, association, partnership, or corporation licensed under Division 3 (commencing with Section 5000) of the Business and Professions Code shall constitute grounds for revocation of the license issued to that person, business, firm, company, association, partnership, or corporation, pursuant to the provisions that provide for the revocation of the license as set forth in Division 3 (commencing with Section 5000) of the Business and Professions Code.

Note that 637.7 pertains to all electronic tracking devices, and does not differentiate between those that rely on GPS technology or not. As the laws catch up with the times, it is plausible that all 50 states will eventually enact laws similar to those of California[original research?].

Of course, other laws, like the common law invasion of privacy tort as well as state criminal wiretapping statutes (for example, the wiretapping statute of the Commonwealth of Massachusetts, which is extremely restrictive) potentially cover the use of GPS tracking devices by private citizens without consent of the individual being so tracked. Privacy can also be a problem when people use the devices to track the activities of a loved one.[12] GPS tracking devices have also been put on religious statues in order to track the whereabouts of the statue if stolen.[13]

Recently, debate ensued about a Georgia proposal to outlaw hidden GPS tracking, with an exception for law enforcement officers but not for private investigators. See Georgia HB 16 - Electronic tracking device; location of person without consent (2009). See [1]; see also [2].

Uses in Marketing
In August, 2010, Brazilian company Unilever ran a promotion where GPS trackers were placed in boxes of Omo laundry detergent. Teams would then track consumers who purchased the boxes of detergent to their homes where they would be awarded with a prize for their purchase. The company also launched a website (in Portuguese) to show the approximate location of the winners' homes.[14]

See also
Automatic Position Reporting System
Electronic tagging
GPS wildlife tracking
GPS Timing
Intelligent transportation system
Real-time locating
Surveillance
Telematics
Vehicle Infrastructure Integration