Exercise 3-3. Multiple-Source Jamming Techniques EXERCISE OBJECTIVE

Transcription

1 Exercise 3-3 Multiple-Source Jamming Techniques EXERCISE OBJECTIVE To introduce multiple-source jamming techniques. To differentiate between incoherent multiple-source jamming (cooperative jamming), and coherent multiplesource jamming. To demonstrate two types of cooperative jamming: blinking jamming, and formation jamming. DISCUSSION Multiple-source jamming techniques can greatly increase a jammer s effectiveness against a tracking radar. These techniques involve the use of more than one jamming source (a source can be a jammer, a decoy, or as simple as a reflector) transmitting toward the radar. They induce an artificial glint onto their combined signal or that of a radar echo, effectively changing the apparent angle-of-arrival of the signal received by the radar. Multiple-source jamming is effective only if the jamming sources intercept the main beam(s) of the tracking radar antenna. Otherwise, if some sources are located outside of the main beam(s), the jamming signal becomes a beacon for the radar. Multiple-source jamming techniques exploit a fundamental weakness found in monopulse radar, and more specifically, in all angle-tracking radar including sequential lobing and conical scan radar. An angle-tracking radar cannot protect itself from artificial glint by simply correcting for a weakness in the radar s design, as is true with jamming techniques such as cross-polarization jamming. Phase Relationship between Jamming Sources The phase characteristics of the jamming signals transmitted by the platforms in a group of spatially dispersed jammers can be categorized as either coherently related, or incoherently related. Multiple-source jamming techniques are more effective when conducted coherently. When multiple-source coherent jamming is performed, a constant relationship over time exists between the phases of the jamming signals. In the case of incoherent multiple-source jamming, there is a time-varying relationship between the phases of the multiple sources. When performed coherently, it is possible for multiple source jamming to make the apparent angle-of-arrival of the signal from the jamming sources to lie in a direction outside of the solid angle in which they are contained. This is done by appropriately adjusting the phase relationship between the jamming signals. This implies that coherent multiple-source jamming can be performed with the jamming sources contained within a relatively small solid angle. For example, an airplane with a repeater jammer located on each wing tip. By adjusting the delay in one of the repeaters, the apparent angle-of-arrival of the combined signal can be made to appear as if it were originating outside of the solid angle formed by the repeaters. The coherence between the signals of the different sources is much easier to maintain when the two sources are located on a single platform. When the 3-49

2 sources are spatially distributed a common reference signal must be used to maintain coherence. The implementation of incoherent multiple-source jamming (also known as cooperative jamming) can take a different form completely to that of the coherent version. Whereas it is easier for the coherent technique to be implemented from onboard a single platform, for the incoherent technique it is often easier for its implementation to be carried out by multiple platforms. The essential reason that incoherent multiple-source jamming is performed by more than one platform is that it is impossible to make the apparent angle-of-arrival of the jamming sources lie in a direction outside of the solid angle in which they are contained. That is, the apparent angle-of-arrival of the combined jamming signal will always lie within the solid angle containing the jammers. Therefore, to create a large angular tracking error, incoherent multiple-source jamming must be implemented within the widest possible solid angle. As stated previously, however, the sources cannot be located outside of the radar antenna main beam(s) otherwise the jammer s signal becomes a beacon. Two of the most common examples of incoherent multiple-source jamming (cooperative jamming) techniques are blinking jamming and formation jamming. In blinking jamming, which is illustrated in Figure 3-19, a noise or repeater jamming signal is cooperatively transmitted toward a radar antenna. The signal is turned on and then off, one at a time, by several closely grouped platforms. Formation jamming is implemented by positioning two or more closely spaced platforms that are transmitting a false-target jamming signal toward a tracking radar. It is important to note that both blinking and formation jamming can be implemented coherently. However, because the techniques involve multiple independent platforms it is often difficult to do so. Figure Blinking jamming. Blinking Jamming The effectiveness of blinking jamming against a specific radar depends entirely on the on-off commutation rate (the blinking rate) between the jammers. The ideal situation for a group of jamming platforms, i.e., the situation that is the most effective at deceiving the radar s angle tracking, is when the blinking rate is on the order of the bandwidth of the radar s angle tracking servomechanism. At this rate, typically between 0.1 Hz and 10 Hz, significant amplitude perturbations are induced into the 3-50

