Radar System Engineering

SEC. 63] SUMMARY OF INDICATOR TYPES 163
directly as a cartesian coordinate, or (3) as the basis for resolving a range
sweep in a particular direction. This will be further discussed in connection
with specific types of display.
Indices or Scale Markers.-Both mechanical and electrical means
are used for making geometrical meamrements on the displays.
In measuring range, “electronic” range markers are practically
always used. These consist of artificial video signals introduced into
the display by a precision timing circuit; thus, inaccuracies in the display
do not enter as sources of error in the measurements. Almost every
display entails a set of discrete, regularly spaced markers derived from
an oscillator properly phased with respect to the firing of the modulator.
Fairly accurate ranges can be read from these markers at a glance. In
many cases, however, interpolation errors are larger than can be tolerated;
the fixed markers are then supplemented by a manually controlled, continuously
movable, calibrated index. This index has the advantage of
extremely high precision, especially on an expanded sweep, but it requires
appreciable time in its use. Mechanical indices which move in front
of the CRT face are seldom used in measuring range.
For the determination of angle, it has been most usual in the past
to use mechanical indices: either a set of fixed indices engraved on a
transparent overlay, or a movable mechanical cursor. These can give
errors due to parallax and to inaccuracies in the display. More recently,
electronic indices have begun to replace mechanical range markers. A
discrete set of fixed indices, or a continuously variable index, may be
provided; for some techniques of display synthesis, the latter is not easy
to achieve.
The controls of movable markers, bo~h of range and of angle, are
often connected to devices providing remote data transmission.
6.3. Summary of Indicator Type s.-Many considerations enter into
the choice of the display geometry. In a three-dimensional problem, the
designer must decide how to divide the coordinates between two displays,
or how to present all the information on a single display, if this is feasible.
Even a two-dimensional display is complicated, and often involves conflicting
requirements, such as the need for high resolution in angle and
range ]vithout sacrifice in the total field of ~iew. In some cases, the
needs can best be met by deliberately deforming the picture; in others
it may be necessary to use more than one display, either alternately on a
single tube or simultaneously on different ones.
Many different display schemes have been invented to deal with
these problems. The following summary, which classifies them according
to the spatial geometry represented, includes the important geometries
actually used. Although many others are possible, they have had little
or no practical application to date.