GPS Receiver
GPS is short for Global Positioning System was funded by and controlled by the United States Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U. S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. There is 24* satellites circles the earth and 12 is available at any onetime. But all it needs is 4 GPS satellite signals** are used to compute positions in three dimensions and the time offset in the receiver clock. The 3 major GPS Receiver manufacturers are Garmin, Magellan and Lowrance.
Note: * actually there are often more than 24 operational satellites as new ones are launched to replace older satellites. ** GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
More GPS technical reference visit colorado.edu/geography site
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