3 radar s angle error signal causing the radar antenna to oscillate in angle erratically. If too low a blinking rate is used, then the radar antenna has time to settle on each of the jammer s angular positions, an undesired situation from the point of view of the jammers. In this case, each jamming signal acts as a beacon signaling to the radar the angular positions of each jammer. If too high a blinking rate is used, then the radar angle tracking servomechanism tends to average the angular perturbations produced by the jammers, minimizing the possible tracking error (antenna oscillation). A radar able to change its angle tracking servo bandwidth, using a bandwidth limiter circuit, can render blinking jamming ineffective. Formation Jamming Recall that formation jamming was stated as taking the form of multiple closely spaced platforms transmitting a false-target jamming signal. The parameters that are involved in producing the angular error created by formation jamming are more apparent than those responsible for creating blinking jamming s angular deception. Consider Figure 3-20 which shows two platforms conducting formation jamming against a tracking radar. The important parameters related to formation jamming can be characterized as follows: I. The relative amplitude of each jammer s false-target signal. II. III. IV. The angle ( ) between the radar antenna s pointing axis and a perpendicular to the midpoint of the line between the jammers. The distance (L) between the jammer s. The range (R) between the tracking radar and the midpoint of the line between the jammer s. Figure Formation jamming geometry. The jamming signals are combined with each other and the radar echoes from the sources, before being received by the radar. The superposition of the phase characteristics of these signals before radar reception is what induces artificial glint onto the signal being tracked by the radar. As the radar s looking angle to the jammers changes, and as the various signal strengths (from one jammer, from another, from the radar) change, then the apparent direction of arrival of the 3-51

4 composite signal experiences a displacement. Thus, effective formation jamming causes the angle-of-arrival of the composite false-target signal received by the radar to wander back and forth between the jammers. Procedure Summary During the first part of the exercise, you will set up and calibrate the Tracking Radar. You will also position the Target Positioning System with respect to the Tracking Radar. During the second part of the exercise, you will set up the Radar Jamming Pod Trainer to be able to demonstrate blinking noise jamming (a form of incoherent cooperative jamming) against the Tracking Radar. By conducting blinking noise jamming against the Tracking Radar, you will approximate the value of the angle tracking servomechanism s bandwidth. You will observe the relationship between the radar antenna s angular tracking error (erratic oscillations) and the commutation rate of the blinking jamming signal. During the third part of the exercise, you will set up the Radar Jamming Pod Trainer to be able to conduct coherent formation jamming against the Tracking Radar. PROCEDURE Setting Up the Tracking Radar G 1. Before beginning this exercise, the main elements of the Tracking Radar Training System (i.e., the antenna and its pedestal, the target table, the RTM and its power supply, the training modules, and the host computer) must be set up as shown in Appendix A. On the Radar Transmitter, make sure that the RF POWER switch is set to the STANDBY position. On the Antenna Controller, make sure that the MANual ANTENNA ROTATION MODE push button is depressed and the SPEED control is set to the 0 position. Turn on all modules and make sure the POWER ON LED's are lit. G 2. Turn on the host computer, start the LVRTS software, select Tracking Radar, and click OK. This begins a new session with all settings set to their default values and with all faults deactivated. If the software is already running, click Exit in the File menu and then restart the LVRTS software to begin a new session. G 3. Connect the modules as shown on the Tracking Radar tab of the LVRTS software. For details of connections to the Reconfigurable Training Module, refer to the RTM Connections tab of the software. 3-52

5 Note: Make the connections to the Analog/Digital Output Interface (plug-in module 9632) only if you wish to connect a conventional radar PPI display to the system or obtain an O-scope display on a conventional oscilloscope. Note: The SYNC. TRIGGER INPUT of the Dual-Channel Sampler and the PULSE GENERATOR TRIGGER INPUT of the Radar Transmitter must be connected directly to OUTPUT B of the Radar Synchronizer without passing through BNC T-connectors. Connect the hand control to a USB port of the host computer. G 4. Make the following settings: On the Radar Transmitter RF OSCILLATOR FREQUENCY CAL. PULSE GENERATOR PULSE WIDTH... 1 ns On the Radar Synchronizer / Antenna Controller PRF Hz PRF MODE SINGLE ANTENNA ROTATION MODE... PRF LOCK. DISPLAY MODE POSITION On the Dual-Channel Sampler RANGE SPAN m In the LVRTS software System Settings: Log./Lin. Mode Lin. Gain as required Radar Display Settings: Range m G 5. Connect the cable of the target table to the connector located on the rear panel of the Target Controller. Make sure that the surface of the target table is free of any objects and then set its POWER switch to the I (on) position. Place the target table so that its grid is located approximately 1.2 m from the Rotating-Antenna Pedestal, as shown in Figure Make sure that the metal rail of the target table is correctly aligned with the shaft of the Rotating-Antenna Pedestal. 3-53

