Exercise 4-1. Chaff Clouds EXERCISE OBJECTIVE

Transcription

1 Exercise 4-1 Chaff Clouds EXERCISE OBJECTIVE To demonstrate chaff as a method of denying target information to a radar. To verify whether MTI processing is an effective anti-chaff processing technique for radars. DISCUSSION Corridor Chaff The objective of corridor chaff (illustrated in Figure 4-3) is similar to that of noise jamming. By dispensing large quantities (bundles) of chaff in corridor-like formations and over very wide areas within the surveillance volume of an enemy radar, the expected flight paths of infiltrating aircraft are concealed from radars. Figure 4-3. Corridor chaff distribution. To effectively deny information regarding the composition and timing of infiltrating aircraft, chaff corridors must originate outside the maximum radar detection range and be uninterrupted throughout their length. To make certain that the penetrating aircraft are screened from radar detection, the RCS associated with the chaff must be greater than the RCS associated with the target. With radars that use moving target detection (MTD) processing, the return power from the chaff that enters the detection Doppler filters and competes with the target return must be stronger than the target return power. 4-5

2 Launched from outside a radar s maximum detection range via the use of rockets, or dispensed by advanced small RCS aircraft, chaff corridors take hours to reach the ground and are able to confuse radar operators during this entire length of time. Chaff Shielding Effect The theoretical maximum RCS that a chaff cloud can have is subject to degradation because of an effect known as shielding. Shielding occurs when the dipole density is such that it prevents every dipole in a chaff cloud from receiving the full amount of energy incident from the radar. Depending on the average spacing between the dipoles in a chaff cloud, this may vary significantly. At one extreme, the dipoles may be so closely spaced that only the dipoles at the surface of the chaff cloud (surface dipoles) are responsible for reflecting an incident radar signal. In this case, the chaff cloud s RCS is significantly smaller than its theoretical maximum. Maximum RCS, that can be no larger than the sum of the individual dipole radar cross sections, occurs when the average distance between dipoles is approximately twice the wavelength of the incident radar signal. Chaff Clouds The actual values for the dimensions and density of a chaff cloud vary widely, depending upon the chaff dispenser design, the type and dynamics of the launching platform, and the deployment point on the platform. After the initial blooming effects attributed to the turbulence, if any, generated by the dispensing platform, the chaff cloud continues to grow as a result of three factors: 1. The distribution of the fall rates of the elements (partially due to lack of uniformity in size and coatings), 2. the prevailing winds, 3. and, air turbulence. Anti-Chaff Techniques When chaff is dispensed from an aircraft, the drag on the individual chaff dipoles is so great that the chaff cloud comes to air mass velocity almost instantaneously. Chaff s characteristic lack of motion is exploited by radar Doppler filtering systems (such as MTI or MTD) to discriminate chaff echoes from moving target echoes, and thus, reject the chaff echoes. MTD processors are usually more effective than MTI processors in rejecting chaff echoes. Procedure Summary In the first part of the exercise, you will observe a sample of chaff fibers included with the EWT accessories. In the second 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. 4-6

3 During the third part of the exercise, the Tracking Radar will be used in search mode and in conjunction with the Variable-Density Chaff Cloud (VDCC) provided with the EWT, to demonstrate the effect of chaff on radar target detection. By varying the density of the VDCC in exercise part four, you will demonstrate the effect of chaff shielding. Lastly, in exercise part five, you will enable MTI processing and verify whether this is an effective protection against chaff clouds. You will also observe, on the O-Scope Display, the echo signal coming from the VDCC. PROCEDURE Chaff Dipole Sample G 1. Included in the EWT Connection Leads and Accessories is a sealed container holding true chaff fibers (part number 33203). Each one of these fibers has been cut to approximately one half of the wavelength of an X-band (10.0-GHz) radar. CAUTION! Do not open, or puncture the container holding the chaff fibers. Avoid contact with chaff fibers. One chaff bundle launched from an aircraft contains billions of these chaff fibers. It is interesting to note that a chaff bundle launched from an aircraft has a weight in the hundreds of grams, where as the chaff bundles used aboard naval ships often weigh-in at over 5 kg. The reason is that a naval ship has a much larger RCS to cover than does an aircraft. Setting Up the Tracking Radar G 2. 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 3. 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 4-7

4 running, click Exit in the File menu and then restart the LVRTS software to begin a new session. G 4. 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. G 5. 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 6. 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 4-4. Make sure that the 4-8

5 metal rail of the target table is correctly aligned with the shaft of the Rotating-Antenna Pedestal. Figure 4-4. Position of the Rotating-Antenna Pedestal and target table. G 7. 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 Replace the semi-cylinder target, installed on the target table mast during the Tracking Radar calibration, with the cylinder target placed in the upright position. Position the cylinder target at the following target table coordinates: X = 45 cm, Y = 65 cm. G 8. Select the SCAN mode of the Tracking Radar to make the antenna rotate. 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 radar antenna. Target blips should appear on the Radar Display (PPI display) of the Tracking Radar. 4-9

