You are correct seaweed. It is a useless measure to keep pinging. A ping takes time to travel to and from the uboat. If the escort is too fast, the return could be off quite a bit.
If there are two escorts, one will ping, while the other passively listens. If the other escort keeps pinging, it can throw off a listener, who is trying to identify machine noises in ambient sound.
And as you said, the device is directional, so at some point the ping is useless. The "pinger" will turn off his device, and let the "passive" listener keep track, so he can advise the next D/C runner if the uboat turned or is going fast, etc etc.
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