is the dependance of the effective propagation speed upon the electrical characteris- 
tics of the ground over which the signals pass'?. Where the ground is homogeneous 
and/or of good conductivity, a performance can already be secured that is sufficient 
at least to reduce to a very small number the control points required in small-scale 
photogrammetric mapping", bearing in mind that every individual nadir point can 
be assigned a Decca fix devoid of cumulative error. A trial carried out over south. 
east England in which the nadir points of 17 photographs at a scale of 1:20,000 
were determined by orientation with a Wild A6, gave mean errors of --15.5 metres 
and 18.5 metres respectively. The location of the trials area (Thames Estuary) 
with respect to the chain was such that the results represented a ratio of relative to 
absolute accuracy of about 1.7:1. 
5.14. Inasmuch as the area covered by the photographs just referred to was 
only some 50 sq. miles, however, it would be unwise to regard this ratio as prac- 
ticably achievable over the whole coverage of a chain even with very favourable 
ground characteristics; on the basis of all evidence at present available, a ratio of 
2.5:1, corresponding to a standard deviation of .025 mean lanes, is estimated as 
the highest degree of absolute accuracy that a land based Decca chain could at 
present be expected to provide given that only the station-positions and one control 
point were known. To see what this performance means in relation to area of 
coverage and distance from the stations, reference should be made to the contour 
diagrams at Figs. 3-5 with the error values multiplied by 2.5 (e.g. the outer contours 
would represent an r.m.s. error of 500 feet). That this level of absolute accuracy 
could be obtained in practice over a considerable portion of the earth's surface is 
suggested by the results of the Decca reconnaissance survey in North Africa already 
referred to (para. 4.9) in which the discrepancies between Decca and visual fixation 
appear to have been of this order. In terrain where there are sharp discontinuities 
and very low conductivity soil, the ratio of relative to absolute accuracy has been 
found, by observations made over the whole service area of a trail chain, to be as 
large of 1:7; in such areas the use of Decca for fixation (as opposed to navigation 
and tracking) would be impracticable except for reconnaissance or exploratory 
purposes. 
5.15. While the present Decca system is of value for fixation in favourable 
terrain, its use for this pre-supposes that the positions of the three stations and at 
least one additional point in the coverage are accurately known. Whereas it is 
possible for the system to determine the relative positions of these points itself by 
a process analogous to the Shoran line-crossing, the precision with which this can 
be done with land transmission-paths is insufficient for the requirements of geodetic 
measurement, and could lead to large errors in the subsequent use of the system 
for fixing due to the expansion of the lanes and the relatively large positional shift 
that this can cause for a small error in station position. 
5.16. It is hoped that current trials will yield information on an important 
factor for which little experimental data yet exists, namely the relationship between 
accuracy and aircraft height. On theoretical grounds it appears certain that some 
improvement in accuracy will be gained in an aircraft flying at say, 20,000 feet, due 
to the reduction that must take place at altitude in the phase variations associated 
with the passage of the signals across boundaries between different levels of ground- 
conductivity. It has already been experimentally confirmed that no such gain results 
in an aircraft at 2,000 feet (the mean wavelength of the Decca signals is about 
10,000 feet). What practical gain will result at high altitudes, having regard to other 
unknown quantities such as the overall effective velocity along the inclined path 
16 
   
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