Showing papers on "iRobot Seaglider published in 2019"

Preliminary Results in Current Profile Estimation and Doppler-aided Navigation for Autonomous Underwater Gliders

Abstract: This paper describes the development and experimental results of navigation algorithms for an autonomous underwater glider (AUG) that uses an on-board acoustic Doppler current profiler (ADCP). AUGs are buoyancy-driven autonomous underwater vehicles that use small hydrofoils to make forward progress while profiling vertically. During each dive, which can last up to 6 hours, the Seaglider AUG used in this experiment typically reaches the depth of 1000 m and travels 3-6 km horizontally through the water, relying solely on dead-reckoning. Horizontal through-the-water (TTW) progress of AUG is 20-30 cm/s, which is comparable to the speed of the stronger ocean currents. Underwater navigation of an AUG in the presence of unknown advection therefore presents a considerable challenge. We develop two related formulations for post-processing. Both use ADCP observations, through the water velocity estimates, and GPS fixes to estimate current profiles. However, while the first solves an explicit inverse problem for the current profiles only, the second solves a deconvolution problem that infers both states and current profiles using a state-space model.Both approaches agree on their estimates of the ocean current profile through which the AUG was flown using measurements of current relative to the AUG from the ADCP, and estimates of the AUGs TTW velocity from a hydrodynamic model. The result is a complete current profile along the AUGs trajectory, as well as over-the-ground (OTG) velocities for the AUG that can be used for more accurate subsea positioning. Results are demonstrated using 1 MHz ADCP data collected from a Seaglider AUG deployed for 49 days off the north coast of Alaska during August and September 2017. The results are compared to ground truth data from the top 40 meters of the water column, from a moored, upward-facing 600 kHz ADCP. Consequences of the state-space formulation are discussed in the Conclusions section.

A New Recommender System for Determining Trim and Flight Parameters of Seagliders

Abstract: Currently, pilots maximise the performance of Seaglider underwater gliders by selecting their set-up parameters. Building on existing procedures based on the assumption of steady-state motions, a recommender system for the trim and flight parameters is developed to aid trainee pilots and enable round-the-clock operations. The system has been validated with data from 16 missions run in waters off the United Kingdom and Australia. Whereas close agreement to the actual decisions by pilots is found for the trim parameters, the prediction of the flight parameters requires further smoothing.

Effects of Incorporating Inertial Measurements on the Localization Accuracy of the Seaglider AUV

Abstract: Seaglider is a buoyancy driven autonomous underwater vehicle (AUV) primarily used as a platform for oceanographic measurements of salinity, temperature, and oxygen, where precise localization is not crucial. A recent experiment tracked a Seaglider, instrumented with an attitude and heading reference system (AHRS), on an acoustic tracking range in an effort to quantify the accuracy of the Seaglider’s two localization solutions and determine the effects of incorporating additional inertial measurements into the solution.Preliminary analysis of results has shown the Seaglider’s glide slope model (GSM) is more accurate and reliable than the hydrodynammic model (HDM) during typical flight dynamics. Errors in the GSM solution did not exhibit a clear drift behavior but remained on average within 22 m of the ground truth acoustic track over dives with a maximum depth of 90 to 125 m. Errors in the HDM solution exhibited linear growth until the apogee point of the dive when errors began to linearly reduce. On average the error increased at a rate of about 5 m/min between the surface and maximum dive depth, with maximum errors in excess of 100 m during dives with a maximum depth of 90 m. The substitution of attitude estimates from the AHRS into the localization models provided similar but less consistent results with slightly higher errors.

Near-ice hydrographic data from Seaglider missions in the western Greenland Sea in summer 2014 and 2015

Abstract: . During summer 2014 and summer 2015 two autonomous Seagliders were operated over several months close to the ice edge of the East Greenland Current to capture the near-surface freshwater distribution in the western Greenland Sea. The mission in 2015 included an excursion onto the East Greenland Shelf into the Norske Trough. Temperature, salinity and drift data were obtained in the upper 500 to 1000 m with high spatial resolution. The data set presented here gives the opportunity to analyze the freshwater distribution and possible sources for two different summer situations. During summer 2014 the ice retreat at the rim of the Greenland Sea Gyre was only marginal. The Seagliders were never able to reach the shelf break nor regions where the ice just melted. During summer 2015 the ice retreat was clearly visible. Finally, ice was present only on the shallow shelves. The Seaglider crossed regions with recent ice melt and was even able to reach the entrance of the Norske Trough. The data processing for these glider measurements was conducted at Alfred Wegener Institute (AWI). The first part consists of the Seaglider Toolbox from the University of Each Anglia; the second was exclusively composed for the data from the Greenland Sea. The final hydrographic, position and drift data sets can be downloaded from https://doi.org/10.1594/PANGAEA.893896 (Latarius et al., 2018).

