The squat and lift of a vessel changes as the speed changes. For some vessels, if the squat/lift is not accounted for, significant errors are introduced into the soundings. In this section, you can specify up to 10 speed-draft value pairs. During merge, the difference between the instantaneous draft and the static draft is computed and the final depth compensated.
Draft is measured in metres/feet, and speed is measured in knots. All draft values entered should be relative to the same reference, but what that reference is, is not important. The first Speed-Draft pair must correspond to a state where no draft correction is necessary. The delta draft values, relative to the initial draft, are computed and used to correct observed soundings.
The following formula is used to correct soundings for dynamic draft:
Depth = observed depth – waterline + delta draft.
Delta draft is computed from the present vessel speed, as derived from the navigation system.
You can also create your own dynamic draft model and load Delta Draft information directly which will override the Dynamic Draft table in the HVF. (See “DELTA DRAFT” ONPAGE172.) 1. Click Dynamic Draft in the sensors list box so the selection is highlighted
and the dynamic draft data fields are displayed.
2. Type data (as needed) in the following fields:
• Date: The year and Julian day of the current time stamp for the draft table.
• Time: The hour and minute of the current time stamp for the draft table.
• Apply: Select Yes to apply the table during merge (or No to not apply the table).
3. Click Edit in the Error field. A dialog box containing a table with up to 10 speed-draft pairs is displayed.
Note: Simrad systems apply dynamic Heave, Pitch, and Roll values to the swath data during survey. Therefore, the HVF must be set up with the Apply switches for Heave, Pitch, and Roll set to '”No” so these values are not applied twice.
Create Vessel File: Sensors
4. Click inside the Speed cell and type a speed value (in knots).
5. Click inside the Draft cell and type a corresponding draft value.
6. Continue entering as many speed /draft pairs as needed.
7. Click OK.
The speed-draft table is saved in the HVF.
SVP
A sound velocity profile (SVP) records the speed of sound at various depths in the water column. HIPS enables you to apply SVP data in sound velocity corrections. However, this procedure is not available for all types of sonars because some sonar data logging systems compensate for sound velocity during data acquisition.
To ensure the SVP is accurately applied in HIPS, the transducer X-Y-Z offset values must be entered.
1. Click SVP 1 in the Sensors list box so the selection is highlighted and the SVP transducer data fields are visible. If there is a second transducer then click SVP 2 and complete the same procedures as listed below.
2. Type data (as needed) in the following fields:
• Date: The year and Julian day of the current SVP pole time stamp.
• Time: The hour and minute of the current SVP pole time stamp.
3. Select Yes if there are dual transducers (or No if there is only a single transducer).
The following three fields are for entering the pole’s X-Y-Z coordinates. All coordinates are measured from the Reference Point.
4. Type the transducer’s X-Y-Z offsets in the following fields:
Create Vessel File: Sensors
• X: The athwart-ship distance of the transducer, positive to starboard.
• Y: The along-ship distance of the transducer, positive to the bow.
• Z: The vertical distance of the transducer, positive into the water.
The next three fields refer to the alignment of the transducer.
The values you entered in the wizard when creating the HVF are displayed in these fields. The fields are only to be used for large transducer mounting offsets.
• Pitch: Pitch offsets of the transducer.
• Roll: Roll offsets of the transducer.
• Yaw: Rotation of the transducer (either 0 or 180 degrees).
See also “SOUND VELOCITY CORRECTIONS” ONPAGE33.
To apply SVP in processing see “SOUND VELOCITY CORRECTION” ON PAGE133.
Sweep
Sweep systems typically contain multiple vertical beam
transducers mounted on a boom and pointed straight down. The reference point for a sweep system can be anywhere on the X-Y plane, but must be on the water surface.
1. Click Sweep in the Sensors list box so the selection is highlighted and the data fields are visible.
2. Type data (as needed) in the following fields:
• Date: The year and Julian day of the current sweep time stamp.
• Time: The hour and minute of the current sweep time stamp.
• Time Correction: The time correction value.
• Transducers: Number of beams mounted on the boom.
3. Click the Edit button in the Transducer Status field to view a table for entering data for individual beams on the boom.
Note: The Reference Point for Simrad data is the centre of rotation. It should be possible to retrieve offset values from the Simrad Installation Datagram.
Create Vessel File: Sensors
4. Type values in the fields and click OK.
5. Click Edit in the TPU Status field to enter TPU values for each beam on the boom and to edit values entered in the new vessel wizard.
6. Type values as needed and click OK.