Radar System Engineering

514 THE RECEIVING ISYST13fi{-I,VDICA l’ORS [SEC. 1311
helpful in the case of multiple indices, but their principal usefulness isin
connection with plotting and the use of charts.
“ Electronic” marks eliminate or reduce many of these difficulties.
Parallax errors are entirely eliminated. Since theindices are generated
by precision methods that are independent of the formation of the
display geometry, they automatically fall in their proper place on the
display regardless of where it may be centered or how muchit may be
deformed or distorted. In general, electronic methods of producing
interpolation indices are far less cumbersome than mechanical ones.
However, in the case of a slow scan the intermittent nature of the display
makes it difficult to set an index on the echo, unless rather complicated
switching methods are introduced to provide the indices at more frequent
intervals.
Electronic indices areinvariably used for range determinations, since
they can be readily provided with a high degree of precision and since the
radar data are inherently capable of providing great accuracy in range.
In cases where the fundamental data are not so precise, the choice depends
upon circumstances. Electronic marks are always used if the display
is to be continuously movable in position or in size, but in many of the
cases where only a few discrete changes are involved, external markers
are sufficiently simpler to warrant their use.
The following two sections will describe methods of providing directly
viewed electronic indices; auxiliary optical aids have already been
described in Chap. 7.
13.11. Angle Indices.—Because of their simplicity, fixed angle
indices ruled on a transparency are widely used in spite of the inaccuracies
described above. The inherent data are usually far less accurate
than those of range. Furthermore, in many applications, such as that
of homing, the angle of interest changes slolvly if at all and there is time
for repeated observations. Parallax errors are made as small as possible
by placing the scale very close to the tube face, and in many cases by
ruling on both sides of a rather thick transparency in order to provide a
line of sight.
Movable angle indices of the same type are little used except in the
important case of the centered PPI, where the motion is one of simple
rotation. The index or “cursor” may consist of a thin metal strip
viewed edgewise or of a transparency with a ruled line or a thin slit
milled through it. It is supported by a ring bearing larger than the tube
face and is usually turned by means of a hand crank to which a data
transmitter may be attached if desired. Readings are made from a
circular scale at the edge of the tube.
Fixed Electronic Angle Indices.—An electronic angle index can be
provided by brightening the cathode-ray tube for a few sweeps so that a