6 Figure Position of the Rotating-Antenna Pedestal and target table. G 6. Calibrate the Tracking Radar Training System according to the instructions in Appendix B. Set the RF POWER switch on the Radar Transmitter to the STANDBY position. Make sure that the Tracking Radar is adjusted as follows: Operating Frequency GHz Pulse-Repetition Frequency single, 288 Hz Pulse Width ns Observation Range m Blinking Jamming G 7. Remove the semi-cylinder target, used for the Tracking Radar calibration, from the target table mast. Turn off the target table. Move the metal rail to either end of the target table. The metal rail will not be used during the exercise. Place the Radar Jamming Pod Trainer support (part number ), provided with the Connection Leads and Accessories, onto the target table. Position it so that it is in the center of the target table grid. Place a fixed target mast (part number 30409) onto the target table. Position the mast at the target table grid coordinates X = 35 cm, Y = 26 cm. 3-54

7 G 8. Install the Radar Jamming Pod Trainer onto its support (in the horizontal position) using the long support shaft (part number ). Align the Radar Jamming Pod Trainer so that its horn antennas are facing the Tracking Radar antenna and aligned with the shaft of the Rotating- Antenna Pedestal. The longitudinal axis of the Radar Jamming Pod Trainer should be aligned with the shaft of the Rotating-Antenna Pedestal. Rotate the infrared receiver on the Radar Jamming Pod Trainer toward the direction from which you will use the remote controller. Install the Power Supply (Model 9609) of the Radar Jamming Pod Trainer on the shelf located under the surface of the target table. Connect the Power Supply line cord to a wall outlet. Connect the power cable of the Radar Jamming Pod Trainer to the multi-pin connector located on top of the Power Supply. G 9. Assemble a small horn antenna (part number 9535), a waveguide-to-sma coaxial adaptor (part number ), and a support pin (part number ) as shown in Figure Note that two quick-lock fasteners (part number 28132) are required to join together the small horn antenna and the waveguide-to-sma coaxial adaptor. Install the horn antenna assembly on the fixed mast located on the target table. Figure Horn antenna assembly. G 10. Remove the 50- load connected to the Radar Jamming Pod Trainer COMPLEMENTARY RF OUTPUT. Using a medium-length ( 75 cm) SMA cable (part number ), make a connection between the RF input of the horn antenna assembly installed on the fixed mast, and the COMPLEMENTARY RF OUTPUT of the Radar Jamming Pod Trainer. 3-55

8 Orient the pointing direction of the horn antenna assembly toward the shaft of the Rotating-Antenna Pedestal. G 11. On the Radar Transmitter, depress the RF POWER push button. The RF POWER ON LED should start to flash on and off. This indicates that RF power is being radiated by the Dual Feed Parabolic Antenna. In LVRTS, turn off the AGC of the Radar Target Tracker. Turn on the Power Supply of the Radar Jamming Pod Trainer. Turn the Radar Jamming Pod Trainer on. Note that the Radar Jamming Pod Trainer status indicates that the Repeater is on. Adjust the remote controller settings to match the Radar Jamming Pod Trainer status (the Repeater is on, all else is off). G 12. Make sure the radar antenna axis is aligned with the Radar Jamming Pod Trainer. This can be done by observing the O-Scope Display of the Tracking Radar while adjusting the radar antenna orientation so that the amplitude of the Radar Jamming Pod Trainer's repeated echo signal is the same for both positions of the antenna main beam. G 13. Observing the O-Scope Display, set the Gain of the MTI Processor so that the amplitude of the Radar Jamming Pod Trainer's repeated echo signal is approximately 0.20 V. Note the range of the repeated echo signal, as read-off from the O-Scope Display. G 14. Using the remote controller, turn the Radar Jamming Pod Trainer Repeater off. G 15. Retract the Radar Jamming Pod Trainer's target positioning arm and place the large (20 x 20 cm) metal plate target at its tip. Orient the metal plate target so that it squarely faces the Tracking Radar antenna. The target should be perpendicular to the longitudinal axis of the Radar Jamming Pod Trainer. Using the target positioning arm while observing the O-Scope Display, adjust the distance of the metal plate target so that the range of its echo signal matches the range of the repeated echo signal you noted previously (refer to Figure 3-23). If necessary, slightly readjust the orientation of the metal plate target so that the amplitude of its echo signal on the O-Scope Display is approximately the same for both positions of the radar antenna main beam. 3-56

