The NMEA0183 $DBT (Depth Below Transducer) data string from a suitable echo sounder must be connected to a serial port on the system via a separate COM port. It is also possible to include the $DBT string with the normal navigation data, in that case there is no need change any of the navigation data settings, it should be possible to simply enable the Auto Delay function and proceed with the other settings.
Start SB-Logger and configure for data logging using either the Analog input Setup or the Edgetech FSSB setup, see this link for details of the basic setup for both navigation and sonar interfacing, check that everything is working as expected before proceeding.
NOTE: Choose the Record Length and Shooting interval with care, you should try to assess the maximum water depth that will be encountered during the survey, choose a shooting interval (ping rate) that will allow you to record the maximum expected delay time + the required record length. See below for details of the effect of setting a shooting interval that is too fast.
You must be in Acquire Mode (red button)
Click on the serial Configure button:
Use the tabs to select the COM port then choose Single Beam and SB_NMEA (Single Beam NMEA string) from the drop down. In the screen shot the navigation has been setup on COM 3 and the Single Beam Echo-sounder input on COM 4
There are a number of options available here, including Deso25, Knudsen etc. the feature should also work with these Echo-sounder types, although at the time of writing these options have not been tested.
Hit OK to close that dialog.
Hit Configuration > Auto Delay
Check the On box to activate.
The Auto Delay Change dialog has three sliders:
Upper limit - sets the upper (shallow) time limit that controls when the recording delay will be reduced as the water depth decreases. The value is a percentage of the Record Length set earlier.
Lower limit - sets the lower (deep) time limit that controls when the recording delay will be increased as the water depth increases. The value is a percentage of the record length set earlier
Filter Time - sets the shortest allowed time between automatic changes in Recording delay. In areas where the water depth is changing rapidly reduce this time to avoid "clipping" the data, in areas where the water depth is changing slowly this time should be lengthened to decrease the number of unnecessary delay changes should the water depth end up being close to a change boundary
When Auto Delay is active you will see three extra indicators in the SB-Logger signal window:
The red dashed lines indicate the upper and lower limits, the blue line indicates the position received from the echo sounder. (Effectively the position of the seabed )
As the water depth changes the blue indicator will move up and down within the red dashed lines. When either limit is crossed the recording delay is automatically adjusted in the appropriate direction. The Recording Delay is always adjusted such that the seabed appears directly in the middle of the Upper/Lower limits.
If the blue depth indicator crosses either red dashed line the indicator will turn red until the delay changes and the depth indicator again falls within the range set by the Upper and Lower limits:
Below lower limit Above upper limit
Either of the above conditions should only exist if the depth is changing quickly and may indicate that the Filter time needs decreasing.
The Auto Delay feature has the potential to make the logged SEGY data useless. If for some reason the Echo-sounder sends an erroneous depth for more than a few updates the delay may be incorrectly adjusted, resulting in loss of data. The routine includes a median filter that is run over 7 consecutive depth samples, this will remove short term gross errors and prevent the delay jumping around due to noise, also the Time filter should ensure that there are no short term variations in the recording delay. However, if the Echo-sounder should settle on a multiple, start tracking noise or stop working, it could result in data loss. As with all automated features there is no substitute for observation.
As mentioned previously care should be taken when choosing the shooting interval, if a shooting interval is chosen that does not allow enough time for the delay + record length then the following will occur when the lower limit is crossed:
The trigger from the PCI 6601 timer board is fixed and will continue generating triggers at the rate chosen during setup. Should the auto delay adjustment result in the end of the record running into the start of the next shot the trigger will be ignored, and the system will wait for the next valid trigger, effectively the shooting rate will be halved, however the data will still be valid.
Should the same situation occur with this system the illegal update will be ignored, the depth indicator will turn red and the system will keep the last valid delay setting and continue logging, the data will be allowed to "run off" the bottom the scrolling window. If nothing is done eventually data will be lost, and the Party Chief will be fired. The design of the Edgetech FSSB does not allow changes in shooting interval while logging data.