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.
Usually, a GPS tracker will fall into one of these three categories:
A GPS logger simply logs the position of the device at regular intervals in its internal memory. Modern GPS loggers have either a memory card slot, or internal flash memory and a USB port. Some act as a USB flash drive. This allows downloading of the track log data for further analyzing in a computer. The tracklist or point of interest list may be in GPX, KML, NMEA or other format.
Most digital cameras save the time a photo was taken. Provided the camera clock was reasonably accurate, or the GPS was used as a time source, this time can be correlated with GPS log data, to provide an accurate location. This can be added to the Exif metadata in the picture file, thus geotagging it.
In some Private Investigation cases, these data loggers are used to keep track of the vehicle or the fleet vehicle. The reason for using this device is so that a PI will not have to follow the target so closely and always has a backup source of data.
Also known as a GPS beacon, this kind of device pushes (i.e. "sends") the position of the device as well as other information like speed or altitude at regular intervals, to a determined server, that can store and instantly analyze the data.
A GPS receiver and a mobile phone sit side-by-side in the same box, powered by the same battery. At regular intervals, the phone sends a text message via SMS or GPRS, containing the data from the GPS receiver. Newer GPS-integrated smartphones running GPS tracking software can turn the phone into a data pusher (or logger) device; as of 2009 open source and proprietary applications are available for common Java ME enabled phones, iPhone , Android, Windows Mobile, and Symbian.    
Most of the modern GPS trackers provide data "push" technology, enabling sophisticated GPS tracking in business environments, specifically organizations that employ a mobile workforce, such as a commercial fleet. Typical GPS tracking systems used in commercial fleets have two core parts: location hardware (or tracking device) and tracking software. This combination is often referred to as an Automatic Vehicle Location system. The tracking device is most often hardwire installed in the vehicle; connected to the CAN-bus, Ignition system switch, battery. It allows collection of extra data, which later get transferred to the tracking server, where it is available for viewing, in most cases via a website accessed over the internet, where fleet activity can be viewed live or historically using digital maps and reports.
GPS tracking systems used in commercial fleets are often configured to transmit location and telemetry input data at a set update rate or when an event (door open/close, auxiliary equipment on/off, geofence border cross) triggers the unit to transmit data. Live GPS Tracking used in commercial fleets, generally refers to systems which update regularly at 1 minute, 2 minute or 5 minute intervals, whilst the ignition status is on. Some tracking systems combine timed updates with heading change triggered updates.
The applications of these kind of trackers include:
GPS data pullers are also known as GPS transponders. Contrary to data pushers, that send the position of the devices at regular intervals (push technology), these devices are always-on and can be queried as often as required (pull technology). This technology is not in widespread use, but an example of this kind of device is a computer connected to the Internet and running gpsd.
These can often be used in the case where the location of the tracker will only need to be known occasionally e.g. placed in property that may be stolen, or that does not have constant source of energy to send data on a regular basis, like freights or containers.
Data Pullers are coming into more common usage in the form of devices containing a GPS receiver and a cell phone which, when sent a special SMS message reply to the message with their location.
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.
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. GPS tracking devices have also been put on religious statues in order to track the whereabouts of the statue if stolen.
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 ; see also .
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.