Hoe Does GPS Work??

January 3, 2008 at 10:03 pm

The Global Positioning System (GPS) is the most significant recent advance in navigation and positioning technology. In the past, the stars were used for navigation. Today’s world requires greater accuracy.  The new constellation of artificial stars provided by the Global Positioning System serves this important need.GPS is an aerospace technology that uses satellites and ground equipment to determine position anywhere on Earth.  Anyone with a small receiver can use the system at no cost.  GPS has drastically changed methods of navigation and is fast becoming important in everyday life.

GPS Constellation


Global Positioning System satellites transmit signals to equipment on

the ground.GPS receivers passively receive satellite signals; they do not

transmit.GPS receivers require an unobstructed view of the sky, so they

are used only outdoors and they often do not perform well within

forested areas or near tall buildings. GPS operations depend

on a very accurate time reference, which is provided by atomic

clocks at the U.S. Naval Observatory. Each GPS satellite has

atomic clocks on board.

Each GPS satellite transmits data that indicates its location and

the current time. All GPS satellites synchronize operations so that

these repeating signals are transmitted at the same instant. The signals,

moving at the speed of light, arrive at a GPS receiver at slightly different

times because some satellites are farther away than others. The distance

to the GPS satellites can be determined by estimating the amount of

time it takes for their signals to reach the receiver. When the receiver

estimates the distance to at least four GPS satellites, it can calculate its

position in three dimensions.

There are at least 24 operational GPS satellites at all times. The satellites,

operated by the U.S. Air Force, orbit with a period of 12 hours. Ground

stations are used to precisely track each satellite’s orbit.

Determining Position
A GPS receiver “knows” the location of the satellites, because that

information is included in satellite transmissions. By estimating how far

away a satellite is, the receiver also “knows” it is located somewhere

on the surface of an imaginary sphere centered at the satellite. It then

determines the sizes of several spheres, one for each satellite.

The receiver is located where these spheres intersect.

<!–Because GPS receivers do not have atomic clocks, the distance to the satellites can only be roughly estimated at first. However, this error is exactly the same for all satellites because their transmissions are synchronized. This allows a GPS receiver to determine its position by precisely calculating the distances to GPS satellites relative to each other. –>

GPS Accuracy
The accuracy of a position determined with GPS depends on the type

of receiver. Most hand-held GPS units have about 10-20 meter accuracy.

Other types of receivers use a method called Differential GPS (DGPS)

to obtain much higher accuracy. DGPS requires an additional receiver

fixed at a known location nearby. Observations made by the stationary

receiver are used to correct positions recorded by the roving units,

producing an accuracy greater than 1 meter.

When the system was created, timing errors were inserted into GPS

transmissions to limit the accuracy of non-military GPS receivers to

about 100 meters.


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