CHAPTER 7. TARGET TRACKING
2. RANGE TRACKING
In most TTR applications, the target is continuously tracked in range, azimuth, and elevation. Range tracking can be accomplished by an operator who watches an “A” scope presentation and manually positions a handwheel to maintain a marker over the desired target return. The setting of the handwheel is a measure of target range and is converted to a voltage used by the fire control computer.
As target speeds and maneuvers increase, the operator may have extreme difficulty maintaining manual target range tracking. To avoid this situation, most TTRs employ an automatic range tracking loop. All pulse TTRs, which includes conical scan, track-while-scan, monopulse, and pulse Doppler radars, employ either a split gate or leading-edge automatic range tracking system. In a TTR, automatic range tracking serves two essential functions: (1) it provides the critical value of target range, and (2) it provides a target acceptance range gate that excludes clutter and interference from other returns. Since radar range is normally the first target discriminator used to initiate automatic target tracking, the second function is essential to the proper operation of the other tracking loops.
a. A range gate circuit is simply an electronic switch that is turned on for a period of time after a pulse has been transmitted. The time delay for switch activation corresponds to a specific range. Any target return that appears inside this range gate is automatically tracked. The most common type of automatic range tracking is accomplished by a split-gate tracker. Two range gates are generated as shown in Figure 7-1. The automatic range tracking loop attempts to keep the amount of energy from the target return in the early gate and late gate equal. The range tracking error is computed by subtracting the output of the late gate from the output of the early gate. The amount of the range tracking error signal is the difference between the center of the pulse and the center of the range gate. The sign of the error signal determines the direction in which the gates must be repositioned to continue to track the target.
Figure 7-1. Split-Gate Range Tracker
b. Leading-edge range tracking is an electronic protection (EP) technique used to defeat range-gate-pull-off (RGPO) jamming. Figure 7-2 illustrates the application of leading-edge tracking. The leading-edge tracker obtains all range data from the leading edge of the target return. All RGPO cover pulse jamming tends to lag the target return by some increment of time (see (a) in Figure 7-2). By differentiating the entire return with respect to time, the target return can be separated from the jamming pulse (see (b) in Figure 7-2). Employing a split-gate tracker electronically positioned at the initial pan, or leading edge, of the returning pulse, the range tracking loop can track the target return and ignore any jamming signals. The range tracking loop then uses split-gate tracking logic to determine the magnitude and direction of range tracking errors and reposition the range gate.
Figure 7-2. Leading-Edge Range Tracker
c. The width of the tracking gate is an important radar design consideration.
The range gate should be sufficiently narrow to effectively isolate the target from other returns at different ranges. It should be wide enough to allow sufficient energy from the target echo to be displayed. The width of the range tracking gate is normally equal to the pulse width of the radar.
d. Nearly all range tracking gates employ some form of automatic gain control (AGC). AGC is designed to limit target clutter and glint. It is also designed to avoid excessive false alarms.
3. CONICAL SCAN
A conical scan tracking system is a special form of sequential lobing. Sequential lobing implies that the radar antenna beam is sequentially moved between beam positions around the target to develop angle-error data. For a conical scan radar to generate azimuth and elevation tracking data, the beam must be switched between at least four beam positions as shown in Figure 7-3.
Figure 7-3. Conical Scan Positions
a. One of the simplest conical scan antennas is a parabola with an offset rear feed that rotates, or nutates, to maintain the plane of signal polarization. The radar beam is rotated at a fixed frequency around the target. The angle between the axis of rotation (normally the axis of the antenna) and the axis of the antenna beam is called the squint angle.
b. A conical scan radar first tracks the target aircraft in range. For azimuth and elevation tracking, the target return is modulated at a frequency equal to the rotation frequency of the radar beam. This results in a target signal output that resembles a sine wave (Figure 7-4). The azimuth and elevation tracking loops
drive servo motors to position the antenna to keep the energy level in each of the four positions equal. The amount of the modulated signal determines how far the target is off the antenna boresight while the phase of the modulation (positive or negative) determines the direction.
Figure 7-4. Conical Scan Modulation
c. In Figure 7-5, most of the target energy is in position 1, with a small amount of energy in position 4. The output from the elevation tracking loop is positive and drives the antenna servos upward. The output from the azimuth tracking loop moves the antenna to the right.
Figure 7-5. Conical Scan Tracking Errors
d. Once a balance of target energy in each scan position is achieved, the target is in the central tracking area (Figure 7-6). The azimuth and elevation tracking circuits continue to drive the antenna servos to maintain this energy balance which keeps the radar beam on the target.
Figure 7-6. Conical Scan Tracking
e. The primary advantage of a conical scan radar is the small beamwidth which provides extremely accurate target tracking information. The primary disadvantages of conical scan include the following: (1) the narrow beamwidth makes target acquisition a problem. Even using a Palmer-helical scan, it may take considerable time to find and initiate track on a target; (2) conical scan radars are vulnerable to inverse gain modulation jamming based on the scanning frequency of the rotating beam; (3) a conical scan radar must analyze many radar return pulses to generate a tracking solution.