Deceptive Jamming Using Amplitude-Modulated Signals

Transcription

1 Exercise 3-1 Deceptive Jamming Using Amplitude-Modulated Signals EXERCISE OBJECTIVE To demonstrate the effect of AM noise and repeater inverse gain jamming, two angular deceptive EA used against sequential lobing radars. To outline the reasons why radar PRF agility is an effective EP against inverse gain jammers. DISCUSSION Introduction As stated in this Unit's Discussion of Fundamentals, conical scan and sequential lobing radars, unlike monopulse radars, are vulnerable to amplitude-modulated jamming signals. These radars operate by moving their antenna beam to induce an amplitude modulation onto the target echo signal for the purpose of angular tracking. When the radar antenna is pointed directly toward the tracked target, the amplitude of the target echo signal is constant over time. Non-radar induced amplitude variations onto the target echo signal, due to scintillation or a jamming signal, are a cause of error and uncertainty in the target s angular position as seen by the radar. Inverse Gain Jamming Inverse gain jamming, an angular DECM, consists in directing either a false target signal (repeater inverse gain jamming) or a noise jamming signal (AM noise jamming) toward the radar antenna with an amplitude modulation in phase opposition to that generated by the scanning (or lobing) radar beam. The transmitted amplitude modulation can be implemented either as a direct inverse of the radar s amplitude modulation, or as on-off modulation, whereby the "on" period is made to coincide with the interval when the radar-signal amplitude is minimum. Either of these types of amplitude modulation can cause an apparent angular error in the radar s tracking system. This apparent tracking error is opposite in sign to the actual tracking error needed to correct for the radar antenna s angular position, as is illustrated in Figure 3-2 (a). In this figure, erroneous angular tracking information is introduced into the radar angle tracking loop, because the amplitude of the target echo signal, received through one lobe, has been made greater by the presence of the jamming signal. 3-5

2 Figure 3-2. Inverse gain jamming against a sequential lobing radar. 3-6

3 Synchronicity For inverse gain jamming to be effective, the frequency at which the jamming signal is amplitude modulated must be near the victim radar s lobing (or scan) rate. Inverse gain jamming is effective when the jamming signal is synchronized with the radar antenna s lobing (or scan) rate. However, it can also be effective in an asynchronous (sweeping) mode which searches for the most suitable amplitude modulation frequency that perturbs the radar angle tracking loop. Figure 3-2 (a) shows repeater inverse gain jamming synchronized to the radar lobing rate. Figure 3-2 (b) shows AM noise jamming transmitted asynchronously to the radar lobing rate. When transmitted synchronously, the jammer s amplitude modulation frequency is set equal to the radar's lobing (or scan) rate. This introduces a fixed angular error into the radar s angle tracking loop, resulting in an angular offset that deviates the radar antenna axis from the tracked target s direction. When transmitted asynchronously, the amplitude modulation frequency of the jamming signal may be slightly higher or lower than the radar's lobing (or scan) rate. This effectively creates a radar angular error signal with a beat frequency. The beat frequency is equal to the difference between the amplitude modulation frequency of the jamming signal and that induced onto the target echo signal by the radar (i.e., the radar lobing or scan rate). The jammer must set its amplitude modulation frequency carefully so that the beat frequency is low enough to pass through the angular servomechanism s bandwidth (typically in the range of a few Hertz). Otherwise, the effect of the received jamming signal is greatly attenuated by the angular tracking loop and, consequently, angular tracking is affected barely, or not at all. Inverse Gain Jamming with the Lab-Volt Radar Jamming Pod Trainer The Radar Jamming Pod Trainer, as stated in Exercise 2-1, can on-off modulate either the repeater signal, or the noise jamming signal. Therefore, the Radar Jamming Pod Trainer can produce either repeater asynchronous inverse gain jamming, or asynchronous AM noise jamming. The on-off amplitude modulation frequency for these techniques can be set close to the tracking radar's lobing rate. The modulation frequency is changed using the Radar Jamming Pod Trainer remote controller. Radar Electronic Protection Against Inverse Gain Jamming Asynchronous and synchronous inverse gain jamming can be rendered ineffective against a conical scan or sequential lobing tracking radar by randomly changing, on a pulse-to-pulse basis, the radar s pulse repetition frequency (PRF). Radars that have this ability are said to have PRF agile emitters. PRF agility is a useful radar electronic protection to inverse gain jamming. Patterned radar emitters, such as those that can enable a staggered PRF, are only effective in reducing the amplitude of the angular error caused by inverse gain jammers. They cannot eliminate the angular error completely. To completely eliminate the angular error, a PRF agile emitter is required. Unfortunately, truly PRF-agile radar emitters pose many design challenges when implemented in pulsed Doppler radars. These radars require their PRF to remain constant during their coherent processing interval (at least several consecutive pulse periods long). 3-7

