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How to clean up the SRC 'Deadzone?

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Fishton:
I have been trying to get a 'fix' for the dead zone where the water column has been removed by the Slant Range Correction.

We know that the sonar beam is not just a 90 side to 90 down, so why can't each side collect just a few feet more, where left overlaps slightly into right and right slightly into left?

This will allow the SRC to clean up that fuzzy dead zone where the left and right mosaic meet, thus removing that unsightly 'trail'.

To remove this with a photo editor and only use a single side to 'tile' the mosaic is just too time consuming, especially on large bodies of water.

Have been asking Tore (support@sonartrx.com) for a few years now for any help regarding this matter but to no avail.

Has anybody got any ideas regarding this?

Regards
John

Rickard:
Hi John,

As far as I know there is no reliable way to clean up SI data from the sector right below the transducer. Firstly, a sonar image is primarily composed of shadows, but in that direction it's almost impossible to get any shadows. Secondly, all positions on the surface in that sector is located at nearly the same distance from the transducer, so the raw image is extremely compressed. Thirdly, since a SI channel collects some data from the opposite SI channel's area, using it's own transmitted signal, those data are mixed with data from "it's own" side. The transducer doesn't know from which direction echoes arrive, it just records time and intensity. There is no way that I'm aware of to separate data afterwards. A photo editor doesn't help, because the image can't show anything to cut away because the mixed data are displayed at the same place. Slant range corrected images will always be blurred below the boat.

Regards,
Rickard

Fishton:
Hi Rickard,

What you said makes total sense, I must confess, I forgot all about the shadow.

My intension now is to place the outer pontoons on my trimaran mapping craft around 10ft apart, with the left scanning transducer on the right and the right scanning transducer on the left.

As my mapping is shallow water only, I believe this will create just enough of a shadow to cleanup the SRC.

Then surely just a small software tweak will put it all together quite nicely?

Regards
John

Rickard:
Yes, this will give better shadows from the area below the middle of your trimaran in each channel. But the channels will also interfere with each other, if they use the same frequency. To avoid this you could locate one channel at the bow and the other at the stern. But then the channels will recode data from a specific position at different points in time, so that time lag must be compensated to make it possible to join the left and right images. After slant range correction the worst part of the "dead zone" can be removed. Just cut away a strip equally wide as half the distance between the left and right transducer from each image. But, the critical point in this process is compensation of time lag. I think this is a hard thing to solve for the software people. Speed and position data must be very precise to make it meaningful.

/Rickard

Fishton:
I record off a single unit with the GPS puck dead centre and above the P66 200kHz 9 cone on the centre hull stern.

Then for side I use two transducers but wired to the same single plug the P66 uses, so the unit still thinks it is using a single transducer.

This will not be difficult to correct on SonarTRX, all it needs is to process the left and right separately but allowing a 'Crop' at the beginning of the mosaic, not just the end as it currently has.

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