2.3. The continuous position-fixing service provided by the permanent Decca
Navigator Chains forms a useful aid, in areas where accuracy is sufficient, to 4
variety of survey operations. An interesting example is the control of accurate
tracking of the aircraft engaged on photographic flying for Ordnance Survey;
another, the use of the system for position-fixing and navigation in various aero-
magnetic surveys. The first Decca chain to be erected for peacetime operation,
covering SE. England and the Thames Estuary, was used from its inception by
the Hydrographic Department of the British Admiralty as an aid to survey work
and as the coverage has extended the system has been used for this purpose on an
increasing scale around the coasts of Great Britain. À new North Scottish Chain,
centred on the Orkneys, will give further scope in this direction. It was thus in the
field of hydrography that the system’s potentialities as a servey aid were first put
to practical test and so far it is in the marine rather than the air field that it has made
the primary contribution to the art. The reasons for this are not far to seek: to
mention three, the conditions for the ground-wave mode of propagation used by
the Decca signals are near-ideal when operating over the uniform conducting
medium of sea water; the logistical effort involved in deploying a coastal chain of
stations, even if these are small and portable, is less than in siting a similar chain in
the interior of an undeveloped country; and the cost and time involved in carrying
out trials of the accuracy and performance of a radio aid to air survey are vastly
greater than in corresponding tests at sea.
2.4. Much has already been done to adapt the equipment to the needs of sur-
veying in remote areas. The initial step, of course, was to produce miniature versions
of the ground transmitting stations capable of being carried in a single vehicle or
taken ashore in sections in a small boat. The first such chain was supplied in 1947
to the Danish Government for Hydrographic work off Western Greenland?®, and
about a dozen, progressively refined in design, have since been put to work in parts
of the world as varied as the Gulf of St. Lawrence?, the Sahara Desert* and Dutch
New Guinea. Before considering the present and potential application of the system
to aerial surveying, however, the working principles and basic characteristics of
Decca should be briefly recalled.
3. OUTLINE OF THE SYSTEM
3.1. The Decca Navigator is a hyperbolic fixing system in which a chain of
three ground transmitting stations, forming two pairs with the central ‘master’
station common to each, provide two intersecting patterns of position lines (Fig. 2).
These lines are of hyperbolic form, being the /oci of points of constant distance-
difference from the two stations generating the pattern. The position-lines are in
turn defined by comparing at the receiver the phase of the two received signals,
the assumption being made that the signals travel at a known speed of propagation.
The receivers carried in the ships, aircraft or vehicles using the system drive a pair
of indicators (Decometers) one for each pattern, which display continuously the
numerical values of the two position-lines intersecting at the receiver and so enable
a fix to be plotted on a map or chart bearing the hyperbolic grid. In addition, the
currents actuating the Decometers can be amplified and caused to operate a fully
automatic plotter, known as the Flight Log in its aircraft form, which records the
position and progress of the aircraft on a chart whose projection is such that the
hyperbolic co-ordinates of the system appear as a rectilinear grid.
3.2. A hyperbolic pattern obviously contains an infinite number of constant-
phase-difference lines. Conventionally only those of zero-phase difference are drawn
6
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