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and these recur at intervals which, along the interstation baselines, are constant 
and equal in width to half a wavelength at the phase-comparison frequency. The 
intervals between adjacent zero-phase lines are termed ‘lanes’ and the rotor of the 
Decometer makes one revolution per lane and drives a pointer which divides the 
lanes into hundredths. The phase comparison frequencies average about 300 kc/s 
(1,000 metres wavelength) for the ‘red’ and ‘green’ patterns produced by a chain, 
so that the lane-width on the interstation baseline is equal to about 500 metres. 
Each hundredth division of the Decometer fraction scale thus corresponds to 5 
metres along a baseline. Whereas the existence of a constant-phase pattern pre- 
supposes that the same frequency is radiated by the two stations of a pair, in practice 
the receiver could not discriminate between them if this were done; instead, the 
transmitted frequencies are harmonically related so that multipliers in the receiver 
can raise them to a common frequency for phase-comparison. 
3.3. Characteristics. Decca uses transmitted signals having a very long wave- 
length—some 3,000 metres. Signals of this type have the property of following 
around the earth's surface with very small attenuation, with the result that the 
system can be used by surface vessels at distances of several hundred miles from 
the station, and, more important in the present context, by aircraft at all altitudes 
down to ground level. Another useful characteristic of longwave transmissions is 
that in areas subject to certain types of radio interference, such as that experienced 
in northern Canada and associated with the aurora borealis, it is possible to secure 
reliable operation at times when quite a large portion of the radio spectrum is 
severely affected. On the other hand, in tropical areas near the equatorial thunder- 
storm belt, radio ‘noise’ has a marked effect at the Decca frequencies and care has 
to be taken to furnish sufficient power to secure reliable operation at the distances 
desired’. 
3.4. The long-wavelength signals have a characteristic which, with the system 
in its present stage of development, precludes its use in certain types of terrain for 
accurate fixing of the order required for the horizontal control of aerial photography. 
It was stated above that the operation rests on the assumption that the speed of 
propagation of the signals is known; this parameter, however, is effectively a function 
of the type of terrain over which the signals pass and is modified also by the incidence 
of discontinuities or boundaries between one type of ground and another® ’. The 
poorer the electrical conductivity of the ground, the slower will be the effective 
velocity, and the figure for this can lie between the extremes of 299,750 km/sec. 
for sea water and 297,000 for rocky terrain. In many cases this characteristic would 
not prove a serious disadvantage, but inasmuch as it represents a fundamental 
limitation it is necessary to develop some means of applying the necessary corrections 
without taking the obvious but expensive step of specially surveying a large 
number of control points. Reference to this work is made later. 
3.5. The scale represented by the drawings in Figures 3-5 is subject to consider- 
able variation. In survey operations, the interstation baselines are seldom less than 
40 miles in length or greater than 120. Very broadly speaking, the service area of 
a Decca chain used in surveying is bounded by an arc of radius equal to twice the 
interstation baseline length for the front cover of the stations. The accuracy require- 
ment, i.e., the acceptable minimum angle of cut and maximum lane widths, as well 
as considerations of signal strength, are operative in deciding the service area, the 
relative importance of these two factors varying in different cases depending on the 
type of survey, the type of terrain involved and the power of the stations. 
7 
  
  
SETZE s