being the consumption of a station of the smallest lightweight type; stations having 
two control units and two transmitters consume 2.4 kilowatts (these figures refer 
to the slave stations, which consume slightly more power than the master). Some 
margin must be allowed for test-gear soldering irons, etc., and to cover these items 
the basic station load (i.e. Decca equipment only) should be taken as not less than 
1.5 and 3 kW for the above examples. The final choice of a suitable generator 
depends, of course, on the additional load imposed by heating, air-conditioning, 
cooking and communication equipment installed at the stations. 
4.5. Itis with equipment of the form outlined above that Decca-assisted surveys 
have been carried out in various parts of the world outside the coverage of the 
permanent chains. As already mentioned, most of the work thus performed has 
been in the hydrographic field, the system being employed as a means of fixing the 
position of the vessels carrying out depth measurements and also for steering along 
straight section-lines of pre-determined separation. By the visual methods the pro- 
cesses of taking fixes, running section lines and navigating to specified points are 
liable to be slow. They are also subject to delays whenever weather interferes with 
visibility and become progressively more difficult as the limit of visibility to shore 
is approached. Decca has made a valuable contribution to the technique of off-shore 
and ocean hydrography by providing the facility of continuous position-fixing; it 
is also extensively used in the allied field of marine oil exploration and here the 
facility of repeatability implicit in the high relative accuracy of the system pays 
valuable dividends by enabling any desired point to be recovered without delay on 
successive occasions. 
4.6. In one overseas survey at present in progress Decca is to be used for the 
measurement of long lines—of the order of 100-300 km—between islands and the 
mainland; the performance over sea water is such that this method of trilateration 
is expected to make a valuable contribution to the work of coastal charting. À line 
measurement is carried out by placing a master and slave station at the extremities 
and counting the total number of lanes plus the residual fraction in the hyperbolic 
pattern thus generated. This is done by carrying a receiver in an aircraft or boat 
around the pair of stations, with the set operating continuously from the time of 
crossing the baseline-extension beyond one of the stations to the time it crosses 
the other. The act of crossing a baseline-extension is revealed unambiguously by 
a change of sense in the Decometer rotation and the whole operation can be per- 
formed without any initial knowledge of position. It has been shown" that a line 
can be measured in this way over land with an accuracy as high as 1:90,000, provided 
that the geological structure along the path is known in great detail; it is expected 
that over sea water paths an accuracy in excess of this figure will be achieved on the 
present survey. 
4.7. The measurement of a line by placing a master station at one end and the 
slave at the other is the basis of the recently introduced and already well-established 
‘Two Range’ Decca system'?, which it is worth mentioning briefly here since the 
technique has a direct relevance to future developments, referred to at the end of 
this paper, directed towards the use of Decca for overland airborne operations of 
the kind in which Shoran is currently employed. In the Two-Range Decca layout 
(Fig. 9) the master transmitting station is installed on the survey ship, together 
with a receiver, with the two slave stations on suitable sites ashore. Given the 
velocity of propagation the number of lanes in the two master/slave baselines is a 
function of the baseline lengths, so that the shipborne Decometer readings can be 
translated directly into units of distance. To the simplicity of the computations in- 
10 
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