Marco Morgado

Position USBL/DVL sensor-based navigation filter in the presence of unknown ocean currents

This paper presents a novel approach to the design of globally asymptotically stable (GAS) position and velocity filters for Autonomous Underwater Vehicles (AUVs) based directly on the sensor readings of an Ultra-short Baseline (USBL) acoustic array system and a Doppler Velocity Log (DVL). The proposed methodology is based on an equivalent linear time-varying (LTV) system that fully captures the dynamics of the nonlinear system, allowing for the use of powerful linear system analysis and filtering design tools that yield GAS filter error dynamics. Numerical results using Monte Carlo simulations and comparison to the Bayesian Cramer Rao Bound (BCRB) reveal that the performance of the proposed filter is tight to this theoretical estimation error lower bound. In comparison with other approaches, the present solution achieves the same level of performance of the Extended Kalman Filter (EKF), which does not offer GAS guarantees, and outperforms other classical filtering approaches designed in inertial coordinates instead of the body-fixed coordinate frame.

USBL/INS Tightly-Coupled Integration Technique for Underwater Vehicles

This paper presents a new ultra-short baseline (USBL) tightly-coupled integration technique to enhance error estimation in low-cost strap-down inertial navigation systems (INSs) with application to underwater vehicles. In the proposed strategy the acoustic array spatial information is directly exploited resorting to the extended Kalman filter implemented in a direct feedback structure. The determination and stochastic characterization of the round trip travel time are obtained resorting to pulse detection matched filters of acoustic signals modulated using spread-spectrum code division multiple access (CDMA). The performance of the overall navigation system is assessed in simulation and compared with a conventional loosely-coupled solution that consists of solving separately the triangulation and sensor fusion problems. From the simulation results it can be concluded that the proposed technique enhances the position, orientation, and sensors biases estimates accuracy

Tightly coupled ultrashort baseline and inertial navigation system for underwater vehicles: An experimental validation

This paper presents a new ultrashort baseline (USBL) tightly coupled integration technique to enhance error estimation in low-cost strapdown inertial navigation systems (INSs), with application to underwater vehicles. In the proposed strategy, the acoustic array spatial information is directly exploited in an extended Kalman filter (EKF) implemented in a direct feedback structure. Instead of using the USBL position fixes or computed range and elevation/bearing angles to correct the INS error drifts, as in classical loosely coupled strategies, the novel tightly coupled strategy directly embeds in the EKF the round-trip-time and time-difference-of-arrival of the acoustic signals arriving at the onboard receivers. The enhanced performance of the proposed filtering technique is evidenced both through extensive numerical simulations and with experimental data obtained in field tests at sea. The tightly coupled filter is also shown to be able to operate closer to theoretical performance lower bounds, such as the posterior Cramer-Rao lower bound, using Monte-Carlo simulations. This paper details the design and description of an USBL/INS prototype to be used as a low-cost navigation system, including the acoustic processing and positioning system, fully developed in-house. The developed system validates the usage of the proposed technique with real data in real world operation scenarios, and its enhanced performance compared to classical strategies is evaluated experimentally (median improvement level of 15% in typical operating conditions). Improved and faster convergence to nominal trajectories from multiple initial conditions, as well as enhanced accelerometer and rate gyros estimation capabilities, are also demonstrated experimentally for the new tightly coupled filter.

Embedded Vehicle Dynamics Aiding for USBL/INS Underwater Navigation System

This brief presents an embedded vehicle dynamics (VD) aiding technique to enhance position, velocity, and attitude error estimation in low-cost inertial navigation systems (INSs), with application to underwater vehicles. The model of the VD provides motion information that is complementary to the INS and, consequently, the fusion of both systems allows for a comprehensive improvement of the overall navigation system performance. In this brief, the specific VD equations of motion are directly embedded in an extended Kalman filter, as opposed to classical external vehicle models that act as secondary INSs. A tightly-coupled inverted ultrashort baseline is also adopted to enhance position and attitude estimation using measurements of relative position of a transponder located in the vehicle mission area. The improvement of the overall navigation system is assessed in simulation using a nonlinear model of the INFANTE autonomous underwater vehicle, resorting to extensive Monte Carlo runs that implement perturbed versions of the nominal dynamics. The results show that the vehicle dynamics produce relevant performance enhancements, and that the accuracy of the system is robust to modeling uncertainties.

