74 SURVEY NAVIGATION, CORTEN 
however, a mean error of 1:1000 of distance (e.g. mean scaleof flight line axis) is sufficient 
for construction of semi-controlled mosaics. On further improvement, particularly in 
combination with inertial navigation, it may eventually develop to a system suitable for 
trilateration. 
2.4. Aerodist positioning. 
The Tellurometer system of distance measurement, based on radio microwave rang- 
ing, has proven to be most useful for the establishment of geodetic lines. An accuracy of 
1 : 300.000 + 5 cm can be obtained by this method of phase comparison between the con- 
tinuous wave travelling from Master to Remote station and a pulsed return wave from 
Remote to Master. This system, designed for ground distance measurements, has now 
been modified for use in aircraft to measure ground-to-air distances and is called Aero- 
dist. It is a new method, undergoing its flight trials in U.S.A. 
Principle of the Method. 
Masters (M) are placed in the airplane and Remotes (R) are positioned at the 
ground. Information about distance M...R is obtained by phase comparison between 
two continuous waves of approx. 1 Ke frequency (in the standard Tellurometer, phase 
differences of one continuous wave and one pulsed return wave are measured). These 
phase differences increase and decrease with distance and are measured at the Master 
stations in the airplane. Their transmitter characteristics — including transmission power 
and antenna beam width — have been modified to adapt to aircraft operation. The ground 
based operators must keep the aircraft trapped within the Remote's antenna beam. A 
Duplex system consists of two, a Triplex system consists of three sets Master-plus-Remote. 
Principles of operation. 
To solve the distance triangles, the planimetric position of two ground stations R, 
$ 
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(75 MILES) — 
  
  
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‘POSITIONING BY CONTINUOUS TRILATERATION 
Fig. 19. 
and R, and the heights of these stations and of the airplane M, and M, must be known. 
The accuracy of these elevations necessary to exploit the Aerodist method to the full 
extent, may well present a practical problem to the surveyor.