Underwater Acoustic Localization For Uasns And Target Based On Isogradient Sound Speed Profiles

With the improved understanding of human on marine strategic position,marine resource development,marine resource research,all countries in the world have raised their attention to the ocean to an unprecedented strategic height.China also proposes the strategic goal of building a “maritime power”.The completion of important tasks such as marine security,marine search and rescue support,and marine biological monitoring and protection is depends on large-scale water monitoring.As one of the key technologies of water monitoring,underwater acoustic localization is the focus of attentions.However,there are numerous challenges of underwater acoustic localization posed by the harsh underwater environments.Time synchronization has to be carried out due to the absence of GPS and clock differences of underwater receivers.Underwater sound speed(USS)varies with the temperature,salinity and hydrostatic pressure of seawater.The temperature is the most important factor which impacts the USS.In addition,the temperature exhibit not only seasonal variations but also diurnal variations.The combined effects of the above three factors conduce the spatiotemporal variation of USS.Therefore,there are uncertainties in the USS.The temperature,salinity and hydrostatic pressure of seawater vary with the depth of the seawater.therefore,the USS can be expressed as a function of the depth.The underwater sound ray(USR)bends according to the varying USS.The uncertainties of the receiver locations which result from the ocean currents lead to the decline of localization performance.The new distributed multi-base sonar system considered in this dissertation comprises surface buoys and an underwater sensor network.There are two types of surface buoys.The first type of surface buoys act as the “underwater GPS” and gateways,to provide services,such as time synchronization,localization and data collection,for the underwater devices.The second type of surface buoys,namely the sonobuoys,act as active sonar and provide sound source for underwater target localization.The underwater sensor network act as receiving base station in the form of cluster.The clusters of the underwater sensor network and the sonobuoys constitute single-input multiple-output(SIMO)sonar systems.In this dissertation,based on the new distributed multi-base sonar system,we carry out underwater target localization in two steps.The first step is to perform the underwater sensor localization,time synchronization and the estimation of the USS to support the underwater target localization based on the underwater localization system consisted of the first type of surface buoys.The second step is to perform underwater target localization based the SIMO sonar system consisted of a sonobuoy and and a cluster of the underwater sensor network.As the receiving based stations of the distributed multi-base sonar system,the accuracy of the locations of the clusters of the underwater sensor network relates to meaning of the collected data and the performance of the underwater target localization.In addition,the tight coupling between the time synchronization and localization of the underwater sensors requires the combination of the time synchronization and localization to improve the performance of localization.Therefore,in the first step,we propose two underwater acoustic localization and time synchronization algorithms for underwater sensors with the consideration of the uncertainties of the USS and the curve of the USR.1.In shallow water,the USS can be approximated with an uncertain constant.Based on the UALS consisted of the surface buoys,with the consideration of the constant USS and asynchronous clock,the underwater sensors perform the estimation of USS,time synchronization and self-localization utilizing the measurements of the received signals broadcasted by the surface buoys.2.In deep water,the real USS is assumed to be approximated suitably with linear USS function with respect to(w.r.t.)the depth of seawater.Based on the UALS consisted of the surface buoys,taking the unknown linear USS,the curve of the USR and the asynchronous clock into consideration,the underwater sensors perform the estimation of USS,time synchronization and self-localization based on the measurements of the received signals broadcasted by the surface buoys.The innovation of them is that the affects of the uncertainties of the USS and the asynchronous clock on the localization of the underwater sensors are considered during the process of sensor localization,the parameters of the clock and USS are estimated,and the performance of the sensor localization has been improved.In the second step "underwater target localization",the USS is assumed to be a linear function w.r.t.the depth,the parameters of the the USS function are assumed to change so slowly that the USS in the second step is the same as the USS in the first step.Therefore,in the second step,the USS is assumed to be known as a prior.With the consideration of the curve of USR,the uncertainties of underwater sensor locations and the asynchronous clock parameters,we proposed three underwater target localization algorithms for three situation.1.Assume that the center sensor(the underwater submersible vehicle)of the cluster of the SIMO sonar system need to keep silent.With the consideration of the curve of USR and time synchronization,we propose a space-alternating generalized expectation maximization for underwater target localization(SAGE-UTL)algorithm to estimate the position of the target and the clock skew of the central sensor by utilizing the statistic properties of the clock skews of the underwater sensors.2.Assume that the security of the central sensor is unnecessary.We propose a nonlinear weighted least squares based underwater target localization(NWLS-UTL)algorithm to perform target localization and estimate the clock skews of the underwater sensors with the consideration of the curve of USS and time synchronization.3.Assume that the security of the central sensor is unnecessary.We propose an underwater target localization and synchronization(UTLS)algorithm for underwater target localization and time synchronization with the consideration of the uncertainties of the underwater sensors caused by the ocean currents,the curve of USS and time synchronization.The innovation of them is that the adverse effects of the factors(asynchronous clock,the curve of USR and the uncertainties of underwater sensors)on the underwater target localization have been reduced.