Frequency Agility and Barrage Noise Jamming

Transcription

1 Exercise 1-3 Frequency Agility and Barrage Noise Jamming EXERCISE OBJECTIVE To demonstrate frequency agility, a radar electronic protection is used against spot noise jamming. To justify the use of barrage noise jamming against frequency-agile radars. To compare the jammer induced noise level produced in a radar receiver by a barrage noise jamming signal with that created by a spot noise jamming signal. DISCUSSION Introduction A noise jamming signal is always directed toward the receiver system of a radar. This system is made to discriminate, to an extent, against all signals but the one it transmits and is expected to receive from targets. A directional antenna limits the locations of the possible sources of desired echo signals. The receiver is designed to accept only signals present within a certain frequency range known as the radar receiver passband. As shown in Figure 1-19, the passband is centered on the radar s operating frequency. Its width is adapted to the transmitted radar pulse width. The narrower the transmitted pulse width (which implies a better range resolution), the wider the receiver passband. Figure Spot noise jamming signal bandwidth and radar receiver passband represented in the frequency domain. In the case of the Lab-Volt radar receiver system (Radar Receiver and Dual-Channel Sampler), the passband width, also known as the radar receiver bandwidth, is approximately 1.3 GHz. The small transmitted pulse width used by the radar is the 1-43

2 reason that the receiver has a wide passband. The Radar Receiver and the Dual- Channel Sampler accept all signals with a frequency found within a 1.3-GHz band centered on the radar s operating frequency. As stated, spot noise jamming consists of a narrow bandwidth ( 30 MHz in the case of the Radar Jamming Pod Trainer) RF signal transmitted by the jammer toward a radar antenna. For it to be effective against the radar, the jamming signal frequency must be tuned to the radar s operating frequency. The narrow bandwidth of a spot noise jamming signal allows it to be contained entirely within the radar receiver passband limits. This permits nearly all jamming signal power inserted through the radar antenna to pass unhindered into the radar receiver system. However, if the radar were to suddenly change its operating frequency such that the bandwidth of the jamming signal was no longer contained entirely within the radar receiver passband limits, then only some or none of the jamming signal power would be inserted into the radar receiver system. Frequency Agility Certain radars are capable of changing their transmitted carrier frequency on a pulse-to-pulse or burst-to-burst basis to eliminate the effects of narrowband jamming signals (e.g., spot noise jamming signal). Illustrated in Figure 1-20, this capability is known as frequency agility and when implemented takes the form of abrupt, random frequency changes over the entire frequency tuning range. When used as a radar electronic protection against jamming, effective frequency agility forces potential spot noise jammers to spread their jamming signal power over a wider bandwidth. One which, for the jammer to no longer be susceptible to the radar s frequency changes, must cover the entire radar frequency tuning range. Frequency agility cannot be used by coherent pulse-doppler radars (radars with MTD processing). These radars use Coherent pulse-processing Intervals (CPIs). A CPI is a pre-detection period over which the radar s operating frequency must stay the same so that returned radar pulses can be coherently integrated together to generate basic target information. In effect, this type of radar must maintain a constant frequency for at least a few consecutive pulses. If this type of radar were to change its carrier frequency, a jammer conducting look-through jamming could detect and match the radar s frequency change before the radar could switch its frequency again. Look-through jamming consists of periodically pausing jamming to evaluate jamming success and to ensure the correct jamming signal properties (such as frequency, modulation, and bandwidth). Conversely, coherent radars with analog MTI processing, such as the Lab-Volt Tracking Radar, can use frequency agility as an effective countermeasure against spot noise jamming. This type of radar does not use CPIs and so can continue target detection while changing its operating frequency. A spot noise jammer confronted with a frequency-agile radar cannot predict the radar s next frequency change and does not have the time necessary to match the current frequency. In such cases, the jamming platform must resort to barrage noise jamming. That is, transmitting its jamming signal over the entire radar frequency tuning range. 1-44

