AP 3302 Pt. 3
Section 1
CHAPTER 3
Factors affecting the performance of Pulse-Modulated Radars
In
early-warning search radar, echoes produced by anything other than aircraft
or missiles are unwanted. We are then faced with the problem of removing the
'clutter' on the c.r.t. display due to these unwanted echoes (Fig 5). Such clutter
may be so pronounced, especially from objects near the ground radar, as to completely
hide the wanted echoes. Fortunately modern radar contains circuits which can
differentiate between moving objects and stationary objects. By using moving
target indication radars (MTI) the appearance of stationary objects on the display
may be suppressed so that the majority of the clutter is removed. Such systems
will be considered in more detail later.
Dimensions of Targets
All pulse-modulated primary radars depend for their success upon the reflection of e.m. energy from objects which have been illuminated' by a radar beam. All objects in the path of the beam will reflect energy to some extent. The amount reflected depends upon the material of which the object is made, the shape of the object and its size (Fig 6).
If
we have two identical objects at different distances from the radar the one
nearer the radar reflects more energy.
A metal object will reflect more energy than an object of the same size and shape made of wood or plastic. The better the conductor the greater is the reflection.
The shape of the object will deter-mine how the energy is reflected. If the object has a flat side facing the radar transmitter it will reflect more of the energy back towards the radar than an object of any other shape.
Large objects will reflect more energy than small objects of the same material and shape at the same distance from the transmitter. The object however must be greater than a certain minimum size, in terms of wavelength of the radiated energy, to produce a reasonable reflection of energy. Generally targets must have a size greater than about a quarter of the radar wavelength being used before a detectable echo is received. Thus for the detection of small objects the radar wavelength must also be small, i.e. the frequency must be very high. This is one reason for the use of high frequencies in radar.
