phase velocity of the waves in vacuo is obtained. The published
result is C0 299792.50 i 0.10 where the variation quoted is the
standard deviation of a single observation modified by the inclusion
of an allowance for estimated systematic errors. Incidentally the
value C0 299792.5 0.4 km/s has been adopted by the Inter
national Scientific Radio Union (U.R.S.I.) and by the International
Union for Geodesy and Geophysics (I.U.G.G.).
The refractive index of air at three different microwave frequen
cies has also been measured to high precision by Essen and Froome
(11, 12, 13). Formulae are given for the calculation of refractive
index from observation of atmospheric pressure, temperature,
and humidity.
The newest device using the velocity of electromagnetic waves
for the measurement of geodetic distances is Wadley's "Telluro-
meter". I see one in a cupboard before me so I assume you are
familiar with its appearance. In theory it is very similar to the
Geodimeter except that it uses a modulated beam of microwaves
(wavelength 12.5 cm) in place of light waves. Because microwave
detectors are relatively sensitive compared with photoelectric cells
the apparatus is very light and easily portable. In addition it can
be used in daylight or in mistwhich is a most useful asset. How
ever the refractive index of the air at microwave frequencies is more
difficult to obtain accurately than at optical frequencies because of
the very great effect of water vapour. Consequently the humidity
must be measured with the utmost care, using a really good forced
draft wet-and-dry bulb hygrometer. Nevertheless the Tellurometer
is undoubtedly the most important development in geodetic
surveying for many years.
no
Fig. 12.