3 Figure Frequency agility as an EP against spot noise jamming. Barrage Noise Jamming Barrage noise jamming makes certain that jamming induced noise is created in the radar receiver for whatever the radar s operating frequency. Thus, a frequency-agile radar can no longer eliminate the receiver jamming noise by changing its frequency. When a radar is confronted with barrage noise jamming, the RF jamming signal power accepted by the radar receiver is significantly less than if the radar were confronted with spot noise jamming. A platform conducting barrage noise jamming uniformly spreads its available jamming power over the entire frequency tuning range of the victim radar. Implying, as shown in Figure 1-21, that the bandwidth of a barrage noise jamming signal is much wider than the radar receiver passband limits. The receiver accepts only a fraction of the total power of a barrage noise jamming signal. 1-45

4 Figure Jamming power received by a radar. The RF signal power (P BARRAGE ) accepted by a radar receiver from a barrage noise jammer can be represented by Equation (2). This equation is valid provided that the jamming is sufficiently noiselike, and that the jamming signal power is uniformly distributed over the jamming signal bandwidth (i.e., a uniform spectral power density). (2) P SPOT is the RF signal power accepted by the radar receiver from a spot noise jammer of equivalent radiated power. The wider the barrage noise jammer signal bandwidth (B BJ ), the less jamming signal power is accepted into the radar receiver. Maximum barrage noise jamming efficiency is achieved by making the jamming signal bandwidth (B BJ ) only slightly wider than the radar receiver system bandwidth (B R ). The Radar Jamming Pod Trainer Noise Signal Be it for narrowband or wideband noise jammers, the method taken to generate a noise jamming signal can be critical to the jammer s success. It is important that the jamming induced noise in a radar receiver resembles thermal noise as closely as possible. This implies that the noise amplitude has a Gaussian distribution with time, and that the noise spectral power density is uniform. The Lab-Volt Radar Jamming Pod Trainer does not produce thermal noise. The Radar Jamming Pod Trainer uses a Voltage Controlled Oscillator (VCO) to generate a frequency-swept Continuous Wave (CW) signal. A VCO is an electronic device that generates a CW (sinusoidal) output signal whose frequency is proportional to the voltage level of an input signal. The Radar Jamming Pod Trainer can be configured to input an internal triangular signal, or a randomly generated signal to the VCO, as shown in Figure

5 Figure Randomly controlled VCO input signal for generating an output signal spanning a 4-GHz frequency range. By randomly varying the VCO input signal over a small voltage range, a narrowband noise jamming signal can be generated. This corresponds to the Radar Jamming Pod Trainer spot noise jamming signal. By similarly modulating the VCO input signal over a wide voltage range, a wideband noise jamming signal is generated. This produces the Radar Jamming Pod Trainer barrage noise jamming signal. Procedure Summary The Tracking Radar is set up in the first part of the exercise. During the second part, the efficiency of frequency agility as an electronic protection against spot noise jamming is demonstrated. In the third part of the exercise, the effect of barrage noise jamming and spot noise jamming on the radar video signal are compared. 1-47

6 In the fourth part of the exercise, quantitative measures are made of the level of noise induced in the radar receiver system of the Tracking Radar, using spot noise jamming and barrage noise jamming. Four different bandwidths are used when conducting barrage noise jamming. The levels of noise induced in the radar receiver system using spot noise jamming and barrage noise jamming are compared, and a conclusion is drawn on the pertinence of using frequency agility when a radar is confronted with noise 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. 1-48

7 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. 1-49

8 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. G 7. 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 Frequency Agility G 8. 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. 1-50

9 G 9. 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. Retract the Radar Jamming Pod Trainer's target positioning arm and place the medium-size (15 x 15 cm) metal plate target at its tip (refer to Figure 1-24). 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 Installing a target on the target positioning arm of the Radar Jamming Pod Trainer. G 10. 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. 1-51

10 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. G 11. Turn the repeater of the Radar Jamming Pod Trainer off by making the following settings on the remote controller: Noise Off AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off Verify that the Radar Jamming Pod Trainer status, indicated on its rear panel, shows that no jamming signal is being transmitted. 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 natural echo signal (radar echo signal of the metal plate installed on the target positioning arm) 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 natural echo signal is approximately 0.25 V. Lock the Tracking Radar onto the Radar Jamming Pod Trainer's natural echo signal. In LVRTS, add persistence to the O-Scope Display by setting the corresponding parameter to 6 traces. G 13. 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 Maximum Attenuation db AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off This sets the Radar Jamming Pod Trainer VCO to an operating frequency of 8.0 GHz, the same frequency as the Tracking Radar. The Radar Jamming Pod Trainer is transmitting a spot noise jamming signal toward the radar antenna. 1-52

