The glider guidance system (ggs): a model-guided path-planning program for advancing glider operations through depth-averaged ocean currents
Creators: name: Fricano, Salvatore Thomas
Publisher: name: No Publisher Supplied
Year: 2024

description: The Global Ocean Observing System (GOOS) is assessing the use of autonomous underwatervehicles (AUVs) to provide a sustained sampling presence underwater. One class of AUVs arebuoyancy-driven gliders which can conduct long-duration missions at sea under the control of shoresideoperators (Schofield et al. 2007). Gliders utilize pilot-programmed missions to sample target areas of theocean as they fly in a sawtooth pattern beneath the waves, typically down to depths of 1000 meters. Atsea, gliders are subject to advection by ocean current forces which can impact their flight path. Given thatbuoyancy-driven flight mechanisms yield slow horizontal speeds over ground (approximately 20 cm s-1), strong currents can be particularly problematic. Therefore, knowing ocean currents conditions around anoperating environment is vital for the success of glider operations. Surface currents alone do not representthe net hydrodynamic forces exerted on a glider, as it will experience the water column average current.Due to a limited number of existing glider flight tools, developing modern operational methods to provideoperational forecasts of modeled depth-averaged ocean currents would be of great value. To improve glider operation confidence, we have developed the “Glider Guidance System”(GGS) - a Python-based program that computes 2-dimensional depth-averaged current (DAC) datasetsfrom existing 3-dimensional ocean models (RTOFS, CMEMS, and GOFS). To correctly calculate DACdata from ocean model outputs, which provide unevenly spaced data points over depth, GGS utilizes a1-meter resolution bilinear interpolation method to calculate data points for each meter of depth (figure1). Furthermore, GGS employs a user-configuration file which defines information about the glidermission (such as maximum working depth and mission extent) to automatically compute DAC for themission and subsequently provide real-time data visualization products. GGS has been used to support active glider operations contributing to the National AcademiesUnderstanding Gulf Ocean Systems (UGOS) Initiative. Over the course of the UGOS project, mappingproducts such as magnitude contour plots and threshold zone contour plots (depicting regions ofprominent DAC fields which approach and surpass the average horizontal speed of a glider) wereproduced daily to assist in the identification of zones which were both safe and optimal to fly gliders inreal-time (Figure 2). Beyond UGOS, GGS is being used to identify the optimal path for the SentinelMission - the first circumnavigation mission by an AUV, championed by the glider “Redwing”. UsingDAC to identify advantageous current fields for Redwing to fly through allows pilots to confidentlydefine target waypoints for the mission which maximize battery efficiency and minimize risk. Elevatingthis capability, GGS also contains a pathfinding algorithm which calculates the most time-efficient path between waypoints in DAC grid. This optimal path is further used to calculate the distance, effective glider speed over ground,and estimated duration for the mission. Insights related to DAC and derived estimations for AUV speed and transit time are critical forincreasing operational confidence. Providing this information from multiple ocean models additionallyincreases the capacity for pilots to identify clear regions of uncertainty (disagreement between oceanmodels) which serves as a proxy for areas that may be unreliable; even under circumstances whereconditions seem mild. As gliders continue to grow as a core oceanographic observation platform, thedevelopment of tools such as GGS will play a vital role in ensuring that users have the capacity toperform remotely piloted research missions with the support of representative real-time data.