Underwater Positioning

Författare: Patric Jansson; Kth.; [1994]

Nyckelord: ;

Sammanfattning: The project underwater positioning was initiated by the Swedish National Road Administration (SNRA) in 1990. The aim of the project is to study positioning techniques and methods which can be used to position and locate man-made structures and features, especially engineering structures, piles for the foundation'laying of bridge pillars, in shallow water. In many years, SNRA has used a taut wire system, which today cannot meet their accuracy demands. In the very beginning of the project, acoustic, light and inertial techniques were considered to be the alternative techniques, which already would have been able to meet the demands. Or would be able to meet them in the nearest future after some modifications or developments. Therefore, a study of these techniques have been performed by the author in this thesis.The behavior and the proprieties of acoustics in water are described. The main phenomena and effects influencing underwater sound transmission, like different kinds of transmission losses and noise, are explained. Long, short and ultrashort baseline method and their different modes, inherent properties and errors sources are described. Among these three different methods, each of the three has distinct advantages and disadvantages. Long baseline and short baseline system have reached an advanced level of maturity and little further development of basic hardware components is anticipated in the future. The design of systems combining two of the three different methods, long and short baseline or long and ultrashort baseline systems, have advanced dramatically in the last years and seem to provide the best of both worlds, convenience, speed and accuracy.    An alternative to acoustics for accurate underwater positioning is electromagnetic radiation. This technique applied at frequencies high enough to be useful is highly absorbed in the sea because of the electrical conductivity of seawater. The only useful range is the optical one, but even there, one’s capabilities are limited to approximately 100 meters. Optical properties of the ground water environment are explained and the laser technique and its applications in water are described. In spite of the fact that the working range is not by far as long as for acoustics or electromagnetic radiation.After the review of existing techniques, some numerical investigations are carried out in order to estimate the precision of transponder coordinates on or near the seabed when four different parameters or coordinates are changed, i.e. surface positioning technique, network configuration, platform tracking pattern and transponder depths. Although optimization of the tracking pattern and network configuration are not thoroughly investigated, several patterns and configurations are examined in the simulations.The results of the numerical investigations show only slight differences between the two different surface positioning techniques. The considered standard deviation of the vertically transferring may be optimistic, which affect the results to a great extent. The lowest precision values of seabed sites are yielded assuming vertically transferred transponder sites and with a total number of four transponders at the seabed. On the contrary, including an additional transponder, from 5 to 6 and 6 to 7 transponders.From the results of the concept GPS as the surface positioning technique one can conclude that the surface track pattern should be symmetrically spread over and around the seabed transponder network and be made as dense as possible. Subsequently Kalman filtering and optimum smoothing should be paid attention in this type of analysis.

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