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providing the service can be regarded as straight and parallel; accordingly the 
‘homing’ method of tracking is used in which the pilot flies the aircraft so as to 
maintain the appropriate Decometer fraction pointer at a selected reading. The 
required separation between lines is given by inspection of a hyperbolic chart of 
the area (or by direct computation) and choosing the fractional Decca position 
lines that have the required location and separation. The use of the system to home 
along a single position line in this way secures a precision sufficient for the control 
of tracking in relation to photographic scales as large as 1:2,500. Here the flight 
line separation may be 220 yards with a tolerance off track of only plus/minus 
20 yards. Under these exacting conditions, in which an adequate standard of visual 
navigation is very difficult to achieve, the use of Decca tracking has virtually 
abolished gap-filling flights and has also enabled considerable savings to be made 
in the subsequent analysis of the photographs by virtue of the uniform overlap 
obtained. Fig. 11 shows a group of lines flown by this method. 
5.5. Where it is required to cover a large area, the homing method is no longer 
applicable owing to the expansion and curvature of the lanes. Straight flight-lines 
of any orientation can be flown, however, irrespective of the disposition of the 
hyperbolic patterns in the area to be surveyed. The essence of the method is to draw 
on the Decca chart the desired flight lines and to steer the aircraft along them by 
reference to a series of Decca fixes plotted on the same chart. The fixes may be 
obtained either as a succession of points plotted manually, or continuously by 
means of a Flight Log. In the former case the observer passes course-corrections 
to the pilot over the intercom expressing these as changes in aircraft heading. The 
process is therefore one of compass-course flying with repeated monitoring of the 
track by means of the Decca fixes. The accuracy of the control thus exercised 
depends on the speed at which the observer can translate the Decometer readings 
into plots and it has been found better to use a simplified chart in the aircraft rather 
than the normal hyperbolic lattice. The basis of this so-called track graph is the 
replotting of the relevant part of the Decca grid as a rectilinear grid, as in the Inverse 
Lattice used in the Flight Log. By reference to the basic hyperbolic co-ordinates 
the required flight lines are plotted on the track graph together with the magnetic 
headings of each line and any other relevant navigational data; flight trials have 
shown that after some two hours' practice by a pilot unfamiliar with Decca an 
aircraft can be held by this method within plus/minus 200 yards of the wanted 
track at the 90 per cent probability level. 
5.6. An obvious extension of the technique just described is the Decca Flight 
Log, which is an automatic plotter replacing the human operator and directly 
visible to the pilot. The Flight Log takes the form of a pen moving over a chart 
(Fig. 12). The pen itself represents the position of the aircraft and traces the track 
followed, thereby making a graphical map of the flight and enabling the aircraft 
to be steered along any desired track by flying so as to keep the pen on the pre- 
drawn line. The complete equipment in its present form (Mark 01) adds approxi- 
mately 60 1b. in weight and 12 amps. (maximum) consumption to the airborne 
receiver installation. A lighter version of the instrument is being produced for 
helicopters and single-seat aircraft. The display-head placed in front of the pilot 
contains the charts in the form of a continuous roll, a chart area roughly 10 inches 
wide by 4 inches high being exposed on the face of the instrument. The chart paper 
18 perforated with sprocket holes along the sides and one of the selected Decca 
co-ordinates controls a servo mechanism which moves the paper in a vertical 
direction as seen by the pilot; the other co-ordinate moves the pen laterally by means 
13 
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