4 Silent lobing, also known as COSRO or LORO, and as briefly discussed in this Unit's Discussion of Fundamentals, is a radar EP designed to counter EA using amplitudemodulated jamming signals. However, silent lobing is not a 100% effective countermeasure to inverse gain jamming. The reason is, that a potential asynchronous inverse gain jammer could slowly scan through the range of possible pulse repetition frequencies and locate the value of the actual radar PRF. This is done by observing which amplitude modulation frequency causes the greatest angular tracking error for the radar. Certain conical scan and sequential lobing radars use an additional receive-beam that is scanned or lobed (depending on the radar) at the same rate as the main beam. However, the additional receive-beam is displaced in angle by half a revolution (180 ) with respect to the main beam (it is pointed in a direction about the antenna's axis that is opposite to that of the main beam in the case of sequential lobing radars), as is illustrated in Figure 3-3. The outputs from the main beam and additional receive-beam are used to create the radar s angular tracking error signal. This technique, known as scan-with-compensation, is an effective EP that reduces the amplitude modulation of the target echo signal produced using amplitudemodulated jamming signals. Figure 3-3. A secondary receive-antenna beam is used in the scan-with-compensation technique. Procedure Summary During the first part of this exercise, you will set up and calibrate the Tracking Radar. You will also position the Target Positioning System with respect to the Tracking Radar. In exercise part two, the equipment setup for demonstrating asynchronous inverse gain jamming is made. The Radar Jamming Pod Trainer is positioned on the target positioning table. A metal plate target is added to the Radar Jamming Pod Trainer so as to provide it with a natural radar echo. The effect which inverse gain jamming has on the radar video signal is observed on the Oscilloscope. 3-8

5 During the third part of the exercise, you will observe and list the effects that an amplitude modulated noise jamming signal (AM noise jamming) has on the radar s angular tracking. You will verify whether staggered PRF and PRF agility are appropriate radar EP to AM noise jamming, which is a form of asynchronous inverse gain jamming. During the final part of the exercise, you will demonstrate asynchronous inverse gain jamming using the Repeater of the Radar Jamming Pod Trainer. You will verify the efficiency of staggered PRF and PRF agility as radar EP against this form of asynchronous inverse gain jamming. 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. 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. 3-9

6 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 3-4. Make sure that the metal rail of the target table is correctly aligned with the shaft of the Rotating-Antenna Pedestal. 3-10

7 Figure 3-4. Position of the Rotating-Antenna Pedestal and target table. G 6. Calibrate the Tracking Radar Training System according to the instructions in sections I to V of Appendix B. In LVRTS, decrease the Clockwise and Counterclockwise Friction Compensation settings by about 10%. This should provide good radar performance while allowing convincing demonstrations of asynchronous inverse gain jamming to be performed, using either noise or the repeated echo signal. 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 Asynchronous Inverse Gain Jamming Set Up G 7. Remove the small metal plate 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. 3-11

