Exercise 2-6. Target Bearing Estimation EXERCISE OBJECTIVE

Transcription

1 Exercise 2-6 EXERCISE OBJECTIVE When you have completed this exercise, you will be able to evaluate the position of the target relative to a selected beam using the A-scope display. You will be able to verify your result using the same method with the adjacent beam. DISCUSSION Beam radiation patterns are parabolic when plotted using Cartesian coordinates, with the angle in degrees on the horizontal axis and the relative power in decibels on the vertical axis. If for a given beam B of the PAA, the angle at the center of the beam is B and the maximum power is considered to be 0 db, the equation for the parabola, with the vertex at (0, B ), is There is a half-power point on each side of the maximum. These points are located at, where HP is the half-power beam width (HPBW), as shown in Figure 2-5. To calculate the factor m, which determines the "width" of the parabola, consider that the relative power is -3 db at either half-power point. Then Therefore and 2-35

2 Figure 2-5. Radiation patterns for two beams. When a target is centered in a beam whose direction is B, the amplitude of the echo pulse will be at a maximum V max. When the target is situated at an arbitrary bearing of T with respect to the radar line of sight (0 or Z in Figure 2-5) and the measured 2-36

3 amplitude of the echo pulse is V T, the target bearing can be determined using the following relation (the factor 10 is used in this equation instead of 20 because the radar signal goes through the antenna twice, once at transmission and once at reception): This gives Rearranging gives Solving for T gives Since the radiation pattern of each beam is defined by the equation of a parabola, in order to determine the exact target bearing with respect to a specific beam, you must first determine on which side of the beam the target lies. To do this, simply verify the adjacent beams to see in which one the target echo is present. Procedure Summary In this exercise, you will use the Target Controller to place the target between two center beams and you will measure and record the return signal for these two beams. Finally, using these values, you will calculate the exact angular position of the target. PROCEDURE Set-up and calibration G 1. Before beginning this exercise, the main elements of the Radar Training System (the antenna, the target table and the training modules) must be set up as shown in Appendix A. Turn on all modules and make sure the POWER ON LEDs are lit. G 2. Make sure that the LVRTS software has been started and that the Radar Training System has been connected, adjusted and calibrated according to 2-37

4 the instructions in Appendix B. Then set the RF POWER switch on the Radar Transmitter to the STANDBY position. Note: DO NOT connect the power cable to the MOTOR POWER INPUT of the Rotating-Antenna Pedestal. Target bearing estimation G 3. Place a half-cylinder target on the mast of the target table, with the convex surface facing the radar. G 4. On the Radar Transmitter, turn the RF POWER on. G 5. On the Phased Array Antenna Controller, set the SCAN MODE to MANUAL, the BEAM SEQUENCE to INCREMENTAL and the DISPLAY MODE to BEAM NUMBER. Use the POSITION/SPEED buttons to select beam 0. G 6. Connect probe E to TP3 (PRF) in the Display Processor tab and connect probe 1 to TP14 (VIDEO OUTPUT) in the MTI Processor tab. Show the Oscilloscope and adjust it as follows: Channel V/div (DC coupling) Channel Off Time Base ms/div Trigger Source E Trigger Level V Trigger Slope Trigger Coupling DC Set the oscilloscope to Continuous Refresh (in the View menu, select Continuous Refresh or click in the oscilloscope toolbar.) You should see the VIDEO OUTPUT signal on the oscilloscope screen. G 7. In the System Settings, set the Gain to 4 and set Video Integrator to On in order to reduce the high frequency noise. Set the Video Integrator Pulses as desired. G 8. Using the Target Controller, move the target directly in front of the selected beam (beam 0) in order to maximize the echo amplitude on the oscilloscope screen. Gently turn the Phased Array Antenna from side to side by hand while observing the oscilloscope display until the maximum echo amplitude is obtained. Note the maximum amplitude below. Max. echo amplitude (beam 0) V max 2-38

5 G 9. On the Phased Array Antenna Controller, select beam 15. Using the Target Controller, move the target toward the right of the PAA until you begin to see the target echo on the oscilloscope. Place the target somewhere between beams 0 and 15. When switching between beam 0 and beam 15, you should be able to see the target echo on the oscilloscope screen for both beams. G 10. Select beam 0 and note the amplitude. V T (beam 0) V Select beam 15 and note the amplitude. V T (beam 15) V G 11. The angular position of the target relative to the direction of beam 0 is determined by using V T (beam 0) in the following equation: where = 2 and HP = 6 = degrees Note: Calculation of the target bearing using this method requires accurate knowledge of the half-power beam width and of the direction of each beam. Since these are difficult to determine exactly, the calculated target bearings will only be approximate. G 12. You can verify your result using the angular position equation relative to beam 15. If your result is correct, this equation should yield approximately the same angular value as the previous one. where = -2 (358 ) = degrees 2-39

6 G 13. On the Radar Transmitter, make sure that the RF POWER switch is in the STANDBY position. The RF POWER STANDBY LED should be lit. If no one else will be using the system, turn off all equipment. CONCLUSION In this exercise, you placed the target between two beams and you measured the return signals for these two beams. Then, using these values, you calculated the exact angular position of the target. REVIEW QUESTIONS 1. What type of mathematical curve could represents a simple approximation of the radiation pattern of an array in the angle range close to the maximum? 2. How can you determine on which side of the selected beam the target lies? 3. Briefly explain how to calculate the exact position of a target located between two beams? 2-40