The performance of an ultra-short baseline (USBL) positioning system is limited by noises and errors from physical environment and other sources. One of the major errors in USBL positioning is to neglect the sensor misalignment which produces static yaw, pitch, and roll offsets. In this study, a circular survey observation scheme is first proposed to study the positioning errors of a USBL system with a fixed seabed transponder. The center of the circular survey scheme is assumed to be located over the top of the transponder. Mathematical equations of the transponder positioning with yaw, pitch, and roll offsets are derived, respectively. According to characteristics of positioning errors arose from yaw, pitch, and roll offsets, an iterative procedure of first getting roll offset, next computing yaw offset, and then obtaining pitch offset for sensor misalignment correction is established. Simulation results indicate that the iterative procedure can effectively obtain all offsets with high determination accuracy and the computation can rapidly converge to desired error tolerance in a few iterations. However, the center of circular vessel survey scheme is almost impossible to be exactly located over the top of the transponder. In such a case, the horizontal positioning error resulting from pitch offset or roll offset is no more a circle function. As a result, it will fail to evaluate the angle offsets through above iterative procedure unless the deviation from real and estimate horizontal transponder position is extremely small comparing to the transponder depth. Therefore, in addition to circular survey scheme, this study proposed a straight survey scheme to study the patterns of positioning error resulting from yaw, pitch, and roll offsets. Similar to the philosophy of establishing the iterative procedure described above, the iterative procedure of first getting pitch offset, next computing roll offset, and then obtaining yaw offset for sensor misalignment correction is established. Again, simulation results show that the iterative procedure can find all offsets with high determination accuracy and has the advantage of quick converging. Besides, the iterative procedure can still obtain correct angle offsets even though there is a constant heading deviation from the direction of the straight vessel track during vessel survey.

Positioning an underwater object with respect to a reference point is required in diverse areas in ocean scientific and engineering undertakings, such as marine habitat monitoring, study of sedimentation processes, underwater searching and mapping, data collection, instrument placement and retrieval, and so on. Underwater acoustic positioning systems, including long baseline (LBL) systems, short baseline (SBL) systems, and ultra-short baseline (USBL) systems, are designed to operate from a reference point and employ external transducers or transducer arrays as aids for positioning. Traditional positioning methods rely on measuring of time-of-flight of an acoustic signal travelling from the target to the reference platform by means of the cross-correlation method. The positioning accuracy of LBL systems varies from a few centimeters to a few meters, depending on the operating range and working frequency. LBL systems provide a uniform positioning accuracy for a given transponder array setup, but they suffer the time-consuming instrument deployment on the seafloor, as well as the complicated operating procedures. SBL and USBL systems have relatively simple configurations. But their positioning accuracy is a function of water depth and operating range. To obtain absolute position accuracy, additional sensors such as the ship's gyro or a surface navigation system are needed. In this thesis, a novel positioning method is proposed which takes advantages of a tether cable between the reference platform and the target. This method conducts positioning via continuous phase measurement between a reference signal and the acoustic signal transmitted by the target to the reference platform. It is named the Positioning-based-on-PHase-Measurement method or PPHM method in short. Every 2π change in the phase difference between these two signals corresponds to a one-wavelength range increment along the radial direction from the target’s initial position to its new position. If a receiver array is used, with at least two hydrophones, the target’s bearing information can be also calculated by measuring the phases of the output signals from each of the array hydrophones. Under ideal conditions, the positioning error of the PPHM method is proportional to the phase measurement error. The PPHM method is very sensitive to changes in the underwater medium, such as sound speed variations, ocean currents and multipath interferences. Environmental fluctuations will degrade the positioning performance. These problems will be investigated and solutions will be proposed to minimize their effects. The PPHM method can be used to position an underwater moving object such as a remotely operated vehicle (ROV) or a bottom crawler. Also, it can be used to monitor the ocean currents speed variations over a path, or to monitor the movements of tectonic plates. The last two applications will be addressed in detail in this thesis, whereas the first one is very challenging and needs more work.