9 Figure Adjusting the target positioning arm of the Radar Jamming Pod Trainer. G 16. Observing the O-Scope Display, set the Gain of the MTI Processor so that the amplitude of the metal plate target echo (Radar Jamming Pod Trainer's natural radar echo) is approximately 0.2 V. Lock the Tracking Radar onto the Radar Jamming Pod Trainer's natural radar echo. G 17. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Triangle Attenuation db Attenuation db AM/Blinking On Modulation Frequency Hz Modulation Internal Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now generating a blinking noise jamming signal that is transmitted by the Radar Jamming Pod Trainer horn antenna (jamming source 1) and by the horn antenna assembly (jamming source 2). The noise jamming signal is transmitted by one source at a time. The commutation rate between the two sources is at a frequency of 5.0 Hz. At this value, the blinking rate is greater than the radar angle tracking servo bandwidth. 3-57

10 G 18. Using the remote controller, and while observing the O-Scope Display, slowly decrease the Radar Jamming Pod Trainer Noise Attenuation level until the amplitude of the noise spikes induced in the video signal is as high as possible, without causing the Tracking Radar to unlock. Once the Noise Attenuation level has been adjusted properly, the amplitude of the noise spikes should exceed that of the Radar Jamming Pod Trainer's natural radar echo. Observe that the blinking jamming has almost no effect on the angular position of the radar antenna. You may note, however, the presence of a slight angular offset when compared to the previous angular position of the radar antenna. Furthermore, some very slight angular pertubations may occur due to the high level of noise induced in the radar receiver. Briefly explain why the blinking jamming does not induce significant angular tracking pertubations. G 19. Using the remote controller, decrease the Blinking jamming frequency of the Radar Jamming Pod Trainer in steps until it is equal to 0.25 Hz. While decreasing the blinking rate, observe the effects that the blinking jamming signal has on the radar antenna s angular tracking. Note: If the Tracking Radar unlocks when decreasing the blinking jamming frequency, slightly increase the Noise Attenuation level to decrease the amount of noise induced in the radar receiver, temporarily turn the blinking noise jamming off, lock the Tracking Radar onto the Radar Jamming Pod Trainer's natural radar echo, turn the blinking noise jamming on again, and repeat step 19. At what blinking jamming frequencies are the largest angular tracking pertubations (radar antenna oscillations) induced in the radar? What can be said about the value of the angle tracking servomechanism bandwidth? 3-58

11 Describe the antenna s angular tracking when the blinking rate is significantly less than the frequencies you recorded above. G 20. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise Off AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off Unlock the Tracking Radar. Formation Jamming G 21. Remove the large metal plate target from the Radar Jamming Pod Trainer target positioning arm. G 22. Remove the connection between the horn antenna assembly and the Radar Jamming Pod Trainer. Move the fixed target mast located on the target table to grid coordinates X = 36 cm, Y = 5 cm. Move the Radar Jamming Pod Trainer support to target table grid coordinates X = 45 cm, Y = 35 cm. Align the Radar Jamming Pod Trainer horn antennas with the shaft of the Rotating-Antenna Pedestal. The longitudinal axis of the Radar Jamming Pod Trainer should be aligned with the shaft of the Rotating-Antenna Pedestal. Place a second fixed target mast (part number 30409) onto the target table. Position the mast at the target table grid coordinates X = 48 cm, Y = 5 cm. G 23. Build a second horn antenna assembly. If necessary, refer to step 9 of this exercise when making the assembly. Install the second horn antenna assembly onto the second fixed mast located at target table coordinates X = 48 cm, Y = 5 cm. This horn antenna assembly should be located in front of the transmitting horn antenna of the Radar Jamming Pod Trainer. The Radar Jamming Pod Trainer and the two horn antenna assemblies should be placed as shown in Figure

12 Figure Position of the Radar Jamming Pod Trainer and the two horn antenna assemblies on the target table grid. G 24. Connect the two RF outputs of the power divider assembly to the RF inputs of the two horn antenna assemblies. If necessary, refer to Figure Orient the pointing direction of both horn antenna assemblies toward the shaft of the Rotating-Antenna Pedestal. G 25. Connect the COMPLEMENTARY RF OUTPUT of the Radar Jamming Pod Trainer to the RF input of the power divider assembly (part number 33213) supplied with the Connection Leads and Accessories. If necessary, refer to Figure G 26. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer settings: Noise Off AM/Blinking On Modulation Frequency Hz Modulation External Repeater On RGPO Off False Targets (FT) Off The formation jamming setup described by procedure steps 21 to 25, closely spaces two transmitting antennas (the analog of two jamming sources) within the main beam of the Tracking Radar. 3-60

