Remotely Operated Vehicles (ROV) and Autonomous Underwater Vehicles (AUV) require knowledge of the amount of water beneath them (altitude) as well as their depth below the surface so that they can be positioned at the optimum height for their onboard survey sensors and cameras, to position measurements taken accurately in a 3D space, and as part of hazard avoidance. In an AUV, the measurements are used internally by onboard software to help maintain the intended 3D path. ROV increasingly have high levels of self-guidance to reduce the load on human operators, and again measurement of altitude is critical in this regard.
High resolution, linear surveys such as those undertaken for cable or pipeline route mapping or inspections may use sensors mounted on a towbody attached to the vessel by a power and data cable. High accuracy positioning of the towbody requires a combination of many measurements in order to provide reliability and increased confidence. Acoustic tracking systems that are mounted on the tow vessel (such as Ultra-Short baseline or USBL) can suffer when the geometry of the situation results in relatively faint USBL signals from the beacon on the towbody being lost under the noise of thrusters or propellors or when they are reflected from different layers of the thermocline, especially in shallower waters.
Acoustic tracking systems do not emit a signal continuously but instead there is a “call and response” between the hardware on the vessel and the towbody at intervals (typically referred to as the update rate). At short range, the update rate can be increased so that the navigation software will have a frequent measured update of the towbody position, and the occasional loss of acoustic signal will not have a significant effect on the quality of the calculated position. However, longer tow ranges necessitate a slower update rate and the navigation software predicts the position of the towbody between the received measurements. The longer the interval between “good” measurements, the less reliable that prediction will become, sometimes to an unacceptable level. Altitude and depth sensors in the towbody provide the navigation system with data that can help calculate the 3D-position of the towbody, and may allow for short periods of poor acoustic measurement performance to have less impact on the progress of the survey being performed.