The accuracy of target-range data provided by a radar varies with the use of the radar. For example, a weapons systems radar operating in a search mode is required to be accurate within a small percentage of its maximum range. However, an intercept radar, operating in a tracking mode, must supply range data that is even more accurate; it must be within a few yards of the actual range.
In some applications of radar, the indicator sweep is calibrated by a transparent overlay with an engraved range scale. This overlay enables the operator to estimate the range of targets. In other applications, electronic range marks are supplied to the indicator. They usually appear as vertical pulses on A-scopes and as concentric circles on ppi scopes. The distance between range marks is generally determined by the type of equipment and its mode of operation.
In a weapons systems radar that requires extremely accurate target-range data, a movable range marker may be used. The range marker is obtained from a range-marker generator and may be a movable range gate or range step. When a ppi scope is used, a range circle of adjustable diameter may be used to measure range accurately. In some cases, movement of the range marker is accomplished by adjusting a calibrated control from which range readings are obtained.
The following discussion describes the operation of three types of range-marker generators: the RANGE-GATE GENERATOR, the RANGE-MARKER GENERATOR, and the RANGE-STEP GENERATOR. The range-gate generator, used in conjunction with a blocking oscillator, generates a movable range gate. The range-marker generator and the range-step generator, used in conjunction with an astable multivibrator, generate fixed range marks and a movable range step, respectively.
Figure 3-11 shows a simplified block diagram of a typical range-gate generator. The pulse-repetition frequency is controlled by a master oscillator, or multivibrator, in which the output is coupled to a trigger thyratron (both in the synchronizer). The output of the trigger thyratron is used to trigger the radar modulator and the scope sweep circuits, thus starting the transmitter pulse and the range sweep at the same instant, referred to as time T0.
Figure 3-11. - Range-gate generator.
The PHANTASTRON in the sweep circuits is a variable timing circuit that supplies a sweep sawtooth to the sweep amplifier. The width of the gate and sawtooth is dependent upon the range selected by the radar operator.
The range-gate circuit receives its input pulse from the trigger thyratron and generates a delayed range-gate pulse. The delay of this pulse from time T0 is dependent on either the range of the target when the radar is tracking, or the manual positioning of the range-volts potentiometer when the radar is not tracking (in the search mode). The range-gate triggers the range-strobe multivibrator, from which the output is amplified and sent to the blocking oscillator (which sharpens the pulses), as shown in figure 3-11. This range gate is used to select the target to be tracked. When in the track mode, the range gate brightens the trace or brackets the blip (depending on the system) to indicate what target is being tracked. Range-gate generators are used most often in weapons-control track radar A-scope presentations, but they can also be used with ppi presentations. When used with a ppi presentation, the range gate must be movable in both range and bearing.
The range-gate generator can easily be modified to produce a range strobe instead of a range gate. A range strobe is simply a single brightened spot that is movable both in range and bearing. In operation, the range strobe or range gate control also controls a dial or digital readout to provide a range readout to the operator.
Several types of range-marker generators are in common use. Figure 3-12 shows a simplified version of a circuit that produces both range markers and the basic system timing triggers. The master oscillator in this case is a blocking oscillator that operates at a frequency of 80.86 kilohertz. By dividing 80.86 kilohertz into 1 (t = 1/frequency), we find the time required for one cycle of operation is 12.36 microseconds. Thus the blocking oscillator produces pulses 1 radar mile apart. These are fed to the 5:1 divider circuit. Five of the 1-mile marks are required to produce an output from the divider circuit. These five-mile marks are sent to the indicator for display and to the 10:1 divider circuit. In the latter case, ten of the five-mile marks are required to produce an output from the 10:1 divider. Thus the output triggers are 50 miles apart. These basic timing triggers are for a radar with a range of fifty miles. The period between triggers could be extended through the use of additional dividers for use with longer range systems.
Figure 3-12. - Range-marker generator.
Another version of a range-mark generator is shown in figure 3-13. This circuit provides range marks at 1,000-, 2,000-, or 3,000-yard intervals. Generation of the marks begins with the ringing oscillator, which is started by a delayed master trigger from the synchronizer. A ringing oscillator produces a sinusoidal output of a fixed duration and frequency when triggered. The output is synchronized to the input trigger. In this circuit, the trigger causes the oscillator to produce a 162-kilohertz signal that lasts for 4 1/2 cycles. The emitter follower isolates the ringing oscillator from the countdown multivibrator and clips the oscillator output signals. This action allows only the positive half of each sine wave to reach the multivibrator. The positive triggers from the ringing oscillator are at 1,000-yard intervals. This input signal results in an output from the countdown multivibrator of 1,000-, 2,000-, or 3,000-yard range marks, depending on the position of the RANGE MARK SELECT SWITCH.
Figure 3-13. - Range-marker generator.
The range step is often used to determine target range on an A-scope presentation. The appearance of a range step on an A-scope is illustrated in figure 3-14.
Figure 3-14. - Range-step presentation.
View A of figure 3-15 is a block diagram of a simple range-step generator consisting of a sawtooth generator, a negative clipper, a range potentiometer, and a limiting amplifier. The position of the range step along the indicator's time base is controlled by the range potentiometer. When the range step coincides with the leading edge of a target's echo pulse, the range can be read directly from a calibrated readout associated with the potentiometer.
Figure 3-15. - Range-step generation.
View B shows the time relationships of the voltage waveforms produced by the range-step generator. During the sweep gate, the sawtooth generator produces a sawtooth voltage that is sent to the clipper. The point at which the sawtooth is clipped is controlled by the range potentiometer. The clipped sawtooth is shaped in the limiting amplifier to produce the output voltage waveform. The portion of the output waveform from T1 to T3 is applied to the vertical-deflection plates of the indicator crt to produce the display shown in figure 3-14.
Q.7 What type of ranging circuit is most often used with a radar that requires
extremely accurate range data?