Le positionnement d'objets sous-marins représente un enjeu stratégique pour des applications militaires, industrielles et scientifiques. Les systèmes de positionnement reposent sur des signaux de type SONAR « Sound Navigation and Ranging ». Plusieurs émetteurs synchrones avec des temps d'émission connus sont alors considérés, l'objectif étant que la position d'un récepteur se fasse en fonction des positions des émetteurs. Nous avons la main mise sur la détection des signaux en réception d'une part, et sur le choix des formes d'ondes à l'émission d'autre part. La méthode de détection, basée sur le filtrage adapté, se veut robuste aux différentes perturbations engendrées par le canal de propagation (pertes par transmission, multi-trajets) et par le système lui-même (environnement multi-émetteurs). De plus, la détection restreinte à une somme de tests d'hypothèses binaires, nécessite un fonctionnement en temps réel. A l'émission, les formes d'ondes doivent permettre d'identifier indépendamment les émetteurs les uns des autres. Ainsi les travaux portent essentiellement sur les modulations FHSS, les paramètres de construction de ces signaux étant alors choisis de sorte à optimiser la méthode de détection étudiée. Enfin, l'implémentation des algorithmes issus de ces travaux sur des systèmes embarqués a permis leur validation sur des données enregistrées, puis en conditions réelles. Ces essais ont été réalisés avec l'entreprise ALSEAMAR, dans le cadre de la thèse CIFRE-DGA. / There is an increasing interest in underwater positioning system in industry (off-shore, military, and biology). In order to localize a receiver relative to a grid of transmitters, thanks to the knowledge of positions and transmission time, it needs to detect each signal and estimate the TOA (Time Of Arrival). Thus, a range between a transmitter and receiver can be deduced by estimation of TOA. When receiver knows three ranges at least, it can deduce its position by triangulation. This work takes into account signal detection, and waveform choice. Detection method, based on matched filter, needs to be robust face to propagation channel (transmission loss, multi-paths) and to the system (multi-users environment). Moreover, the detection structure, being a combination of binary hypothesis testing, must work in real time. In a CDMA context which requires to distinguish each transmitter, the FHSS (Frequency Hopped Spread Spectrum) modulation, allocating one code per user, is adapted. FHSS signals performance, depending of the number of frequency shifts N and the time-bandwidth product, are analyzed from detection criterion point of view. Moreover, detection method and adapted signal is tested in a shallow water environment.The research was supported by ALSEAMAR and DGA-MRIS scholarship.

The purpose of this research project was to design, implement, and evaluate a prototype underwater positioning system which extends the reach of the terrestrial Global Positioning System (GPS) underwater. The GPS does not function underwater because the high-frequency low-power signals used by the GPS are not able to penetrate more than several meters in water. The Underwater Global Positioning System (UGPS), presented in this work, provides underwater position data to an unlimited number of underwater assets, such as autonomous vehicles. The user requirements are discussed and a design is presented that incorporates a topside surface buoy (satellite) and a subsurface receiver. The satellite is responsible for receiving GPS data and relaying the data, via acoustic signals, to the subsurface receiver. The receiver calculates its position using the coded acoustic signals. The implementation of the prototype UGPS satellite and subsurface receiver are discussed in detail; the custom electronics, software, data acquisition systems and mechanical housings are described. The key operating characteristics of the UGPS are investigated both experimentally and through the analysis of a model describing the entire UGPS. Employing the prototype UGPS, a series of sea-trials were performed that provides essential design data for developing the next version of the system. The main characteristics that were experimentally investigated were: the long and short-range accuracy; the repeatability; and the resolution. The experimental data was also employed to confirm the UGPS model performance. The prototype system demonstrated the feasibility of the UGPS concept and showed that a position accuracy of 6.5m should be attainable for an unlimited number of underwater receivers operating within a one square kilometer workspace. The accuracy can be enhanced to sub-meter by employing more accurate GPS receivers in the satellites and using a sound velocity meter to measure the sound velocity profile of the acoustic workspace.