<P> Aircraft can be tracked either by ADS - B (primarily airliners and General Aviation aircraft with ADS - B - out enabled transponder), or by FLARM data packets picked up by a network of ground stations (primarily used by General Aviation aircraft, gliders and UAVs), both of which are data pushers . ADS - B is to be superseded by ADS - C, a data puller . </P> <P> GPS data pullers are also known as GPS transponders . Unlike 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 . </P> <P> 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 . </P> <P> 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 . </P>

How gps is used to locate the position of a car