Radar System Engineering

188 THE GATHERING AND PRESENTATION OF RADAR DATA [SEC.612
back-t~back on a type L indicator, or by using integrating methods of
signal comparison and error display.
A simple means for moving the beam in elevation angle is to rotate
the beam rapidly about an axis which makes a small angle with the center
line of the beam; this is called “conical scanning.” It will be seen that a
conical scan permits the simultaneous determination not only of the
elevation error in pointing the axis of the scan at the target, but also of
the azimuth error in such pointing. Full consideration of this method
for angular tracking of a single target is deferred to Sec. 6.14, but it will
be useful here to describe a few principal features of a radar that employs
conical scan solely for the purpose of height-finding.
This principle was used in the British CMH, a mobile 10-cm equipment
designed specifically for height-finding. It used a 6-ft dish, a
power of 500 kw, and was capable of measuring heights to within ~ 500 ft.
The aircraft to be measured was followed manually in range with the
aid of an electronic marker on the range trace. Two other operators followed
manually in azimuth and elevation. The range and elevation
settings were used to operate an automatic height indicator which could
be placed in a remote position adjacent, to a PPI fed by an auxiliary search
radar.
Only a few of these equipments were built owing to progress in the
development of a centimetric scanning system, and to the inherent inability
of the CM H to find height on more than one target at a time. The
SCR-615 was a similar U.S. equipment, except that it was designed as a
fixed installation and employed an 8-ft dish.
The SM equipment used by the U.S. Navy was perhaps the most
widely used of allied radar sets in this category. It was employed in
conjunction with suitable PPI radar for ship control of aircraft. Its
wavelength is 10.7 cm, and it employs an antenna reflector 8 ft in diameter.
The waveguide feed used is a little off the axis of the paraboloid,
so that the beam, whose width is 3°, is off axis by 1°. The feed is rotated
to produce the conical scan. Pulses from the upper half of the antenna
rotation are compared with those from the lower half by means of integrating
circuits, and the output difference is used to displace the spot of an
error indicator (Sec. 6.8). A PPI is provided for general reporting, and
two range scopes, one covering the full range of the set and one an
expanded A-scope, make it possible to determine target range and to
eliminate from the signal-comparison circuits all echoes except those from
the target whose altitude is being determined.
In operation, either a helical scan (Sec. 9.7) or a continuous rotation in
azimuth with manually controlled elevation can be used in searching for
targets. The helical scan can be set to cover any 12° in elevation angle
in the range from 3° below to 75° above the horizon. When it is desired