16
INTRODUCTION
[SEC.1.5
introduced to provide for interpolation and for a moving display, but
this method is usually cumbersome.
Parallax may be largely overcome
by methods of optical superposition (Chap. 16) but in their simplest
form these indices are likely to be confusing, and the more elegant ones
are bulky. Their principal usefulness is in connection with plotting and
with superposing maps or charts on the display.
Electronic markers, produced by modulating the electron beam,
reduce or eliminate most of these difficulties. They completely elimi-
nate parallax and, because they can be generated by precision means
that are independent of the sweeps, they automatically fall in their
proper place on the display regardless of its position, its scale factor,
or any deliberate or unintended distortion.
In general the methods of
producing interpolation indices are far less cumbersome than the mechani-
cal methods but in many cases, especially on slow scans, the intermittence
with which the display is “painted in”
hinders the process of setting the
index on the echo unless rather comp~lcated switching methods are intro-
duced to provide the markers at more frequent intervals.
Electronic indices are invariably used in range determinations. They
consist of sharp pulses, generated by a precision timing circuit, that are
introduced into the display along with the echo signals at the proper
times on each pulse cycle. Almost every display entails a set of discrete
markers derived from a precision oscillator that is properly phased with
respect to the transmission of the r-f pulse. Since the radar data are
inherently capable of providing very great accuracy in range determi-
nations, and since fixed markers must be rather few in number to avoid
confusion, an interpolating index is very often used. Fairly simple
circuits providing an amazing degree of precision have been developed
for thk purpose.
In the case of angle determinations, the fundamental data are not so
precise as in range determinations, and, in general, inaccuracies intro-
duced by the display are less.
For these reasons and in the interests of
simplicity it has been most usual in the past to use a set of fixed indices
engraved on a transparent overlay. In the particular case of a polar
display (PPI) concentric with the tube face, these indices are often
supplemented by a rotatable cursor. More recently, as the inherent
accuracy of the data and the accuracy requirements have increased,
electronic indices have been replacing mechanical angle markers.
These
can be produced most simply by brightening a few range sweep traces
by means of a signal generated when the scanner passes through the
position in question. Thk method is satisfactory for fixed, discrete
indices and for a variable index on
a fairly rapid scan.
On slower scans
the intermittence makes adjustment so slow that the switching methods
alluded to above must be used. These are more costly and in the case
of a mechanically rotating coil they cannot be used at all.