8 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. G 8. Make sure that a 50- load is connected to the Radar Jamming Pod Trainer COMPLEMENTARY RF OUTPUT. Install the Radar Jamming Pod Trainer onto its support (in the horizontal position) using the short support shaft (part number 33125). 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. 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 10. 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. 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 0.2 V. Note the range of the repeated echo signal, as read-off from the O-Scope Display. 3-12

9 G 11. Using the remote controller, turn the Radar Jamming Pod Trainer Repeater off. G 12. Retract the Radar Jamming Pod Trainer's target positioning arm and place the small (10 x 10 cm) metal plate target at its tip (refer to Figure 3-5). 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. Figure 3-5. Installing a target on the target positioning arm of the Radar Jamming Pod Trainer. Using the target positioning arm while observing the O-Scope Display, adjust the distance of the small 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 3-6). 3-13

10 Figure 3-6. Adjusting the target positioning arm of the Radar Jamming Pod Trainer. G 13. Adjust the orientation of the small metal plate target so that the amplitude of its echo signal is between 0.1 V and 0.15 V, while trying to have a similar amplitude for both positions of the antenna main beam. Note: If the echo signal amplitude is less than 0.1 V, replace the small metal plate target with a medium-size (15 x 15 cm) metal plate target. G 14. Using the remote controller, turn the Radar Jamming Pod Trainer Repeater on. G 15. Slightly readjust the distance of the metal plate target to maximize the amplitude of the Radar Jamming Pod Trainer composite echo signal on the O-Scope Display, while trying to have approximately the same amplitude for both positions of the antenna main beam. Using the remote controller, turn the Radar Jamming Pod Trainer Repeater off. The metal plate target echo (Radar Jamming Pod Trainer's natural radar echo) on the O-Scope Display should have an amplitude between 0.1 V and 0.15 V. Furthermore, the echo amplitude should be similar for both positions of the antenna main beam. 3-14

11 Note: If the amplitude of Radar Jamming Pod Trainer's natural radar echo is low or greatly differs from one antenna main beam position to the other, slightly readjust the distance and orientation of the metal plate target to obtain the best compromise between a maximized and balanced composite echo signal when the Repeater is on, and a balanced natural radar echo having an amplitude between 0.1 V and 0.15 V when the Repeater is off. G 16. In LVRTS, disconnect Oscilloscope probes 1 and 2 from TP1 and TP2 of the MTI Processor. Connect Oscilloscope probe 1 to TP14 of the MTI Processor. The signal at TP14 corresponds to the video output signal of the MTI Processor. Make the following settings on the Oscilloscope: Channel mv/div Channel Off Time Base ms/div Set the Oscilloscope to Continuous Refresh. Figure 3-7 shows an example of the radar video signal that you might observe on the Oscilloscope screen. The Oscilloscope now allows the target echo signal received for both positions of the antenna main beam to be observed in the time domain (this type of display is used in Figure 3-2 to explain inverse gain jamming). Figure 3-7. Radar video signal observed in the time domain using the Oscilloscope. AM Noise Jamming G 17. Make sure that the Radar Jamming Pod Trainer Repeater is turned off. Lock the Tracking Radar onto the Radar Jamming Pod Trainer's natural radar echo signal. 3-15

12 G 18. 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 transmitting an amplitudemodulated spot noise jamming signal toward the radar antenna. G 19. Using the remote controller while observing the O-Scope Display, slowly decrease the level of Radar Jamming Pod Trainer Attenuation. Decrease the level of attenuation until the amplitude of the AM noise jamming bursts rolling off the radar video signal is about twice the amplitude of the Radar Jamming Pod Trainer's natural radar echo signal, as shown in Figure 3-8. Note: It may be difficult to observe the AM noise jamming bursts rolling off the radar video signal when the refresh rate of the Oscilloscope is low. If so, temporarily turn the O-Scope Display off. This increases the refresh rate of the Oscilloscope, and thereby helps observing the AM noise jamming bursts rolling off the radar video signal. Observe the Oscilloscope displaying the radar video signal as a function of time. Note that the AM noise jamming bursts are rapidly rolling off the display. 3-16