13 By enabling the Radar Jamming Pod Trainer Repeater and by transmitting the repeated echo signal through the COMPLEMENTARY RF OUTPUT, the transmitting antennas act as two repeater jammers producing a false target signal in a coherent formation jamming position. G 27. Make sure the radar antenna axis is aligned with the direction between the two horn antenna assemblies, i.e., with the receiving horn antenna of the Radar Jamming Pod Trainer. Observe the combined repeated echo signal on the O-Scope Display. Readjust the Gain of the MTI Processor so that the amplitude of the combined repeated echo signal, which may not be the same for both positions of the radar antenna main beam, is approximately 0.25 V. Note: The Radar Jamming Pod Trainer repeated echo signal (combined repeated echo signal) appears at a range greater than normal because of the additional propagation delay due to the power divider assembly. Slightly vary the range and orientation of either one of the horn antenna assemblies while observing the combined repeated echo signal on the O-Scope Display. Describe what happens. Briefly explain why. What does this imply about the apparent angle of arrival of the combined repeated echo signal, as perceived by the Tracking Radar? G 28. Observing the O-Scope Display, slightly vary the range and orientation of the horn antenna assemblies in an attempt to equalize the amplitude of the combined repeated echo signal for both positions of the radar antenna main beam, and maximize the amplitude of the echo signal. Observing the O-Scope Display, set the Gain of the MTI Processor so that the amplitude of the combined repeated echo signal is between 0.4 and 0.5 V. Figure 3-25 is example of what you might observe on the O-Scope Display. 3-61

14 Figure Combined repeated echo signal observed on the O-Scope Display. G 29. Make sure the DISPLAY MODE on the Antenna Controller is set to POSITION. This setting will permit you to quantitatively verify the extent of any jamming induced angle tracking errors. Lock the Tracking Radar onto the combined repeated echo signal coming from the Radar Jamming Pod Trainer. The radar antenna should remain aligned with a point located somewhere between the two horn antenna assemblies. While observing the radar antenna and the POSITION DISPLAY on the Antenna Controller, slightly vary the range and orientation of the horn antenna assemblies. Note: It may take some time before you will be successful in inducing an effect on angular tracking. Try various combinations of range and orientation of the horn antenna assemblies. On the other hand, the radar antenna may start to oscillate when you vary the range and orientation of the horn antenna assemblies. If this occurs, slightly vary the range of one of the horn antenna assemblies until the antenna oscillation (hunting) ceases. 3-62

15 Describe what occurs to the radar antenna s angular tracking as the position of either one of the horn antenna assemblies (jamming sources) is varied. Briefly explain why. G 30. Turn off the Tracking Radar and the Radar Jamming Pod Trainer. Disconnect all cables and remove all accessories. CONCLUSION In this exercise, you demonstrated that noise blinking jamming, when conducted against the Tracking Radar, induces angular tracking pertubations (radar antenna oscillations). You saw that the amplitude of the radar antenna oscillations depends on the commutation frequency (blinking rate) used by the multiple jammers performing the blinking jamming. You showed that the amplitude of the induced radar antenna oscillations is maximized when the blinking rate is on the order of the bandwidth of the Tracking Radar's angular tracking servomechanism. You conducted coherent formation jamming against the Tracking Radar by transmitting the Radar Jamming Pod Trainer repeated echo signal through two closely spaced horn antennas (situation similar to two jamming sources located within the main beam of a tracking radar antenna). You saw that when the position of either one of the transmitting antennas is modified, the apparent angle of arrival of the combined repeated echo signal, as perceived by the Tracking Radar, fluctuates. You demonstrated that when the Tracking Radar is locked onto the combined repeated echo signal, the radar antenna wanders between the angular positions of the two antennas transmitting the repeated echo signal. REVIEW QUESTIONS 1. What is the difference between formation and blinking jamming? 3-63

16 2. Briefly explain how multiple-source jamming techniques are conducted, and how they affect a radar s angular tracking. 3. What are the differences between the coherent implementation of a multiplesource jamming technique and its incoherent implementation? 4. Briefly explain the effects of blinking jamming on a tracking radar when the blinking rate is equal, more than, and less than the bandwidth of the radar s angle tracking servomechanism. 5. Briefly describe how formation jamming induces artificial glint onto the signal being tracked by the radar. 3-64