28-01-2012, 03:04 PM
Global Positioning System
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Introduction of GPS
The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather, anywhere on or near the Earth.
Where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver with some technical limitations which are only removed for military users.
The GPS project was developed in 1973 to overcome the limitations of previous navigation systems, integrating ideas from several predecessors, including a number of classified engineering design studies from the 1960s.
GPS was created and realized by the U.S. Department of Defense (USDOD) and was originally run with 24 satellites.
It became fully operational in 1994.
Modernization
In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB), conducted developmental flight tests of two prototype GPS receivers over White Sands Missile Range, using ground-based pseudo-satellites.
In 1978, the first experimental Block-I GPS satellite was launched.
In 1992, the 2nd Space Wing, which originally managed the system, was de-activated and replaced by the 50th Space Wing.
In 1996, recognizing the importance of GPS to civilian users as well as military users, U.S. President Bill Clinton issued a policy directive[20] declaring GPS to be a dual-use system and establishing an Interagency GPS Executive Board to manage it as a national asset.
GPS Receiver clock
If we send out a pulse of sound or radio waves then we can determine resistance of object by dividing the time it took for reply by the speed of sound.
Distance = Speed * Time
Time = Distance / Speed
GPS works on much the same principle. GPS satellite only transmit timing data pulse, GPS receive units, only receive.
Let r4 denote the distance from the valid estimate of GPS receiver position to the fourth satellite and let p4 denote the pseudo range of the fourth satellite.
. Working principle of GPS
How GPS works
GPS signals work in microwaves bands.
They can pass through glass, but are absorbed by water molecules and reflect off concrete, steel and rock.
This means that GPS unit have trouble operating in rain forest, urban jungle, deep canyons, inside automobiles and roads and in heavy snowfalls-among other things.
These environmental obstacles degrade positional accuracy or make it impossible to get a fix on your location.