6 Notice that each target displayed on the Radar Display appears as two adjacent blips and is segmented. This is normal and due to the antenna lobe switching performed by the Tracking Radar. In LVRTS, set the Lobe Control Rate of the Tracking Radar to Off (Right). to disable antenna lobe switching. Notice that the targets are displayed normally on the Radar Display. G 9. While observing the Radar Display, adjust the Gain of the MTI Processor of the Tracking Radar so that the cylinder target's echo signal is clearly visible but not immersed in clutter. A VDCC Chaff Cloud G 10. Assemble the Variable-Density Chaff Cloud (VDCC, part number 33155), if this has not already been done. Slide the VDCC support rod into the braces of the streamer support grid, as shown in Figure 4-5. Tighten the screws of both braces to avoid having the streamer support grid move. Figure 4-5. VDCC assembly. G 11. Install the VDCC positioning stand adapter (part numbers and 33160) onto the top of the positioning stand (part number 33179). Install the VDCC onto the positioning stand. Mylar streamers (part number 33159) are included with the Connection Leads and Accessories. Insert the streamers, as illustrated in Figure 4-5, into the support grid of the VDCC. Position them with an average spacing distance of approximately 2 to 3 cm between each streamer. Position the VDCC above the target table grid where the X-values are maximum and the Y-values are minimum, as illustrated in Figure

7 Adjust the height of the positioning stand so that the middle of the VDCC mylar streamers is approximately 55 cm above the surface of the target table. Figure 4-6. VDCC and cylinder target setup. Observe the Radar Display. The radar echo of the chaff cloud (VDCC) should be easily discerned. If not, make sure that the Gain of the MTI Processor is properly set. Figure 4-7 shows an example of what you might observe on the Radar Display. 4-11

8 Figure 4-7. Chaff cloud (VDCC) echo as observed on the Radar Display. G 12. Move the cylinder target into the VDCC. Observe the Radar Display. Can the radar echo from the cylinder target be distinguished from that of the VDCC? G Yes G No Shielding G 13. Move the cylinder target so that it is approximately 30 cm behind the VDCC. If necessary, slightly readjust the position of the VDCC so that it is located between the Rotating-Antenna Pedestal and the cylinder target. 4-12

9 Observe the Radar Display. Is the radar echo of the cylinder target visible on the Radar Display? Briefly explain what you observe, and relate this situation to that of a true chaff cloud. G 14. Gradually decrease the density of the VDCC by resting many of the hanging mylar streamers on top of the support grid while observing the Radar Display. Describe what happens. Note: Select the mylar streamers that you place on top of the support grid so as to keep the density of the VDCC as uniform as possible. MTI Anti-Chaff Processing G 15. Move the cylinder target to target table coordinates X = 45 cm, Y = 45 cm. On the Target Controller, make sure that the X- and Y-axis SPEED controls are in the MINimum position, then make the following settings: MODE SPEED DISPLAY MODE SPEED Set the Y-axis SPEED control so that the target speed is equal to approximately 25 cm/s. G 16. Increase the density of the VDCC by lowering the mylar streamers that are resting on top of the support grid. G 17. In LVRTS, make the following setting: System Settings MTI On 4-13

10 This setting enables the MTI processing circuit. You should be able to observe the moving cylinder target echo on the Radar Display. If not, readjust the Gain of the MTI Processor. Observe the Radar Display and briefly describe the effect that MTI processing has on the presence of the VDCC radar echo. How does this demonstration differ as compared with full-scale radars where the chaff is dispensed at high-altitudes? G 18. Make the following settings on the Target Controller: MODE DISPLAY MODE POSITION POSITION This places the cylinder target in the center of the target table. In LVRTS, turn the MTI function off and readjust the Gain of the MTI Processor as required. G 19. Select the MANUAL mode of the Tracking Radar to stop the antenna rotation. Align the antenna axis with the VDCC. Note: The Lobe Control Rate of the Radar Target Tracker is automatically set to PRF/2 when the MANUAL mode is selected. In LVRTS, set the Lobe Control Rate of the Radar Target Tracker to Off (right). This disables antenna lobe switching. Also, turn off the AGC of the Radar Target Tracker. G 20. Slightly readjust the Tracking Radar antenna orientation so that the amplitude of the VDCC echo signal is maximum on the O-Scope Display. Note: If the VDCC echo signal becomes saturated, decrease the Gain of the MTI Processor. 4-14

11 In LVRTS, set the Gain of the MTI Processor so that the amplitude of the VDCC echo signal on the O-Scope Display is approximately 0.5 V. Move or rotate the VDCC very slightly while observing its radar echo signal on the O-Scope Display. Briefly describe the VDCC echo signal. G 21. Turn off the Tracking Radar. Disconnect all cables and remove all accessories. CONCLUSION In this exercise, you familiarized yourself with the VDCC accessory included with the EWT. You showed that a chaff cloud can deny radar information about a target within the chaff cloud. By varying the density of the VDCC, you were able to demonstrate the effect of chaff shielding. You showed that MTI processing is an effective anti-chaff technique. MTI processing rejects the chaff cloud s radar echo from the radar receiver video signal because the chaff cloud is characteristically moving much slower than targets of interest. You also observed, on the O-Scope Display of the Tracking Radar, the radar echo signal of the chaff cloud simulated using the VDCC. REVIEW QUESTIONS 1. What is the use of dispensing large quantities (bundles) of chaff in corridor-like formations? 2. The theoretical maximum RCS that a chaff cloud can have is subject to degradation because of an effect known as shielding. Briefly explain shielding. 4-15

12 3. What three factors make a bundle of chaff grow in size when released into the air? (Do not consider the initial blooming effects attributed to the turbulence generated by the dispensing platform.) 4. A bloomed chaff cloud has a small velocity compared to that of the aircraft from which it was launched. Why? 5. What differs between the way that the VDCC accessory and a true chaff cloud generate their radar echo? 4-16