11 G 14. Using the remote controller, decrease the Radar Jamming Pod Trainer Noise Attenuation until the video signal on the O-Scope Display is quite noisy, but without causing the Tracking Radar to lose target track. G 15. While observing the O-Scope Display, slowly increase the Tracking Radar s operating frequency from 8.0 GHz to 9.0 GHz, and then from 9.0 GHz to 10.0 GHz. A significant decrease of the noise level in the radar video signal should occur. Briefly explain why the video signal noise level decreases when the Tracking Radar s operating frequency is changed from 8.0 GHz to 10.0 GHz. Barrage Noise Jamming G 16. Make sure that the Tracking Radar s operating frequency is tuned to 10.0 GHz. 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 Random Attenuation No adjustment Attenuation No adjustment AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off This places the Radar Jamming Pod Trainer into a wideband (from 8.0 GHz to 12.0 GHz) noise jamming signal transmission mode, known as barrage noise jamming. Note that the bandwidth (4.0 GHz) of the barrage noise jamming signal produced by the Radar Jamming Pod Trainer is much wider than the radar receiver system passband limits. G 18. While observing the O-Scope Display, slowly vary the Tracking Radar s operating frequency over the entire frequency tuning range (8.0 GHz to 10.0 GHz). 1-53

12 The noise level in the radar video signal should remain virtually constant over the entire Tracking Radar's frequency tuning range. Briefly explain why the noise level in the video signal does not vary over the Tracking Radar's frequency tuning range. G 19. Tune the Tracking Radar s operating frequency to 10.0 GHz. Using the remote controller, slowly decrease the Radar Jamming Pod Trainer Noise Attenuation 1 db at a time until the Tracking Radar loses target track. Note that barrage noise jamming has caused the Tracking Radar to lose target track, even though the Radar Jamming Pod Trainer has spread its noise jamming power over a wide frequency band. G 20. Using the remote controller, turn the Radar Jamming Pod Trainer Noise off. In LVRTS, disable the persistence of the O-Scope Display by setting the corresponding parameter to Off. Induced Noise Level in the Radar Receiver System G 21. Align the radar antenna axis 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 natural echo signal is the same for both positions of the antenna main beam. G 22. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Random Attenuation No adjustment Attenuation No adjustment AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now transmitting spot noise jamming toward the Tracking Radar. 1-54

13 G 23. On the Radar Transmitter, disconnect the BNC-connector cable at the TRIGGER INPUT of the PULSE GENERATOR. By doing this, radar pulse transmission is disabled but reception continues. Therefore, the Radar Jamming Pod Trainer spot noise jamming signal is still received. G 24. Make the following settings on the Oscilloscope: Channel V/DIV Channel Off Time Base ms/div Set the Oscilloscope to Continuous Refresh. The noise induced in the I channel of the radar receiver system by the spot noise jamming signal should be displayed on the Oscilloscope. Slightly realign the radar antenna axis so that the trace on the O-Scope Display does not appear as doubled due to the lobing action of the Tracking Radar. Figure 1-25 shows what you might observe on the O-Scope Display before and after antenna realignment. Figure Noise on the O-Scope Display before and after antenna realignment. G 25. In LVRTS, observe that the true RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar indicates the RMS voltage of the noise induced in the I channel of the radar receiver system. 1-55

14 G 26. Using the RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar, measure the RMS voltage of the noise induced in the radar receiver system by spot noise jamming. Record the value in the first row of Table 1-1. TYPE OF RADAR NOISE JAMMING RMS VOLTAGE OF THE NOISE INDUCED IN THE RADAR RECEIVER SYSTEM (mv) Spot Noise Barrage Noise (1-GHz bandwidth) Barrage Noise (2-GHz bandwidth) Barrage Noise (3-GHz bandwidth) Barrage Noise (4-GHz bandwidth) Table 1-1. RMS voltage of the noise induced in the radar receiver system. G 27. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Random Attenuation No adjustment Attenuation No adjustment AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now transmitting a barrage noise jamming signal having a 1-GHz bandwidth. The available jamming power has consequently been spread over a 1-GHz frequency band centered at 10.0 GHz. Using the RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar, measure the RMS voltage of the noise induced in the radar receiver system by barrage noise jamming. Record the value in the appropriate row of Table

