ecoSUBµ-5 has a 500-metre depth capability, and target endurance of 48 hours with the ability to carry a growing range of highly capable sensors. An important design requirement was that the AUV should maintain a “cylinder rule” i.e., had no projecting control surfaces or antenna which would limit its capability for tube launch from a surface, underwater or sonobuoy tube. This is achieved by using a moving mass within the vehicle to control the pitch angle. Using this technique also allows the combined antennas and location beacons, which are coaxial to the main vehicle body, to be adequately exposed for communications and GPS positioning at the surface. When the mass is fully forward, the vehicle can effectively stand on its nose. An added advantage of this is the ability to dive at extremely steep angles providing a unique profiling capability.
As part of the design process, Planet Ocean sought early engagement with sensor manufacturers to encourage them to consider deployment of their sensors on micro AUV platforms with all that this entails in terms of size, power and weight. An early adopter of this philosophy was Valeport Ltd. Based in the South West of England, Valeport has an enviable reputation for producing high-quality sensors designed and manufactured in house with the inherent ability to adapt quickly to meet the needs of potential end-users such as ecoSUB.
One of the many potential applications of micro-AUV technology identified by the market study was an alternative to the XBT (expendable bathythermograph). The XBT has been used by oceanographers for many years to obtain information on the temperature structure of the water column. Adopted later by naval forces and others to locate the thermocline and by adding conductivity sensors to facilitate calculation of speed of sound for sonar calibrations.
The XBT, is dropped from a ship and measures the temperature and conductivity as it falls through the water. Two very small wires transmit the temperature data to the ship where it is post-processed to provide temperature-salinity profiles. The probe is designed to fall at a known rate so that the depth of the probe can be inferred from the time since it was launched. By plotting temperature as a function of depth, the user can get a picture of the temperature and salinity profile of the water column. Using standard formulae speed of sound can be calculated from this information. Although extremely useful XBT technology has a number of limitations. First, data is provided only directly below the deploying platform (ship), second, important data such as depth and salinity are calculated or inferred rather than measured, and third XBT’s are one-shot devices providing a single profile per XBT. These limitations are addressed using ecoSUB technology combined with a time-of-flight sound velocity sensor provided by Valeport.
Using an ecoSUB-µ5 combined with a specially adapted Valeport SV (sound velocity) sensor provides a real-time, physical measurement of the speed of sound, accurate depth information, and the ability to make multiple SV profiles at several locations autonomously. By clever data compression techniques, ecoSUB can deliver a full sound velocity downcast with speed of sound, depth and lat/long every metre through 290 metres in a single Iridium Short Burst Data (SBD) message before being deployed to a new location to repeat the process. Of course, this can be extended to 500 metres with up and down casts using multiple SBD messages or adjusting the interval from less than 1 metre to more than 1 metre as required in the mission plan. This provides decision-makers with a very detailed 3D map of the underwater acoustic environment over a wide area in near real-time and in advance of the host vessels arrival at the location. Making an accurate physical measurement of sound velocity reduces errors inherent in calculating sound speed using multiple sensors each with its own error band.
The Valeport “time of flight” SV sensor provides an impressive performance:-
- Genuine Accuracy of ±0.02 metres per second (Total Error Budget)
- Precision (peak to peak noise) of ±0.002 metres per second
- Operating range of 1375 metres per second to 1900 metres per second (covering all environments from freshwater to the Dead Sea and the Marianas Trench)
- Data rates up to 60 Hertz (instrument dependent)