Sunday, July 31, 2011
Saturday, July 30, 2011
|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.
This presentation contains the theory behind optical fiber sensing measurement and the latest trends in optical fiber sensing measurement and its applications.Smart structures and materials in which an optical fiber acts as a sensor to measure distributed strain and/or temperature along the fiber have applications in health monitoring functions, such as preventive maintenance, product efficiency improvement, and maintenance cost reduction. Furthermore, optical fiber has the advantage that it is essentially explosion-proof, needs no electric power supply, and is not affected by outside noise, such as lightening or high-voltage electrical power lines. This makes optical fiber suitable for plant and civil applications.
Friday, July 29, 2011