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5.9. Flight Log charts can be drawn for a very large number of possible com- 
binations of primary or secondary pattern, sense of rotation, and scale in lanes per 
inch. With each different chart a small key is provided and these are fitted into a 
turret-switch which forms the principal Flight Log control; the wards on the keys 
operate the appropriate contacts of a multi-contact switch connected with the 
computer unit, so that turning the turret-switch to the code number of the particular 
chart required is the only operation that the pilot has to perform when changing 
charts (other than pressing a button which energises a motor to bring the desired 
chart into view on the display head and positioning the pen). 
5.10. In survey and kindred operations the Flight Log charts consist simply of 
the numbered Decca grid, enabling the pen to be set to the correct initial position 
and checked at will with respect to the Decometer readings, together with lines 
representing the desired flight paths. Topographical and other navigational infor- 
mation can be added as required and the instrument itself represents a ready means 
of incorporating data of this kind; for example, the position of an emergency 
airstrip for possible future use on a survey operation, can be recorded simply by 
flying over it with the Flight Log in operation, the receiver having been previously 
set up at a point of known Decca co-ordinates, e.g., the base airfield. Similarly 
the tracks recorded by the Flight Log on reconnaissance flights through hilly or 
hazardous country can be traced on to the charts used for navigational guidance 
in the subsequent survey. 
5.1l. Fixing. So far we have been considering the processes of navigation and 
tracking, in both of which (except for the homing procedure) is implicit the fixing 
of the aircraft's position by two intersecting Decca position lines; we turn now to 
the function in which the emphasis is placed on position-fixing, namely the use of 
Decca for planimetry and horizontal control. It will be generally conceded that the 
ideal contribution of electronic aids to aerial mapping would be that they should 
wholly eliminate, in all cases, the need for visual surveying on the ground. Of the 
two broad types of radio system so far considered in this paper, those using pulse- 
ranging are the nearer to fulfilling this ideal in the present state of the art and the 
Shoran system has in fact become an established method of geodetic trilateration 
and of horizontal control of photography in the survey of Northern Canada. 
3.12. There is, however, plenty of scope for the improvement and extension of 
electronic fixation methods. Examples of problems awaiting solution are the re- 
duction of airborne installation weight and elimination of the need for a special 
operator; the ability of a given pair or trio of ground stations to serve any number 
of aircraft simultaneously, at any altitude; the elimination of dead areas around the 
ground stations; the reduction of random errors associated with meteorological 
changes; and the extension of the position-fixing service to helicopters and ground 
parties so that they can use it directly as a means of fixing horizontal control points. 
It is fairly safe to predict that if these problems are ultimately solved, this will be 
the result of developments in the electronic technique of phase-comparison rather 
than in that of pulse ranging. Already, in its existing form, the Decca system goes 
some way in this direction: it does not require a specialist aircrew member, the 
existing airborne equipment is much lighter than that of the other systems, there is 
no limit to the number of users, virtually no minimum-range restriction if the 
appropriate corrections for slant distance and induction field are applied, and so 
far random errors of a meteorological kind have not been detected. 
3.13. It will be recalled from the note on accuracy early in this paper (para. 3.7) 
that the factor limiting the performance of Decca as a means of absolute fixation 
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