Bottom Detection page
The purpose of the Bottom Detection settings is to define the upper and lower depth limits most likely to be used during normal operation.
How to open
The page is opened in the Information Pane Options dialog box. To open, select the button on the Active menu.
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You can also open the page by selecting Setup in the information pane.
Prerequisites
The Information Pane Options dialog box is only available when an Inspection view is active.
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Description
Difficult environmental, water or bottom conditions can occasionally prevent a bottom lock. The SN90 system needs this bottom lock to locate the correct depth, and to stay on this depth during operation, even if the depth changes continuously. The Bottom Detection settings are provided to rectify problems with bottom lock.
The Bottom Detection settings provide separate limits for minimum and maximum water depth. These depth limits can be used to obtain a bottom lock on the current depth when the SN90 system transmits.
Tip
If you have problems with bottom detection, you may consider disabling it. This can be useful if you only wish to study targets in the water column.
Details
Minimum Tilt for Bottom Detection
Minimum Tilt for Bottom Detection permits you to set a tilt value that makes the SN90 system stop the bottom detection.
The system’s ability to detect the bottom depends on the tilt angle. A beam that points straight down detects the bottom better than a beam that hits the bottom with an angle.
•  A large tilt value makes the SN90 system detect the bottom with high accuracy. The depth is then also accurate.
•  A small tilt value reduces the bottom detection accuracy, and thus also the accuracy of the depth.
Maximum Depth
The search for the bottom echo extends down to this depth whenever bottom track is lost.
Use a slightly larger depth value than the deepest spot you expect to visit. This will prevent annoyingly long ping intervals every time the bottom track is lost. A depth value of either 0 (zero) or less than the minimum depth disables the bottom detector.
Note
If you set maximum depth to a value identical or smaller than the minimum value, the bottom detection algorithm will be disabled. The SN90 system will not detect the bottom at all, and the displayed depth will be 0.00 m.
Minimum Depth
The bottom detector starts the search for the bottom echo at this depth. The detector will fail in shallow water if you select a too large depth value, and the tail of the transmitting pulse may cause problems if a too small value is set.
Note
If you set maximum depth to a value identical or smaller than the minimum value, the bottom detection algorithm will be disabled. The SN90 system will not detect the bottom at all, and the displayed depth will be 0.00 m.
Bottom Backstep
Bottom Backstep allows you to manually modify where on the bottom pulse the depth shall be detected. The setting does not have an effect on the ability to detect and track the bottom.
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A The peak of the bottom pulse
B -50 dB is the default bottom backstep level
The bottom pulse basically identifies the bottom depth just prior to the peak of the pulse (A). However, this may not be the true bottom. For example, if the bottom pulse is generated by a rock bottom under a thick layer of mud, the actual depth is slightly shallower. For this reason, the SN90 system is by default set up to give you a depth reading a few milliseconds before the peak of the pulse. This is done by setting the Bottom Backstep level to a default value of -50 dB (B).
The actual bottom will never appear at the peak of the bottom pulse (A), it will always be slightly before the peak. By changing the Bottom Backstep value you can detect the bottom earlier.
Show as true Depth
Show as True Depth specifies how the depth is measured. This can either be the horizontal depth (True), or the depth along the tilt angle (Slant).