DSI question

How can the DSI "see" below this cross member of this bridge?

Attachment 231963

Since the signal is shot straight down, doesn't make sense to me. Thanks.

Maybe Rickie will see this and post up his pictures of how the signal fan's out which will explain how it works.

Cray already gave you the answer. Yes, the signal is straight down, but it fans out as it goes. If we only got a 'pencil sized' beam that went down and straight back up, we wouldn't get much of a picture.

Nice pic, by the way!!

Cray pretty much answered the question. The fan shape of a DI sonar ping IS spread out left to right (side to side), and it IS shooting straight down, but it's also ~6 degrees thick .... so the leading edge of the "fan" is actually at an angle (front to back) and therefore will pick up the lack of any return signal "under" the crossmember of the bridge before it picks up the return signal bounced back from the crossmember.

Or, in other words, it actually STARTS drawing the image from the bottom up ... but, is processed so fast in the unit that it shows on the screen almost instantaneously.

At least, that's the way I understand how it can show the "empty space" under any object it passes over/by. If it was sending out a straight down razor thin line of sonar pings, everything it picked up a return signal from would look like solid bottom.

... cp :kewl

I don't think there's anything I can add to what has already been explained ...

Rickie

Quote:

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Originally Posted by CrappiePappy

Cray pretty much answered the question. The fan shape of a DI sonar ping IS spread out left to right (side to side), and it IS shooting straight down, but it's also ~6 degrees thick .... so the leading edge of the "fan" is actually at an angle (front to back) and therefore will pick up the lack of any return signal "under" the crossmember of the bridge before it picks up the return signal bounced back from the crossmember.

Or, in other words, it actually STARTS drawing the image from the bottom up ... but, is processed so fast in the unit that it shows on the screen almost instantaneously.

At least, that's the way I understand how it can show the "empty space" under any object it passes over/by. If it was sending out a straight down razor thin line of sonar pings, everything it picked up a return signal from would look like solid bottom.

... cp :kewl

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Okay thanks, that makes sense.