Design and experimental evaluation of an integrated USBL/INS system for AUVs

This paper addresses the design, development, and test of an integrated Ultra Short Baseline (USBL) and Inertial Navigation System (INS) to be used as a low cost navigation system for underwater robotic vehicles. An architecture for the open prototype is proposed, the acoustic array design and calibration is discussed, the inertial sensors package is presented, supporting acoustic signals to be used are briefly enumerated, and implementation issues are detailed. The system includes also as a by-product the design of a transponder that replies to the interrogations sent out by the integrated USBL/INS system. Preliminary sea tests, conducted in an harbor, are presented to assess the feasibility of the acoustic positioning system, using namely Direct Sequence Spread Spectrum (DSSS) coded signals.

VEHICLE DYNAMICS AIDING TECHNIQUE FOR USBL/INS UNDERWATER NAVIGATION SYSTEM

Abstract This paper presents a Vehicle Dynamics (VD) aiding technique to enhance position, velocity, and attitude error estimation in low-cost Inertial Navigation Systems (INS), with application to Underwater Vehicles (UV). The VD model is directly embedded in an Extended Kalman Filter (EKF) and provides specific information about the rigid body tridimensional motion unavailable from the INS computations, thus allowing for a comprehensive improvement of the overall navigation system performance. A tightly-coupled inverted USBL is also adopted to enhance position and attitude estimation using range measurements to transponders in the vehicles mission area. The overall Navigation System accuracy improvement is assessed in simulation using a nonlinear model of the Infante UV.

Position and Velocity USBL/IMU Sensor-based Navigation Filter

Abstract This paper presents a novel approach to the design of globally asymptotically stable (GAS) position and velocity filters for Autonomous Underwater Vehicles (AUVs) based directly on the nonlinear sensor readings of an Ultra-short Baseline (USBL) acoustic system and two triads of accelerometers and angular rate gyros, from an Inertial Measurement Unit (IMU). The devised solution has its foundation on the derivation of a linear time-varying (LTV) system that is shown to mimic the dynamical behavior of the original nonlinear system, and allows for the use of powerful linear system analysis and filtering design tools, yielding GAS filter error dynamics. Simulation results reveal that the proposed filter is able to achieve the same level of performance of more traditional solutions, such as the Extended Kalman Filter (EKF), while providing, at the same time, GAS guarantees which are absent for the EKF.

Improving Aiding techniques for USBL Tightly-Coupled Inertial Navigation System

Abstract This paper presents two Tightly-Coupled fusion techniques to enhance position, velocity and attitude estimation based on position fixes of an Ultra-Short Base Line (USBL) positioning system with low update rates and the high rate Inertial Navigation System (INS) outputs, subject to bias and unbounded drift. In this framework, the vehicle interrogates transponders placed at known positions of the mission scenario to obtain a measure of distance and attitude of the vehicle relatively to the transponders. Whereas the travel time of the acoustic signals from the transponder to the vehicle is commonly approximated in the literature by half of the Round Trip Time (RTT), a method that exploits the full RTT is presented taking into account the interrogation instant estimates and the associated uncertainty. The relevance of the inclusion of the interrogation instant information and the enhanced performance outcome from the proposed techniques are assessed in Monte Carlo simulations of the overall navigation system.

Attitude Estimation for Intervention-AUVs Working in Tandem with Autonomous Surface Craft

This paper presents the design, analysis, and performance evaluation of an attitude filter for an Intervention- Autonomous Underwater Vehicle (I-AUV) working in tandem with an Autonomous Surface Craft (ASC). In the proposed framework, an ASC aids an I-AUV to determine its attitude by providing an inertial reference direction vector through acoustic modem communications, which is measured in body-fixed coordinates by an Ultra-Short Baseline (USBL) device. The specificity of the intervention mission to be carried out, near structures that distort the Earth magnetic field, renders on-board magnetometers unusable for attitude determination. The solution presented herein includes the estimation of rate gyros biases, yielding globally asymptotically stable error dynamics under some mild restrictions on the vehicle team configurations. The feasibility and performance of the proposed architecture is assessed resorting to numerical Monte-Carlo simulations with realistic sensor noise, and transmission delays and limited bandwidth of the acoustic modems.

A closed-loop design methodology for underwater transducers pulse-shaping

This paper presents a novel methodology for signal generation with application to underwater ranging systems. The proposed technique resorts to a closed loop output voltage feedback, optimal control strategy that modifies the acoustic signals transmitted by the underwater transducers, aiming to improve the detection. Given the signals selected a priori, the design of the pulse-shaping equalizer is further modified to consider a preview term associated with the reference signals available. The performance of this enhancement methodology is assessed from the results obtained with realistic simulations and through the analysis of experimental data acquired in sea trials.