Exotic scientific discoveries of our era-5

Time standards and GPS:

The Global Positioning System (GPS) is a radionavigation system that is available worldwide. GPS signals are broadcast from a constellation of 24 or more earth orbiting satellites. Because the GPS signals are derived from the atomic frequency standards on board each satellite, they are widely used as a reference for time synchronization and frequency calibration.

Heart of the GPS system is the atomic frequency standards and the National Institute of Standards and Technology (NIST) where I am working has developed these standards. Dr. William Philips of NIST won the Nobel Prize for Physics in 1996 for his discovery of cold trapping of atoms which have paved the way in the development of these standards. You may ask what is the connection between this standard and GPS. In GPS the image is processed or place is identified by the time difference in signals sent and received to one or more of the 24 satellites. The standards ensure the accuracy of measurement and the resolution of image depends on the accuracy. Present standard is a caesium atomic clock with which a resolution of 10 cm. only is achieved. But, there is a discovery of Strontium based atomic clock which will improve the resolution. Put it in simple terms, imagine you have parked your car in a big car space and forgotten where exactly you have parked, the present GPS cannot help as it cannot resolve the digits in your number plate. But, with strontium clock it would be possible.

When people talk about "a GPS," they usually mean a GPS receiver. In U.S. allmost all cabs are fitted with GPS and many people have installed a GPS receiver in their cars. Recently, when my daughter visited me from Phoenix, she had brought her GPS and with its help she could go around Washington D.C although she had not been driving here before.

The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.

Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.

A GPS receiver's job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration.

Imagine you are somewhere in the United States and you are TOTALLY lost -- for whatever reason, you have absolutely no clue where you are. You find a friendly local and ask, "Where am I?" He says, "You are 625 miles from Boise, Idaho."
This is a nice, hard fact, but it is not particularly useful by itself. You could be anywhere on a circle around Boise that has a radius of 625 miles, like this:

You ask somebody else where you are, and she says, "You are 690 miles from Minneapolis, Minnesota." Now you're getting somewhere. If you combine this information with the Boise information, you have two circles that intersect. You now know that you must be at one of these two intersection points, if you are 625 miles from Boise and 690 miles from Minneapolis.

If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle will only intersect with one of these points. You now know exactly where you are -- Denver, Colorado.


This same concept works in three-dimensional space, as well, but you're dealing with spheres instead of circles