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aspect 
of that field is a considerable undertaking. However, as technical development has 
proceeded it has become clear that many common elements could be employed 
in the UHF and the long-wave versions of the system. There is in fact a case for 
designing the equipment in such a way that the ground stations could radiate both 
groups of frequencies simultaneously; if this were done, low flying aircraft and 
helicopters, and survey aircraft engaged in tracking or filling-in the control network, 
could employ the conventional frequencies while the interstation lines could be 
measured and horizontal control points established by aircraft capable of receiving 
the same signals via the UHF carrier waves. 
6.7. The aim, however, is to invest UHF Decca with the maximum possible 
degree of flexibility so that, for example, line measurements (by line-crossing) could 
be performed by what would amount to the airborne equivalent of the present 
Two-Range Decca system. In this system the aircraft would carry the master station, 
sending Decca-modulated UHF signals, together with a receiver indicating con- 
tinuously the master-to-slave distances; only two stations would be needed on the 
ground, also working in the UHF band but otherwise operating exactly in the 
manner of Decca slave stations and comprising the same essential units of equipment. 
6.8. The airborne Two-Range Decca will in its initial form be capable of use 
by only one aircraft at a time, but the UHF technique does not preclude and may 
to some extent facilitate the ultimate development of a sharing technique enabling 
two or three aircraft to use the same pair of ground stations. The two-range version 
of UHF Decca is also envisaged as a method of providing fixation for horizontal 
control, and the proposed technique will render negligible in size the dead areas 
around the stations of the kind that are associated with Shoran. While the provision 
of the two-station, two-range system is regarded as essential if UHF Decca is to 
form an economical and easily deployable method of establishing primary and 
secondary control, it is necessary also that the UHF version of the system should 
be capable of use as a three-station hyperbolic chain. At the expense of the third 
ground station this will provide a multiuser system in which the airborne installa- 
tions will weight little if any more than with present Decca. 
6.9. In its two-range form, UHF Decca should furnish high-flying aircraft with 
a precision of fixing comparable with that of the present pulse systems and possibly 
greater. This round assertion is based on the premises that propagation conditions 
in the proposed frequency band have already been thoroughly explored and that the 
equipment will be capable of effecting the basic time measurements by phase- 
comparison at least as accurately as is possible by the pulse technique. The second 
assumption is tenable on technical grounds and there is supporting evidence in the 
observed performance of the Lorac, Raydist and Decca phase-comparison systems. 
Oscillators having a frequency stability of one part in one million are commonplace 
in Decca technique, as are phase-difference measurements with an accuracy of one 
degree at 300 kc/s: the latter corresponds to a time-difference of 0.01 microseconds 
and represents a change of 1.4 metres in the master-to-slave distance. 
April 1956. 
  
  
SEE 
  
  
SEE