Determining Seaglider Velocities Automatically

Abstract: Direct measurement of vehicle speed along its glide path is typically unavailable1. However, pressure is continually measured providing an indication of a vehicle's vertical velocity (w). Given a model of steady flight for a vehicle in relatively still water, multi-variable regressions against the measured w can determine a set of vehicle-specific flight parameters (e.g., coefficients of lift and drag, etc.) that minimize discrepancies between predicted and measured w for various dives. These parameters are then assumed to characterize steady vehicle flight and permit estimates of horizontal and vertical velocities; any residuals presumably indicate oceanographic processes operating on the vehicle, such as internal waves, turbulence, and depth-averaged currents.

A methodology for Thorpe scaling 512 Hz fast thermistor data from buoyancy-driven gliders to estimate turbulent kinetic energy dissipation rate in the ocean

Abstract: A Kongsberg Seaglider with a microstructure package was deployed in the Faroe-Shetland Channel in 2017 as part of the 4th Marine Autonomous Systems in Support of Marine Observations (MASSMO4). Using the FP07 fast thermistor (512 Hz), the standard Seaglider thermistor (0.2 Hz) and potential density calculated from Seaglider conductivity-temperature sail (0.2 Hz) a comparison of the Thorpe Scale method has been made. Through this method turbulent kinetic energy (TKE) dissipation rates are inferred from the length-scale of a turbulent overturn. Comparison of the three physical quantities show that overturns with a comparable length-scale also have a comparable TKE dissipation rate. The range of estimated TKE dissipation rates from the 0.2 Hz data is also comparable to those inferred using the same method applied to potential density calculated from a ship mounted CTD.

Preliminary Results in Current Profile Estimation and Doppler-aided Navigation for Autonomous Underwater Gliders

Abstract: This paper describes the development and experimental results of navigation algorithms for an autonomous underwater glider (AUG) that uses an on-board acoustic Doppler current profiler (ADCP). AUGs are buoyancy-driven autonomous underwater vehicles that use small hydrofoils to make forward progress while profiling vertically. During each dive, which can last up to 6 hours, the Seaglider AUG used in this experiment typically reaches the depth of 1000 m and travels 3-6 km horizontally through the water, relying solely on dead-reckoning. Horizontal through-the-water (TTW) progress of AUG is 20-30 cm/s, which is comparable to the speed of the stronger ocean currents. Underwater navigation of an AUG in the presence of unknown advection therefore presents a considerable challenge. We develop two related formulations for post-processing. Both use ADCP observations, through the water velocity estimates, and GPS fixes to estimate current profiles. However, while the first solves an explicit inverse problem for the current profiles only, the second solves a deconvolution problem that infers both states and current profiles using a state-space model. Both approaches agree on their estimates of the ocean current profile through which the AUG was flown using measurements of current relative to the AUG from the ADCP, and estimates of the AUGs TTW velocity from a hydrodynamic model. The result is a complete current profile along the AUGs trajectory, as well as over-the-ground (OTG) velocities for the AUG that can be used for more accurate subsea positioning. Results are demonstrated using 1 MHz ADCP data collected from a Seaglider AUG deployed for 49 days off the north coast of Alaska during August and September 2017. The results are compared to ground truth data from the top 40 meters of the water column, from a moored, upward-facing 600 kHz ADCP.

Preliminary Exterior Design and Hydrodynamic Evaluation of a Small-Scale Underwater Glider

Abstract: Autonomous underwater gliders (AUGs) use complex control algorithms to navigate that balance power consumption and adherence to mission parameters. Testing of these algorithms on large scale gliders, such as Deepglider and Seaglider, requires deployments that can be labor and time intensive. We examine the feasibility of using a small-scale glider as a test platform to replicate flight dynamics of traditional gliders. In particular, in this paper, we are demonstrating the experimental determination of the coefficients of lift and drag as parameters in the glider's hydrodynamic model using three different regression methods.