Global Positioning System (GPS) refers to a system of about 30 orbiting satellites and four monitoring stations on land that provides radio signals you can use to find your location anywhere on Earth. The more commonl use of the term GPS means a receiver that can read this radio signal and pinpoint exactly where that receiver is located. Like any radio device, a GPS receiver needs an antenna.
The GPS Receiver
A GPS receiver will give information on longitude, latitude, altitude and time of day. It can be mobile, handheld or fixed. But it must receive a direct, line-of-sight signal from more than one satellite, and the more signals it receives, the more accurate it is. Therefore, the more open sky the receiver can "see," the better it will work. So even if there is clear sky directly above, your receiver may only get a strong signal from one satellite but not any other, increasing the need for a more efficient antenna.
Antennas
Your GPS receiver may come with an internal antenna that is sufficient as long as the device is outside and has a clear view of the sky. But if the sky is blocked by a building, a mountain, a ceiling, the roof of a car or a thick canopy of trees in a forest, the signals may be weak and the GPS receiver will work erratically or not at all. In such cases, an external antenna may solve the problem, and there are three types to choose from. Examples can be found at the referenced websites.
Passive
A passive GPS antenna does not need any electrical power to operate. But it does need a cable to connect it to the receiver. Because cables have loss, it cannot be more than one meter (a little over three feet) long. The receiver must also be equipped with a jack to accommodate the external antenna. A passive antenna can be attached to a car using a magnet, but the short cable may restrict placement of the receiver inside the car. A passive antenna's best application may be for use with a handheld receiver.
Active
For applications in which the antenna must be more than one meter away from the receiver, an active antenna will amplify the signal to offset the loss of the cable and provide a higher power signal to the receiver. An active antenna requires external power. It can use batteries, a car cigarette lighter adapter or an AC to DC converter. A mobile antenna can be mounted to a car with a magnet, and the longer cable will allow a more convenient position inside a vehicle.
Reradiating
A reradiating antenna is used primarily indoors or for other types of GPS receivers that do not have a connection jack for an external antenna. It picks up the satellite signals from outside and relays them via cable to an inside unit, which retransmits the signal. Reradiating antennas need electrical power to operate and can use any of the above mentioned sources. For best operation, the receiver should have an unobstructed, direct line of sight with the reradiating element. If so, it can be used up to 100 feet away.
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