The SV or CTD probe is dropped behind the ship and gathers data while it is in freefall. The cast data are logged on an internal datalogger. As the probe drops, high strength tether line is simultaneously paid out from the winch on the moving ship and also from a special tail spool attached to the probe. The tail spool is pre-loaded with line before deployment using the reloader (step #3, below). The line paying out from the probe tail means that the falling probe does not have to pull any line downwards behind it, and achieves a fast descent rate under these low drag conditions. The probe drops vertically for the full cast depth, much like an expendable (XBT, XCTD) probe and does not follow a "sawtooth" path as is the case with towed profilers. The drop speed is independent of the ship speed, and will remain constant at about 4m/s (CTD) or 5.5m/s (SV 700) until all of the line has been paid out from the probe tail spool.



When the downcast is complete and the probe reaches the desired depth, the brake is applied on the main winch and the probe is recovered using the high speed winch. The probe will rise vertically until reaching 50-75m from the surface, when it starts being dragged forwards by the ship. The probe will reach the surface several hundred meters behind the vessel while underway at 10kts, making it easy to see during recovery. If the system is in use simultaneously with towfish such as a sidescan sonar or sub-bottom profiler in the water behind the ship, the path of the UnderwayCTD or UnderwaySV probe is such as to pass over the towfish and tow cables on retrieval. As a result of the tension in the line as the probe is being recovered from the moving vessel, the probe stays in a fairly narrow lateral corridor behind the ship notwithstanding the effect of water currents.

Deployment and recovery of the CTD or SV probe The probe is dropped over the aft rail and allowed to freefall until the target depth is attained, at which point the winch is reversed and the probe recovered.

To obtain deep vertical casts while underway, the removable tail spool on the CTD or SV probe is first loaded with high strength line from the main winch using the reloader. The main winch is rotated so the arm is inboard, and the tail spool is disconnected from the probe by a special quick release coupling. The tail spool is then placed on the reloader and line is automatically loaded to meet the desired cast depth. When the line loading is complete, up to 700m of line is present on the tail spool, with up to 1300m remaining on the main winch. The tail spool complete with line is then re-attached to the probe and it is ready to be deployed. The winch is rotated so that the arm is directly aft, and then the probe is ready to be dropped into the water for the next cast.

Loading tether line onto the probe tail spool using the reloader The first step to prepare the probe for the next cast is to load up to 700m of high strength tether line on the special tail spool of the SV or CTD probe. As the probe falls through the water, this line is paid out.

UnderwaySV
When the UnderwaySV probe reaches the surface, the Bluetooth antenna is switched on and the latest profile is automatically transferred to the survey computer when the probe comes in range. An external Bluetooth antenna is supplied for the survey computer to enable communications with the probe on the aft deck even if the computer is up to 50m away. No operator involvement is necessary, and the process takes about 30 seconds to complete. Alternate sampling modes, such as continuous recording may be selected if required.

UnderwayCTD
The operator can choose to reset the probe and perform another profile immediately after the probe is recovered to the ship, by simply inserting and removing the magnetic switch plug, or download the cast data. Up to 100 profiles may be stored on the UnderwayCTD probe before downloading is required. To download the data, the UnderwayCTD tail spool is removed and then Bluetooth communications can be established using the dedicated UCTDTerm software followed by a convenient file downloading procedure. The operator can choose to use a portable / pocket PC to download the data while on the aft deck or take the probe into the laboratory to download the data onto a PC.




To perform profiling up to 250m depth with the UnderwayCTD, and up to 400m depth with the UnderwaySV 400, the systems are used in a simple operation mode that does not require line to be loaded onto the probe tail spool. The probe is simply dropped behind the ship and allowed to freefall while line is pulled off the winch. The probe will follow a vertical path at the drop location even as the ship moves away. As more line is paid out, the vertical drag of the tether line increases which results in a gradual slowing of the probe as it pulls an increasing length of line down through the water with it. This effect is eliminated with the deep profiling mode described above using line wound on the tail spool. Despite the probe slowing down with increasing depth, the descent rate is consistent between deployments and simple dive tables are used to calculate the required drop duration for the range of target depths.

For deployment in less than100m of water, such as in estuaries or shallow coastal regions, the shallow mode of operation described above can be used with a special tail spool attachment that retards the probe drop rate to about 1m/s. At this drop speed, profiles can be safely gathered in shallow water environments.

Using the UnderwaySV 400 in shallow profiling mode Without using the tail spool reloader, fast and easy shallow water profiles can be obtained with either the CTD or SV systems.