The single largest reason for ship board sound velocimeter or CTD systems is to measure and correct for surface sound velocity variations when using multi-beam or USBL systems.  In some instances, CTDs may be deployed for physical oceanography or meterological studies on research vessels or ships of opportunity.  In either case, instrumentation is most frequently either hull mounted or placed in a sea chest.

Sea chests essentially suck water into the ship via an intake pipe.  The water travels to the sea chest, where it is sampled, and it is then discharged via a second pipe.  The primary advantage of a sea chest is the ease with which sensors can be accessed, maintained, and replaced. Unfortunately, sample quality may be contaminated by temperature offsets and cavitation. 

Surface sound speed measurements should be taken at the multibeam or USBL transducer array location if possible.  Sampling at the same depth as the transducer is the next best location.  Placing the instrument in a seachest in the vessel and sucking water from the same depth as the sonar array through the seachest allows for ease of maintenace.  However the plumbing of water to the seachest must be well insulated to minimize the thermal transfer from the vessel to the water sample.  The sea-chest also slows the response time of the SSS sensor regardless of direct or indirect measurement type.  This makes the sea-chest location less than ideal for highly variable water conditions such as at river mouths.

Do not mount the sensors downstream of any obstructions to the flow of water. This can introduce thermal contamination of the water being measured. Ensure that there is access to the mounting location for sensor maintenance.

An alternative is the placement of the sensor(s) on the hull of the ship.  Such placements are implemented almost exclusively for the purposes of collecting surface sound velocity for multi-beam or USBL correction.  Sensor maintenance is more complicated, and sensors can be exposed to knocks and shocks.  However, water samples are normally taken very close to the transducer location that the sensor is meant to calibrate, and hence are highly representative of field conditions.  Data quality rises as a result.  Disadvantages are maintenance and replacement and exposure to knocks and shocks.

Regardless of which deployment option is selected - sea chest installation or transducer deployment - a ship board sensor will have the following critical requirements:

Unlike vertical profiling or platform systems, ship board sensors are often deployed and left for significant periods of time.  As a result, biofouling is likely to occur.  CTDs should be deployed with a biofouling control mechanism OR placed in a accessible location where maintenance is relatively straightforward.  Fortunately, time-of-flight sound velocity sensors are not as susceptible to biofouling as are CTDs.

Physical protection is not particularly relevant to sea chest deployments.  However, for sensors deployed on the hull of a ship, such protection is critical. Time-of-flight sound velocity sensors can easily withstand the impact of moving water but should have a protection cage to protect the sensor from impacts with hard objects or ice. 

The deployment platform itself - AUV, ROV, MVP or glider - can affect the quality of the water surrounding the sensors.  Types of contamination to watch for include:

• thermal shedding from the vehicle
• cavitation from thrusters or diving planes

Sensor positioning on the vehicle or platform is critical to avoid these types of contamination.  Obstructions upstream of the sensor should be avoided.  Sensors should be located outside of the boundary layer of the vehicle or platform. If ducting is used to protect the sensors, consideration must be given to the possibility of thermal shedding.

Bubbles generated by cavitation will introduce noise to some sensors.  Data spikes may result.

There are two areas of concern in terms of maintaining adequate flow of water over sensors: boundary layer and stagnation points.  Boundary layer is a layer of water along the side of any moving object which is not representative of the ambient water conditions.  The thickness of the boundary layer is dependent upon the speed of the object moving through the water and the distance from the leading edge of the moving object. 

Stagnation point is an area ahead or behind a moving object where the flow of water around the body is zero, that is, the moving object may be pushing or pulling a volume of water with the moving object.  This volume of water will not be representative of the ambient water conditons.    

Practically, the implication of these two areas of concern are as follows: 

• sensors must not be placed in the boundary layer of an instrument or vehicle
  
• sensors must not be placed in the stagnation point of an instrument or vehicle

Electrical noise has the ability to impact underwater sensors and the accuracy of collected data.  Of particular concern:

• noise from the power suppply to the sensor
• noise from slip rings on winches
• electromagnetic interference (EMI) radiated from the platform to which the sensor is mounted

Sensors should be mounted away from any sources of EMI noise, such as motors.  Slip rings on winches should be well maintained to avoid arcing.  Power supplied to the instrument needs to be within the specified voltage range and should have an RMS noise of less than 20 mV.

In any underwater application - particularly seawater applications - corrosion plays a large role in sensor performance and longevity.  Of primary concern is ensuring that the sensor does not become the sacrifical anode for a larger platform or vehicle.  Also of concern is any coupling of the platform or vehicle's cathodic protection to the sensor. Both of these concerns can be eliminated by galvanic isolation of the sensor from the platform.

Depending upon the instrument, the sensors, and the system to which they are being integrated, data output requirements will vary.  While it is impossible to state what system requirements are for all such systems, the best instruments are capable of flexible outputs.