Passive Ballasted Floats for Observing Interior Ocean Flows and More

The Navy has identified a need to observe submesoscale interior ocean velocities down to 500 meters depth, across regions of 105 square kilometers, and over timescales of one month. ARMADA Marine Robotics has assembled a team of experts to address this need with Phase I feasibility study to specify the requirements for and simulate the performance of passively-ballasted floats based on the TideRider profiling float developed at the Woods Hole Oceanographic Institution (WHOI). TideRider significantly de-risks this approach by validating the feasibility of Passive Ballast as a simple, functional, and cost-effective technology where a density change is actuated by the platform’s propulsion rather than an actively pumped ballast system. A Passive Ballast system uses ambient hydrostatic pressure to change a platform’s density in a controlled way, enabling that platform to be positively buoyant at the surface, neutrally buoyant at a tunable depth, and negatively buoyant below that depth. The depth and rate at which this density change occurs is determined by the relative sizes of two volumes: a compressible volume, governing the absolute density change, and a non-compressible volume, governing the rate of density change as the first volume collapses, akin to a spring constant.  We will partner with WHOI developers of TideRider who are also co-inventors of the Passive Ballast concept that underlies its simplicity. In Phase I, we will demonstrate the feasibility of two variants that address surface-permissive and surface-denied environments, respectively. First, we propose a deeper, streamlined TideRider variant that periodically surfaces to relay its GPS position, and possibly sensor data, via satellite. It can be likened to a “faster” ARGO float with a closed-loop thruster controller designed to reach the neutrally buoyant depth with minimal damping and overshoot. This reduces errors in current measurements introduced by shear during water column transits. In the open ocean, surfacing to relay GPS position presents a simple, atomic, infinitely scalable and operational cost-effective alternative to solutions with acoustic payloads that transmit (eg. SOFAR) or receive (eg. RAFOS) acoustic signals from ship-deployed moorings. Second, we propose a float variant deployable by an Unmanned Underwater Vehicle (UUV) based on ARMADA’s External Payload Delivery System (EPADS). Upon receiving an acoustic command, the float  releases itself from the UUV while closing off the passive ballast from ambient pressure and locking in its current density. For the remainder of its battery life, the float behaves like a drifting acoustic transponder, replying to interrogation pings with a unique response so it can be independently tracked by one or more UUVs. In regions where surfacing is not an option, such as under ice, an ability to both deploy and monitor floats at large horizontal offsets conveys a distinct operational advantage.