Chapter 3 — BEARINGS AND MOTION 



BEARING 090 T 



71,6 

 Figure 3-11.— Effect of advance and transfer 

 on bearing. 



target data can be obtained only when the cursor 

 is kept positioned properly on the target echo. 



ATTACK LEAD 



Once target movement is determined, the 

 conning officer turns the ship to point the bow 

 ahead of the target. This maneuver is called 

 the collision lead. If the proper amount of 

 lead is taken, the target bearing ceases its 

 drifting movement and remains steady. The 

 amount of lead depends on the type of attack 

 and the weapon used. Depending on the speed 

 and maneuvers of the target, it may become 

 necessary to increase or decrease the lead 

 so as to maintain a steady bearing during the 

 approach phase of the attack. Drift may be 

 determined correctly only if the cursor is posi- 

 tioned properly to bisect the tai'get echo at 

 all times. The following example should help 

 clarify bearing drift and attack lead. 



Assume that you (the sonar operator) gain 

 contact on a submarine bearing 090° true. (The 

 submarine's course is 180°.) The ship turns to 

 head for the target and steadies on course 

 090°. This maneuver is apparent when the stern 

 line steadies on 270°. Noticing that the bearing 

 is drifting to the right (caused by advance and 

 transfer during the turn and target movement), 

 you move the cursor to the right to remain on 

 target, and report "Bearing drift right." There- 

 upon, the conning officer changes course to the 

 right to establish a collision lead. With colli- 

 sion lead established, there is no bearing di'ift, 

 relative movement of the target on the scope 

 is directly toward the ship as the siiip closes 

 the target, and a true movement in the direc- 

 tion of 180° (course of submarine) can be deter- 

 mined from the video presentation on the scope 

 by observation of the target traces. (NOTE: 

 Remember that true bearing drift can be deter- 

 mined accurately only when the ship is headed 



directly at the target.) In this illustrative case, 

 assuming a depth charge attack, additional lead 

 is applied at 700 yards. This maneuver is called 

 attack lead. The amount of lead depends on 

 target aspect, target speed, and the sinldng 

 time of the depth charges. As this final attack 

 lead is applied to the right, the bearings com- 

 mence to drift left and aft, down the port side, 

 until contact is lost at short range. At this 

 stage of the attack, the operator must train the 

 cursor rapidly aft so as to regain contact as 

 the attacking ship clears the submairine. 



In almost every depth charge attack, you 

 will follow the same procedure, once you detect 

 a target on the scope. The cursor is trained on 

 the target and adjusted for range. You continue 

 to bisect the target, tracking the submarine 

 movements until contact is lost during the final 

 stages of the attack. For an attack using ahead- 

 thrown weapons, the procedures are basically 

 the same, except that contact is not normally 

 lost. The sonai' operator must be alert for 

 rapidly changing bearings at short range. 



DOPPLER AND TARGET ASPECT 



When contact is made on an underwater 

 object, these questions require answers: (1) 

 What is it? (2) Is it moving? (3) In which direc- 

 tion is it moving? 



Classification steps, when executed properly, 

 give the answer to the first question. Doppler 

 helps to answer all questions. Doppler, an 

 acoustic phenomenon, is explained fully in chapter 

 4. At this time it is enough to know that doppler 

 up means the target is headed toward you; 

 doppler down means the target is headed away 

 from you; and no doppler means the target is 

 either broadside to you and neither coming 

 toward you nor headed away, or it is stationary. 

 Assume that you have contact with a submarine. 

 It has doppler, so it must be moving — but in 

 which direction? By ttus tima the ship has 

 turned to head for the target, enabling you to 

 arrive at the answer to the third question. If 

 doppler is up, the target is moving toward the 

 ship. If doppler is down, the target is moving 

 away from the ship. 



The operator's job is to determine the direc- 

 tion of target movement and report it. If the 

 submarine is headed directly at the ship, the 

 target aspect will be direct bow and doppler 

 will be up. If you are headed at the target, and 

 the target has bearing drift to the right with 

 no doppler, the target aspect is starboard beam. 



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