The purpose of a tracking system is to determine the location or direction of a target on a near-continuous basis. An ideal tracking system would maintain contact and constantly update the target’s bearing (azimuth), range and elevation. The output of the tracking system can be sent to the receiving system, which stores the information and derives the target’s motion and therefore its future position. As we shall see, tracking systems not only provide an automatic target following feature but also determine the target’s position with sufficient accuracy for weapons delivery.
Radar Servo Tracking System One of the most basic tracking system designs is the servo tracking system. Here, the radar antenna is initially directed on a target after which it automatically remains pointed at the target as it follows its motion. Furthermore, the system provides continuous position information to the operator. The antenna is rotated by a motor which provides a negative position feedback signal to a controller. This subsystem is known as a servo-mechanism. Fig. 1 shows a basic servo-mechanism system used in radar system.
Figure 1. Servo mechanism system Operation servo-mechanism system: (i)The commanded input signal is the desired azimuth of the antenna. (ii)The error signal drives the motor to reposition the antenna until the position feedback indicates the antenna is at the desired azimuth, at which point the error signal is zero and the motor stops. (iii)This servo-mechanism can be combined with a tracker, which determines the azimuth at the target, which the system now uses as the input. Figure 2. Servo tracking mechanism. Operation of radar servo-tracking system: i)This is the same servo-mechanism block diagram, but its input comes from the tracker. The combination is called a radar servo-tracking system. (ii)The heart of this system is the tracker, which takes the return signal and position information and determines the location of the target. (iii)There are several ways to do this, with varying degrees of complexity and accuracy. Let us consider the following data which might be obtained as a circular sweep radar panned across a target. Figure 3. Return strength as beam sweeps over target. The individual returns are shown as the vertical lines.
As the beam sweeps across the target, there are eight returns generated, whose strength increases until the target is centered in the beam and then falls off. The target location can be determined from the point of maximum return strength. However, the region of maximum return is broad and therefore the exact location of the target cannot be determined with high accuracy. Certainly, one could locate the target within some fraction of the beamwidth, but using this system the tracking accuracy would not be much better than about one-quarter of the beamwidth.