15 G 28. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Random Attenuation No adjustment Attenuation No adjustment AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now transmitting a barrage noise jamming signal having a 2-GHz bandwidth centered at 10.0 GHz. Using the RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar, measure the RMS voltage of the noise induced in the radar receiver system by barrage noise jamming. Record the value in the appropriate row of Table 1-1. G 29. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Random Attenuation No adjustment Attenuation No adjustment AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now transmitting a barrage noise jamming signal having a 3-GHz bandwidth centered at 10.0 GHz. Using the RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar, measure the RMS voltage of the noise induced in the radar receiver system by barrage noise jamming. Record the value in the appropriate row of Table 1-1. G 30. Using the remote controller, make the following adjustments to the Radar Jamming Pod Trainer: Noise On Frequency GHz Frequency Bandwidth GHz Frequency Modulation Random Attenuation No adjustment Attenuation No adjustment 1-57

16 AM/Blinking Off Repeater Off RGPO Off False Targets (FT) Off The Radar Jamming Pod Trainer is now transmitting a barrage noise jamming signal having a 4-GHz bandwidth centered at 10.0 GHz. Using the RMS voltmeter at TP1 of the MTI Processor of the Tracking Radar, measure the RMS voltage of the noise induced in the radar receiver system by barrage noise jamming. Record the value in the appropriate row of Table 1-1. G 31. From the results in Table 1-1, observe that the RMS voltage of the noise induced in the radar receiver system remains almost unchanged when the Radar Jamming Pod Trainer passes from spot noise jamming to barrage noise jamming with a 1-GHz bandwidth. Briefly explain why. G 32. From the results in Table 1-1, observe that the RMS voltage of the noise induced in the radar receiver system starts to decrease when the bandwidth of the barrage noise jamming is greater than 1 GHz. Briefly explain why. G 33. From the result in Table 1-1, what is the relationship between the bandwidth of the barrage noise jamming and the amount of noise induced in the radar receiver system? 1-58

17 What does this relationship imply for a radar operator confronted with noise jamming? G 34. Turn off the Tracking Radar and the Radar Jamming Pod Trainer. Disconnect all cables and remove all accessories. CONCLUSION In this exercise, it was stated and you observed that a radar receiver system is made to discriminate all signals with frequencies found outside its passband from signals with frequencies found within its passband. Only signals present within the passband frequency range, which is centered on the radar s operating frequency, are accepted. You used frequency agility to exploit this fact and to eliminate radar receiver noise introduced by spot noise jamming. Frequency agility is used to force a potential spot noise jammer into a barrage noise jamming mode. Using the Radar Jamming Pod Trainer, you observed that for whatever the Tracking Radar s operating frequency, barrage noise jamming raises the noise level induced in the radar receiver system. However, the noise power induced in the radar receiver system by barrage noise jamming is less than that induced by spot noise jamming. This is because the available signal power of a barrage noise jammer is spread over a much wider frequency band than that of a spot noise jammer. During this exercise, you measured the level of noise inserted into the radar receiver system with spot noise jamming as well as with barrage noise jamming (four different bandwidths were used for the barrage noise jamming). You demonstrated that it is advantageous for a frequency-agile radar to force a spot noise jammer into a barrage noise jamming mode because the available jamming power of a barrage noise jammer is spread thinly over the radar receiver passband. To be as effective as a spot noise jammer, a barrage noise jammer must transmit extremely high levels of RF power. This may be difficult, or even impossible, in certain airborne applications. REVIEW QUESTIONS 1. What is the difference between a spot noise jamming signal and a barrage noise jamming signal? 1-59

18 2. Name two ways through which a radar receiver system is made to discriminate to an extent against all signals but the one it transmits and is meant to receive. 3. Define frequency agility. 4. Briefly explain why frequency agility is an effective countermeasure against spot noise jamming. 5. Briefly explain why it is advantageous for a frequency-agile radar to force a spot noise jammer into barrage noise jamming mode. 1-60