WHAT IS GPS

The Global Positioning System (GPS) is  a constellation of 27 Earth-orbiting satellites (24 in operation and three extras, in case one fails).

The GPS receiver calculates its position by timing very precisely the signals sent by the GPS satellites orbiting above the Earth. Each satellite continually transmits messages which includes:

* the time the message was sent
* precise orbital information
* the general system health and rough orbits of all GPS satellite

The GPS receiver measures the time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the satellites’ locations to obtain the position of the GPS receiver. This position is then displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show more detailed information such as direction and speed which is calculated from position changes.

IS GPS FREE

Yes, once you have some form of receiver and an application on your notebook, PC, P. D. a, etc etc then you are able to acquire the single from each satellite which basically triangulate your position.  The satellites were originally launched by the US military but for a number of years now they have provided signals for any user, so as such there are no fees associated with being able to receive the signals.

SATELLITE COVERAGE

Geo  stationary orbit satellites cover most of the Worlds surface so it is possible to acquire a signal virtually anywhere, most receivers require a minimum of three satellites in order to establish a position but most always tried to acquire a signal from at least seven.  Obviously your receiver will need a line of sight to the satellites, which basically means being able to see a decent portion of sky.  Certain obstacles, such as trees or tall buildings may obscure the signal as well as certain atmospheric conditions such as rain, snow, hail or sleet.  Your GPS receiver will continually try to acquire a signal from as many satellites as it is programmed to receive as part of its normal functioning.

DO THEY USE POWER FROM A PDA etc

No, the Bluetooth GPS receivers are normally rechargeable and have a powerful lithium battery which allows them to operate for anything between six to eight hours, although extremes of temperature, both hot and cold, may reduce the usable time.