at reducing the ratio of relative to absolute fixing when using a Decca chain overland, 
to the point that it can furnish adequate precision of horizontal control in a wider 
range of terrain conditions than at present and provide a means of overland line 
measurement. This programme is in turn divided into two lines of approach; the 
provision of means of correcting the phase shifts due to ground effects, by trans- 
mitting extra information from the ground stations designed to reveal the electrical 
conditions obtaining along the transmission paths from the stations to the receiver, 
and a derivative of the system using the phase-comparison principle and other 
elements of the Decca technique in conjunction with UHF transmissions of the same 
order of frequency as those of the secondary-radar systems. 
6.4. It is too early yet to quote the results of work at present in progress on the 
correction methods. The principle under review can be illustrated, however, by 
supposing that a ‘green’ and a ‘purple’ pattern are generated simultaneously on 
the same master/slave baseline. Taking typical figures, the master station radiates 
85 kc/s, the purple 70.5 kc/s and the green 127 kc/s; the frequency ratio between 
the two transmissions from the slave site is nearly 2:1, and it can be shown that at 
distances exceeding about 100 km. the two hyperbolic patterns, reduced to a common 
lanewidth, will not give an identical Decometer reading owing to differences in the 
propagation of the two slave signals. The lower the conductivity of the soil along 
the transmission paths, the greater will be the difference between the two readings. 
From this simplified outline it will be realised that the essence of the method is to 
transmit at least two patterns from each baseline and to apply corrections to the 
normal Decca readings based on a comparison of the superimposed patterns. 
The factors involved in this proposal are complex and final confirmation of the 
validity or otherwise of the method must await an extensive programme of ground 
observations and theoretical analysis. 
6.5. In view of the proposal just mentioned, it is of interest that the Mark 10 
Decca technique to be implemented on the permanent Decca chains involves the 
superimposition of three patterns—red, green and purple—on each master /slave 
baseline and furthermore that each pattern is radiated with the master station 
acting in its normal capacity and with the master and slave stations reversed in 
function: that is to say, the master, red, green and purple carrier frequencies are 
radiated from all the transmitting sites using a time-multiplex system to prevent 
mutual interference. While the main object of the Mark 10 principle—to improve 
‘lane identification’ under night skywave conditions by reconstituting in the receiver 
the fundamental frequency of which the four radiated frequencies are harmonics— 
has no application in the survey field, it is possible that the greatly augmented 
amount of information transmitted by Mark 10 type stations may allow some 
degree of correction for phase errors due to ground effects on the lines sketched 
above. 
6.6. UHF Decca. A study of the characteristics of Decca and of the secondary 
radar systems leads to the conclusion that a system combining the virtues of both 
would provide the surveyor with a tool of outstanding value. In parallel, therefore, 
with current investigations into the possibility of reducing or correcting propa- 
gational errors at the Decca frequencies it is proposed to produce a system in which 
signals of the Decca type are impressed as a modulation upon carrier waves of 
ultra-high frequency. The idea of modulating UHF carriers in this way is not new, 
but has so far been shelved owing to the limited scope for line-of-sight systems 
compared with conventional Decca. The survey field is one of many in which Decca 
is used and the production of a specialised set of equipment to deal with one aspect 
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