Using Layback in Isis or SS-Logger

Layback is the horizontal distance, in meters, from the towfish to the ship's reference point (usually the ship’s antenna).

Unless you are sending a true towfish position to Isis during data acquisition, you need to account for the difference between the ship's reference position and the towfish position. This can be done using the View>Layback… option in Isis, which will write a layback value into your XTF file either using a manual entry or by reading serial data from a cable counter sensor and the towfish depth sensor. See the description for navigation template tokens:

· o (small “oh”) = cable out

· l (small “ell”) = layback

· 0 (zero) = towfish depth

A layback value can be recorded into a file during a recording session or a new value applied to a file that has already been recorded, however the second option does not affect the value that may exist in the XTF file.

It is important to note that Isis never modifies the original navigation data; instead, the layback data (if any) are simply stored in the XTF file and applied to the raw navigation when required.

All values must be in meters.

Definitions

Offset – a fixed value equal to the distance between the towpoint and the navigation antenna or ships reference point, entered manually.

Height of Towpoint – a fixed value equal to the height of the towpoint above water level, entered manually.

Cable out – a variable value equal to the amount of cable from the towpoint to the towfish. The value can be acquired automatically from a cable counter device via a serial port, or it can be entered manually by the operator.

Fish Depth – a variable value equal to the depth of the towfish. The value can be acquired automatically from a fish mounted depth sensor via a serial port, or it can be entered manually by the operator.

Layback – the total horizontal distance from the tow-fish to the navigation antenna or vessel’s reference point. The value can be acquired automatically from an external navigation computer via a serial port, or it can be calculated by Isis from Offset, Height of Towpoint, Cable out and Fish Depth, or manually input by the operator.

The Layback Correction dialog box.

Clicking View>Layback opens the Layback Correction dialog box:

In this example Isis is applying a total layback of 151.28 m to the navigation data, the value 151.28 could be coming from an external computer (via a serial port) or could have been calculated by Isis.

To override the current automatic value of Layback

Clicking the radio button Enter layback manually allows the operator to modify this value by entering the new value in the Layback window and clicking Accept:

In record mode the new, entered value of 177.00 m, is the one written to the .XTF file.

In playback mode, the Current value is the value applied to the position when creating a mosaic, or to a Target or TargetPro position. Note that entering a value here would not modify the original value in the .XTF file.

Calculating Layback from Cable Out and Other Parameters by entering values manually.

Clicking the radio button Compute layback from Cable Out, allows the operator some further options:

In this example all values have been entered manually. Note that the Cable Out, and Depth checkboxes are checked, allowing manual entry of these values. Height of towpoint and Offset values have also been entered. Clicking Apply causes Isis to use these values to make the layback calculation and display the value in the Current Value display.

In record mode these values are written to the .XTF file.

In playback mode the Current Value is the value applied to the tow-fish position while creating a mosaic or to a Target or TargetPro position. Note that entering a value here does not modify the original value in the .XTF file.

Calculate Layback from Cable Out and Other Parameters by entering values automatically.

If the information for Depth and Cable Out are being received over a serial port using the template tokens 0 (towfish depth) and o (cable out), then the dialog box would appear like this during acquisition:

Notes:

In the example, the values for Cable Out and Depth (arrowed), are being received in real time over a serial port.  As these values change the display updates automatically. The fact that they appear “grayed out” does not mean that the values are incorrect, simply that they cannot be changed by the operator.

Checking the box alongside either Cable Out or Depth allows the operator to override the automatic value, the operator’s entered value would then be saved into the .XTF file.

If for some reason, the Depth value exceeds the Cable Out value (perhaps a zero from the cable counter due to bad connection, or a large incorrect depth value due to bad depth sensor) then the Current Value box will display ERROR.

In playback mode, these values will update in the same way, provided they were recorded in the .XTF during the acquisition process.

Hypotenuse Formula:

Layback = sqrt{^2 – (D-H)^2}+ O

Where D = Towfish Depth, H = Towpoint Height and O = Offset

Calculating the layback using “Percent of cable out”

In some cases it may be preferable to have Isis calculate the layback by applying an operator-entered percentage of cable out. This method would be useful when there is no depth sensor on the tow-fish or where the along track position difference between known objects on lines running in opposite direction can be measured directly from a mosaic.

Clicking the radio button alongside Percent of cable out changes the dialog box:

In this example, a percentage of 80 is being applied to an incoming value of 94.00 m from the cable counter data via the serial port.

Notes

Depth is no longer an option, in this mode the operator enters an estimate of the percentage of cable out and clicks Apply, at this time Isis uses whatever value is present in the Cable Out, Height of Towpoint and Offset displays and calculates the layback. using the formula shown.

The Current Value is the layback value that is being either recorded to the .XTF file in record mode or applied to the position during playback.

Percent of cable out formula:

Layback = {(C-H) * P/100} + O

Where C = Cable out, H = Height of Towpoint and P = Percentage