A RADIO AID TO
AIR SURVEY
1. INTRODUCTION
1.1. The radio aid discussed in this paper is the Decca Navigator position-
fixing system. Much has been written about this system and most air survey
authorities and operators whose activities fall within the scope of electronic aids
will be familiar with its main features. Accordingly, the origin, development and
general characteristics of Decca are dealt with briefly in this paper for reference
purposes, the chief topics to be considered being the application of the system to
survey operations in general and to aerial surveying in particular.
1.2. It is something of a paradox that the Decca system, with its thousands of
users and the long experience it commands in a variety of navigational and survey
roles, is yet in its infancy as an aid to photogrammetric survey; but whereas the
secondary-radar systems now being operated with such marked success in Canada
and elsewhere are based on an electronic technique that may well be nearing the peak
of its potential development, the 'phase-comparison' type of system exemplified
by Decca has scope for extensive development beyond present practice. One of the
objects of this paper is to indicate broadly the lines of research being followed in
connection with the Decca system as applied to aerial surveying.
2. ORIGIN AND DEVELOPMENT
2.1. The basic Decca Navigator system was invented before the second world
war by an American engineer, W. J. O'Brien'. After development in collaboration
with H. F. Schwarz of the Decca Record Company under the sponsorship of the
British Admiralty, the system was first used on the Normandy invasion. In 1945
the Decca Navigator Company was formed and the system rapidly became the
principal navigational aid for shipping in the waters of Western Europe. Through
the medium of permanent chains of transmitting stations (Fig. 1) in Denmark,
France, Germany and the U.K. (a new chain is at present under construction in
Sweden), some 3,000 vessels now use the service. It is interesting that of this total
no less than 900 are fishing vessels, whose activities have been virtually revolu-
tionised by the ability to fix position at will in the open sea and by the facility of
Tepeatability' that the system provides.
22. At its inception Decca was seen also to have considerable potentialities
as an aid to air navigation. Here, development has been slower than in the maritime
field—a fact which is true of its navigation and surveying uses alike—but today
some 200 aircraft employ the European Decca service, including all the propeller-
turbine Vickers Viscounts of British European Airways who recently announced
their intention to equip the entire B.E.A. fleet with Decca. Several of the factors
underlying the success of the system as an air navaid are relevant, as will be shown
later, to the question of its use in aerial survey. Also of interest to surveyors is
Decca's special suitability, by virtue of the propagation characteristics of the radio
Waves used and the method of displaying the information, for the navigation of
helicopters.
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