13 Figure 3-8. AM noise jamming bursts rolling off the radar video signal. Does the AM noise jamming have a significant effect on the angular tracking? Briefly explain why. G 20. Using the remote controller, decrease in 1-Hz steps the amplitude modulation frequency of the Radar Jamming Pod Trainer signal until it is equal to the radar lobing rate (144 Hz). While decreasing the amplitude modulation frequency, observe the effects of the AM noise jamming on: 1) the roll-off rate of the noise bursts displayed on the Oscilloscope, and 2) the radar antenna s angular tracking. Note: If the target tracking lock is lost while performing this step, increase the Radar Jamming Pod Trainer attenuation level by 1 db and lock the Tracking Radar, once again, onto the Radar Jamming Pod Trainer's natural echo signal. Briefly explain the relation between the roll-off rate of the noise bursts in the radar video signal, and the noise jamming amplitude modulation frequency. 3-17

14 Briefly explain how the rate and the amplitude of the antenna oscillations vary with the amplitude modulation frequency of the noise jamming signal. G 21. Using the remote controller, adjust the Radar Jamming Pod Trainer amplitude modulation frequency to 143 Hz. Note: If the target tracking lock is lost while performing this step, increase the Radar Jamming Pod Trainer attenuation level by 1 db and lock the tracking radar, once again, onto the Radar Jamming Pod Trainer echo signal. G 22. Make sure that the DISPLAY MODE on the Antenna Controller is set to POSITION. This setting will permit you to verify quantitatively the extent of any jamming induced angle tracking errors, as illustrated in Figure 3-9. Figure 3-9. Uncertainty in angular tracking due to antenna oscillations. What is the average angle ( ) over which the antenna tracking direction is currently varying? = degrees 3-18

15 PRF Agility G 23. On the Radar Synchronizer, set the PRF MODE to STAGGERED while observing the radar angular tracking, and the radar video signal on the Oscilloscope screen. Is staggered PRF an effective EP to AM noise jamming, a form of inverse gain jamming? Briefly explain why. G 24. On the Radar Synchronizer, set the PRF MODE back to SINGLE. Change the tracking radar lobing rate to 108 Hz, by setting the Radar Synchronizer pulse repetition frequency (PRF) to 216 Hz. While doing this, observe the radar angular tracking, and the radar video signal on the Oscilloscope screen. Describe the effect that changing the tracking radar lobing rate had on the jamming induced antenna oscillations. Briefly explain why. G 25. 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 The Radar Jamming Pod Trainer is no longer transmitting an amplitude modulated spot noise jamming signal toward the Tracking Radar antenna. Momentarily depress the trigger button on the hand controller to unlock the Tracking Radar. 3-19

16 Repeater Asynchronous Inverse Gain Jamming G 26. Make the following settings on the Radar Synchronizer / Antenna Controller: PRF MODE SINGLE PRF Hz Lock the Tracking Radar onto the Radar Jamming Pod Trainer's natural radar echo. G 27. Using the remote controller, turn the Radar Jamming Pod Trainer Repeater on. When the Repeater is turned on, the Radar Jamming Pod Trainer's natural radar echo and the repeated echo signal combine to form a rather strong radar echo. This strong radar echo signal may cause some antenna hunting to occur. If so, decrease the clockwise and counterclockwise Friction Compensation settings by about 10%, realign the antenna with the Radar Jamming Pod Trainer, and lock the Tracking Radar onto the echo signal. Note: If, after decreasing the Friction Compensation settings, there is still some antenna hunting, stop it by gently placing your fingertips on the side of the radar antenna frame. G 28. Using the remote controller, begin to amplitude (on-off) modulate the repeated echo signal, by making the following adjustments to the Radar Jamming Pod Trainer: Noise Off AM/Blinking On Modulation Frequency Hz Modulation Internal Repeater On RGPO Off False Targets (FT) Off Note that the difference between the amplitude modulation frequency of the repeated echo signal, and the radar s lobing rate is too great to cause significant angle-tracking errors. G 29. Using the remote controller, decrease the amplitude modulation frequency of the Radar Jamming Pod Trainer repeated echo signal in 1-Hz steps until it is equal to the radar lobing rate (144 Hz). While decreasing the amplitude modulation frequency, observe the effects of the repeater inverse gain jamming on: 1) the rate at which the target echo signal (displayed on the Oscilloscope) is amplitude modulated, and 2) the radar antenna s angular tracking. 3-20

