１７．曳航体、ROV（Towfish - ROV）
Q. 曳航体用ケーブルの使用(Using a Towfish with the Cable Counter)[英語]
This note briefly describes how to use the cable counter driver to calculate an approximated position for a tow fish.
The cable counter driver can be used either with automatic cable counter devices (currently it supports two formats: Dynapar and MD Totco) or in manual mode (in which case the operator has to update the cable length).
The fish can have either a pressure transducer that outputs the depth of the fish or an altitude transducer that provides the height of the fish above the bottom. Of course it can also be a "dumb" fish without any sensors installed.
In calculating the fish position we make the following assumptions that we know are wrong:
- There is a straight line from boat to fish (i.e. the tow fish is linked using a pole rather than a cable)
- Water depth under the boat and under the fish is the same (you are traveling over an unusually flat bottom)
We have to make these assumptions because otherwise you would have to provide a lot of information regarding the fish and the cable (stuff like drag coefficients, specific weight, etc) and we would have to solve some complicated differential equations. This way, things are a lot simpler for everybody and the only drawback is that the fish might not be where you see it on the screen.
With these assumptions in place the magic layback formula is:
Y_offset is the distance from boat's reference point to the A-frame. It is positive when A-frame is astern of reference point.
corrected_depth is given by the formula:
corrected_depth = fish_depth + Z_offset
Z_offset is the height of the A-frame above water level and it is positive when the A-frame is above the water (it is cumbersome to place the A-frame under water)
- corrected_cable is calculated from the cable length using the following formula:
corrected_cable = catenary_factor x cable_length
It can be seen from these formulas that once user enters a cable length (or we get a reading from the cable counter) we can calculate the layback if we know the tow fish depth. The driver setup box provides four options for determining the fish depth:
- Shallow fish-- the fish depth is always 0 (this is more of a raft than a fish)
- Depth sensor -- the fish is fitted with a pressure transducer that sends out directly the fish depth. You have to interface this device using any of the available echosounder drivers (try using the generic one). You must also configure the mobile associated with the fish to use that device as depth sensor.
- Altitude sensor -- some side-scan sonars provide the fish altitude above the bottom. In this case you have again to configure another echosounder device to provide this information. In addition you have to have a second echosounder that provides water depth for the boat (anyhow it is safer to have an echosounder on board while towing a fish at least to know how deep your diver has to go to rescue the fish). The cable counter driver calculates the fish depth as the difference between the water depth under the boat and the fish altitude (here comes into play that second assumption that the bottom is flat).
- Deep fish - your fish is extremely heavy and hangs straight under the A-frame. In this case the layback is always equal to Y_offset.
A note regarding the offsets set in the "Offsets" dialog box: they represent offsets on the tow fish between the cable anchoring point and the reference point of the tow fish. If you choose the origin of the side scan as reference point on the tow fish and if you care about the difference between the anchoring point and the side-scan origin (after the wild approximation of the rigid pole) than you can enter some values here. However, as a general rule, I would suggest to leave them to 0.