Trackers in the sky

Santosh Kushwaha        Print Edition: February 2012

In case you are lost with no clue of where you are, getting directions could tend to be a bit tricky. But you will be saved the bother of such an eventuality if you have a GPS-enabled mobile in your hand or a GPS system in your car. With these devices it is easy to figure out where you are and get turn-by-turn directions to your destination.

Like a cell phone, a GPS receiver relies on radio waves. But instead of using towers on the ground, it communicates with satellites that orbit the Earth. There are currently 27 GPS satellites in orbit-24 are in active use, with the rest meant as backup in case one or the other fails.

In order to determine your location, a GPS receiver has to determine:
  • The locations of at least three satellites above you
  • Where you are in relation to those satellites

The receiver then uses trilateration to determine your exact location on earth. Basically, it draws a sphere around each of the three satellites it locates. These three spheres intersect at two points-one in space, and one on the ground. The point on the ground where the three spheres intersect is your location.


The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
A GPS satellite weighs approximately 2,000 pounds and is about 17 ft across with the solar panels extended.
Their transmitter power is a mere 50 watts or less.
There are some 2,500 satellites of all types and purposes orbiting the earth.
Each GPS satellite is built to last about 10 years.
GPS car navigation systems can be factory-installed on new vehicles or purchased as an add-on accessory. Combining the use of signals from the satellites with interactive on-board maps, these systems can plot routes of travel to a given destination based on a number of variables. Some of these systems are interconnected with sources of traffic information, enabling them to automatically account for congestion en route while determining the best options to reach a destination.

If a driver misses a turn, these devices can quickly correct for the error and present an updated route option. Providing voice or visual instructions, these units also can help drivers find the nearest gas station or give tips on restaurant or shopping options.

However, GPS isn't foolproof. GPS receivers use a combination of signals from a network of satellites and ground stations to figure out where you are and where you'd like to go. It is really only as good as the satellite network and its map data. Without a clear and strong signal, your device can't accurately establish your location. Tall buildings, dense foliage, mountains and even reflective objects can cause errors.

Courtesy:Gadgets and Gizmos

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