17 Note: It may be difficult to observe the rate at which the target echo signal is modulated in amplitude when the refresh rate of the Oscilloscope is low. If so, temporarily turn the O-Scope Display off. This increases the refresh rate of the Oscilloscope, and thereby helps observe the rate at which the target echo signal is modulated in amplitude. Briefly explain the relationship between the rate at which the target echo signal is amplitude modulated, and the amplitude modulation frequency of the repeated echo signal. Briefly explain how the rate and the amplitude of the antenna oscillations vary with the amplitude modulation frequency of the repeated echo signal. G 30. Using the remote controller, adjust the Radar Jamming Pod Trainer amplitude modulation frequency to 145 Hz. G 31. On the Radar Synchronizer / Antenna Controller, set the PRF MODE to STAGGERED while observing the radar angular tracking, and the radar video signal on the Oscilloscope screen. Is staggered PRF an effective EP to repeater inverse gain jamming? G Yes G No G 32. On the Radar Synchronizer / Antenna Controller, set the PRF MODE back to SINGLE. Change the tracking radar lobing rate to 108 Hz by setting the pulse repetition frequency (PRF) to 216 Hz on the Radar Synchronizer / Antenna Controller. While doing this, observe the radar angular tracking, and the radar video signal on the second oscilloscope display. Is PRF agility an effective EP to repeater inverse gain jamming? G Yes G No 3-21

18 G 33. Turn off the Tracking Radar and the Radar Jamming Pod Trainer. Disconnect all cables and remove all accessories. CONCLUSION In this exercise, you demonstrated the effects of asynchronous inverse gain jamming against a tracking radar by first using an amplitude-modulated noise jamming signal, and then using an amplitude-modulated repeated echo signal. You learned that asynchronous inverse gain jamming can either introduce a deflection in the radar antenna s angular tracking, or cause the antenna to oscillate about the tracked target s direction. While implementing this angle deception EA technique against the Tracking Radar, you were able to verify the effectiveness of PRF agility, and of staggered PRF as radar EP to AM noise jamming as well as repeater inverse gain jamming. REVIEW QUESTIONS 1. Briefly describe how inverse gain jamming affects the angle tracking loop of a sequential lobing radar. 2. Figure 3-3 graphically shows how in certain conical scan radars the scan-withcompensation technique is implemented. What is the purpose of the additional receive beam? 3. What is the difference between synchronous and asynchronous inverse gain jamming? 3-22

19 4. Inverse gain jamming against a sequential lobing radar, can be achieved by a repeater jammer in the following manner. The weaker radar signals received at the jammer, before being repeated back to the radar, are amplified. The stronger signals, before being repeated back to the radar, are attenuated. Briefly explain if this implementation of inverse gain jamming is done synchronously or asynchronously with respect to the radar lobing rate. 5. Asynchronous and synchronous inverse gain jamming can be rendered ineffective against a sequential lobing tracking radar by randomly changing the radar s pulse repetition frequency (PRF) on a pulse-to-pulse basis. Briefly explain why this electronic protection is effective. 3-23