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.

Say whether the following statements are true or false.

1. GP systems only become of practical value when the accuracy of time measurement reaches a certain threshold.

2. The Global Positioning System comprises a constellation of 24 satellites orbiting the earth at an altitude of about 10 000 km

3. Each satellite has onboard caesium beam atomic clocks.

4. The clocks on the satellites are all set to the different time.

5. The time difference between four satellites in four different parts of the sky is measured.

6. It is not an easy task to programme the satellites into the receiver memory.

7. In practice, the overall accuracy for real-time position determination is about 10 m.

8. The cost of setting up and maintaining a system such as GPS is very high.

9. There are many civil users of GPS.

10.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.

Describe how GPS works.

Unit 2

HISTORY OF METROLOGY

Practice reading the following words. Translate them into Ukrainian.

Stature, consequently, measures, commerce, endeavour, decimal, purify, extension, conversion, require, liquid, increase, bureau, dimension, linear, recreate.

Think of an important event in the history of metrology and discuss it in a group.

Read and translate the following text.

TEXT A

The stature of the human body, according to the Talmudists, contains about 3 cubits from the feet to the head. Now the ordinary stature of men, when they are barefoot, is greater than 5 Roman feet and less than 6 Roman feet. Take a third part of this and the vulgar cubit will be more than 20 unicae¹ and less than 24 unicae of the Roman foot; and consequently the Sacred Cubit will be more than 24 unicae and less than 28 + (4/5) unicae of the same foot.

Sir Isaac Newton

Although standardization of weights and measures has been a goal of social and economic advance since very early times, it was not until the 18th century that there was a unified system of measurement. The earliest systems of weights and measures were based on human morphology. The names of units often referred to parts of the body: the inch or pouce², the hand, the foot, and the yard or cubit corresponded to dimensions of the human body. Consequently, these units of measurement were not fixed; they varied from one town to another, from one occupation to another, and on the type of object to be measured.

¹unica - Inch, lit. one twelfth (pl. unicae), also pollex for thumb, is 1/12 pes, or 2.47 cm.

²pouce -- Inch, 1 / 12 pied

This lack of a standardized system of measurements was a source of error and fraud in commercial and social transactions, putting a brake on international commerce and prevented the development of science as an international endeavour. With the expansion of industry and trade, there was an increasing need for harmonization of weights and measures between countries. Politicians and scientists resolved this situation by adopting a standard of measurement (distance or weight) by comparison with a standard (étalon) taken from Nature.

One of the first such natural measures was the metre, which was defined in a decree of the French National Assembly (7 April 1795) as being equal to the ten millionth part of one quarter of the terrestrial meridian, but specified by measurements undertaken between Dunkerque and Barcelona. Such a unit was not arbitrary, being based on the size of the Earth. Once the base unit of length had been decided upon, it was possible to establish the resulting units of measure: the square metre (for area) and the cubic metre (for volume). The kilogram was originally defined as the weight of a certain volume of water, a convenient and readily purified liquid.

Such a system of simple multiples of base units lends itself naturally to extension. The decimal metric system was introduced in France on 7 April 1795 by the law "On weights and measures". This caused a major change in the everyday life of ordinary people, readily allowing the calculation of, for example, areas and volumes.

The first standards (étalons) of the metre and the kilogram, against which all future copies were to be compared, were deposited in the Archives of the French Republic in 1799, dedicated to "all men and all times". Because of its simplicity and universality, the decimal metric system spread rapidly outside France. The development of railways, the growth of industry and the increasing importance of social and economic exchange all required accurate and reliable units of measurement. Adopted at the start of the 19th century in several Italian provinces, the metric system became compulsory in the Netherlands from 1816 and was chosen by Spain in 1849. In France, the decimal metric system was exclusively adopted with the law of 4 July 1837.

After 1860, the countries of Latin America took up the metre, and there was a steady increase in the adoption of the metric system by other nations during the latter half of the nineteenth century (for example, the United States of America, 1866, Canada, 1871, Germany, 1871). However, these countries were dependent for their national standards on copies of the original prototypes. This dependence, together with the lack of uniformity in making copies, limited the desired international standardization. To overcome these difficulties, the Bureau International des Poids et Mesures (BIPM) was founded by the terms of the diplomatic treaty known as the Metre Convention on 20 May 1875. To celebrate the signing of the Metre Convention, the date of 20 May is known as World Metrology Day.