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This layout reduces greatly the number of lanes in the patterns, so reducing ambiguity
(at the expense of sensitivity) but without the extra equipment involved in the ‘lang
identification’ system of ambiguity-resolution used on the permanent Decca chains,
The position lines are effectively straight over the operational area, which greatly
simplifies the preparation of lattice charts. The relative accuracy of a typical Short
Base chain falls within the range 100-300 metres; the absolute or geographical
accuracy is of secondary importance in operations of the type for which the layout
is envisaged but would be of the order of $ km. in suitable country and in cases
where the station positions could be determined with precision.
5. THE DECCA NAVIGATOR AS AN AID TO AIR SURVEY
5.1. Electronic position-fixing systems can perform a wide variety of functions
of assistance to the air surveyor, ranging from the general navigation of aircraft
engaged on the survey to the control of parallel tracking and the provision of partial
or total horizontal control. So far as aerial photographic survey (as opposed to
geomagnetic and exploration work) is concerned, Decca has so far been employed
in the first two of these categories; in the third, the Shoran and Hiran systems have
made an outstanding contribution. At the present time the two broad types of
technique—CW phase comparison and pulse-ranging—are complementary in their
capabilities and limitations. The use of the pulse systems for trilateration and
horizontal control will be covered in other Papers presented to the Congress and the
available literature is extensive.
5.2. Navigation. Returning to a consideration of the Decca Navigator system
in relation to the various requirements, the first mentioned of these— general
navigation—offers wide scope for the use of an aid of the Decca type to effect
savings in time and cost on a survey operation in poorly or un-mapped terrain.
Particularly is this true where ground features are sparse or repetitive, rendering
difficult the process of navigation by reconnaissance photographs. In country where
cloud formations are a constant problem (such as British Guiana), the ability to
navigate straight to the start of a flight line, or to take photographs along such parts
of the line as are seen to be clear of cloud, obviously represents a very great potential
saving in delay. Here the relatively low weight-penalty of the Decca type of airborne
installation compared with the pulse receiver/transmitter combination is noteworthy
since it is possible to install the equipment in any type of aircraft in use or at present
contemplated for photogrammetric work. The contribution this can make to
flight-safety, particularly in difficult country, is self-evident.
5.3. Latterly a new requirement has arisen for a radio aid to survey operations,
namely the navigation of helicopters and possibly of ground vehicles carrying sur-
veyors engaged in establishing visual control. At present this function is the purely
navigational one of getting the survey party to within a few hundred feet of the
desired location for the control point, but is none the less of considerable potential
value. The possibility that with further development the system could itself provide
fixation for the establishment of ground control points, is discussed below.
5.4. Tracking. While the actual process of tracking or flying parallel strips can
be performed by experienced aircrews with precision by purely visual means, there
is undoubtedly great scope for radio assistance to this work. Prominent examples
have been the use of the GH system overseas for the Directorate of Colonial Surveys
as a means of flying concentric tracks, and of Decca in connection with the Direc-
torate of Ordnance Survey's programme in U.K. The latter work involves coverage
of small urban areas over which the position lines of the permanent Decca chain
12
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