Radar System Engineering

t$Ec. 9,18] DATA STABILIZATION 311
let the antenna scan umi’ormly during search and to control the deflecting
coil of the indicator to correct the deck-tilt error. This is a species of
data stabilization; it is not commonly used. The other cure for the
deck-tilt error is to drive the azimuth sweep of the radar indicator at a
constant rate during search and to control the train angle of the antenna
by means of a computer and servomechanism. As we have seen, it is one
of the functions of the computer to provide the deck-tilt correction.
This computation is easily mechanized, because the deck-tilt error is
mathematically similar to the error in transmitting angular motion
through a universal joint.
The gyroscopes used in shipborne antenna stabilization are accurate
to within 2’ to 7’; the error seems to be a function of the roll, pitch, and
heading of the vessel. The large synchros used in this work are liable
to err by about 10’ to 30’. It follows that a good shipborne equipment,
using synchros geared as high as 36 to 1 for increased accuracy, holds the
beam with an average error of as little as 5’, although momentary errors
of more than twice this magnitude may al ise.
9.18. Data Stab~zation. ‘—It has been remarked that even after the
mechanical engineer has provided a stabilized antenna mount and thereby
ensured that the radar will correctly perform its scanning function despite
rolling or pitching of the vehicle, the indicator may still display the position
of targets falsely. We are concerned in this section with the problem
of improving the indicator in this respect.
The older PPI radars, both airborne and shipborne, were designed to
display targets at their bearings relative to the heading of the vehicle.
More recent design allows a display of targets in their true bearings, that
is, with north at the top of the screen regardless of heading. The relative-bearing
indication is bad for two reasons: (1) the entire display
rotates and becomes confused if the course of the vehicle is altered; (2)
the natural random changes in the heading of the vehicle cause a corresponding
blurring of the display on the persistent screen. The truebearing
indication is free of these faults. It is a data stabilization in
regard to yaw and changes of heading.
Yaw stabilization is implemented with the aid of a horizontal gyroscope
which is kept pointing north by manual or automatic reference to
the earth’s magnetic field. Through a mechanical or electrical differential
there is provided at the indicator a voltage signifying the true bearing
of the beam, that is, the difference between the relative bearing of the
beam and the relative bearing of the true north.
Deck-tilt error is serious in airborne as well as in shipborne radar.
Let us imagine an airplane equipped with a bombing radar having a fan
beam; the base of the scanner is not stabilized, and the antenna may or
1Sees. 9.1S-9.20 by W. M. Cady.