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volved in the Two-Range method is added the advantage that only two shore stations 
are used. Furthermore, the area over which a high accuracy is achieved is much 
greater than with the hyperbolic layout, since the lanes that the system meas- 
ures are always at their minimum (constant) width. The survey carried out in 1955 
by the Canadian Hydrographic Service in the Gulf of St. Lawrence was an interesting 
example of the use of the Two-Range technique and enabled substantial savings 
in time and cost to be made by comparison with the classical methods?. 
48. With both the hyperbolic and Two-Range layouts in hydrography and 
marine oil exploration, a marine derivative of the Flight Log, known as the Track 
Plotter, is employed in addition to the Decometers. While the latter are sufficiently 
rapid to interpret for all ordinary marine navigational purposes, the Track Plotter 
is a valuable aid to steering a cross-lattice course since it avoids the need for con- 
tinuous manual plotting. This instrument, which can also make annotation-marks 
at specified time intervals for co-ordination purposes, forms a useful method of 
recording the complex movements and operations of a seismic or other exploration 
vessel (Fig. 10). 
49. Before considering the airborne applications of the system in surveying, 
reference should be made to its use at ground level. This is made possible, as already 
indicated, by the fact that the signals can be received on the earth's surface at long 
distances. While the accuracy is not of the same order as that which is associated 
with the classical methods of visual ground surveying as generally understood, the 
performance is sufficient for certain branches of reconnaissance-surveying and 
exploration with little or no visual control, in terrain of a favourable character for 
wave propagation. This has been clearly demonstrated in a survey carried out by 
an oil company in North Africa in which Decca was used for the navigation and 
fixation of a group of vehicles engaged in geophysical measurements. The facility 
of repeatability that the use of radio aids has introduced is all-important in this 
work, enabling a vehicle to proceed to any spot at which an observation has pre- 
viously been taken with none of the delays that would be involved in even the crudest 
visual observations. The absolute accuracy attained on this particular survey was 
difficult to check, since the precision of the rather sparse geodetic control available 
was itself open to question, but it was reported that the discrepancies between the 
two systems where these could be observed lay within the range 10 to 100 metres. 
Among the various types of Decca receivers available is a ‘manpack’ set driven 
by dry batteries and weighing approximately 50 Ib. The receiver, battery and Deco- 
meter units are attached to a framework supported by a harness for manpack 
operation but can be withdrawn from the carrying frame for installation in a vehicle, 
boat or aircraft. 
4.10. The fact that the Decca Navigator can be used by vehicles and ground 
parties means that it is also available to aircraft flying at very low altitudes; the 
implications of this are important in survey and exploration work, not least in 
relation to the use of helicopters. The latter type of aircraft are finding increasing 
application in the survey field and their navigational control over distances exceeding 
visual range can only be achieved by means of a long-wavelength radio aid. With 
the particular requirements of terrestrial reconnaissance by aircraft and ground 
parties in mind, and the co-ordination of these two classes of user in a given operation 
—€.g. the use of aircraft to drop supplies to an exploration party—another variant 
of the basic system has been evolved known as ‘Short-Base’ Decca. In this, two 
master /slave pairs of stations are used, each pair being on a short baseline about 
10 km. in length with a separation of about 100 km. between the baseline centres. 
11 
  
  
  
  
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