Application of accurate time measurement

A different type of requirement for accurate measurement is in systems for global positioning and surveying using time signals from earth satellites. Such systems only become of practical value when the accuracy of the key measurement, in this case time, reaches a certain threshold.

The Global Positioning System (GPS) comprises a constellation of 24 satellites orbiting the earth at an altitude of about 20 000 km. Each has onboard caesium beam atomic clocks and transmits coded time signals that can be picked up by small receivers anywhere on the surface of the earth. The clocks on the satellites are all set to the same time, to within about 30 ns. The principle of GPS positioning calls for the simultaneous observation of four satellites in different parts of the sky and measurement of the apparent time differences between them. The position of the receiver on the ground can be found from these apparent time differences knowing the positions of the satellites at the time the observations were made. The satellites are sufficiently high for atmospheric drag to be negligible and are in sidereal orbits so that their positions can easily be programmed into the receiver memory. The accuracy of the satellite position in space must be at least as good as the required position accuracy, namely about 10 m. In practice, the overall accuracy for real-time position determination is not as good as this and is only about 30 m, due principally to the difficulty of making accurate corrections for ionospheric delays in signal propagation. The cost of setting up and maintaining a system such as GPS is very high: each satellite has a life of about eight years and to complete the constellation of 24 satellites has cost more than 10 billion dollars, the second most expensive space programme after the manned landing on the moon. This whole system relies completely on the performance of the clocks.

In addition to the military users of GPS there are many civil users. The commercial applications of worldwide positioning and navigation to an accuracy of 100 m are very wide and there are now many producers of GPS receiving equipment. It is estimated that in 1992 the sales of GPS hardware reached 120 million dollars and the new industry based on GPS and producing digital maps and associated navigation systems had sales approaching 2 billion dollars. Within a few years civil aviation navigation is sure to be based on GPS. New generations of atomic clocks, having accuracies perhaps two orders of magnitude better than those now available, will undoubtedly lead to corresponding improvements in global positioning and corresponding commercial applications.

Accurate measurement of these delays, however, can provide information on the atmospheric composition and temperature that are of interest to climatologists. Recent studies have indicated that if the ionospheric delays are measured with an accuracy of about 3 ns, equivalent to 1 m of path, which is already possible, GPS could become a valuable tool for the study of long-term changes in atmospheric water and carbon dioxide content as well as temperature in the upper atmosphere.

A substantial increase in accuracy in the measurement of any important quantity almost always leads to unexpected applications in fields quite different from that for which the advance was made.

Date: 2016-04-22; view: 748