Subsea positioning by merging inertial and acoustic technologies

Abstract

Inertial sensors have high data rate and smooth drifts, while acoustic positioning systems have low data rate and bounded errors. Merging the two technologies requires that the user can access (i) the knowledge of all error sources of both technologies, (ii) the detailed interfaces of the products, (iii) the technical keys and the funding for the calibration of the coupling. By doing this once for all, IXSEA provides innovative fused solutions ldquooff the shelfrdquo for the benefit of the users. The fully portable high performance USBL ldquoGAPSrdquo has an embedded inertial system, with pre-calibration of misalignments at the factory. It offers new and unprecedented applications for ships of opportunity, buoys, shallow and very shallow water... in addition of a usual high end USBL. The technology of GAPS will be presented together with detailed applications where it makes the difference: shallow and very shallow water operations such as pipe survey, noisy environments like ploughing operations, positioning from a barge where traditional USBL cannot be calibrated, etc. Since the GAPS includes all the necessary technologies with pre-calibration of the system (USBL, INS inside for the attitude and the position of the antenna, GPS, embedded computation of data), it makes it a fully plug&play solution. The performance is the following: 4000 m range, 0.2% of range for the position accuracy, acoustic head aperture of 200 degrees. For applications where smoothness, accuracy, and ease of use are even more stringent, a new concept called RAMSES will be presented. RAMSES is a Range Navigation System with positioning data fusion engine. It can be coupled to an Inertial Navigation System. It is able to stabilize the drift of the INS just by dropping a single transponder on the seafloor at unknown position, with no calibration of any kind. The coupling technology comes from robotics, and is called SLAM (Simultaneous Localization and Mapping). This technique has been adapted to subsea positioning. The main applications will be presented: AUV survey (traditional or very large one), pipe route survey, stringent ROV positioning and navigation. A new way of thinking is now possible in reducing transponder deployment. RAMSES is tolerant to acoustic shadows; there is no calibration of the transponder array. In the case of an AUV, the users generally have an INS and a DVL. Just by plugging a RAMSES to the positioning system, it allows to perform a straight line survey of hundreds of kilometres with just one beacon every 10 kilometres, keeping a positioning accuracy of around 10 m. For this application, the positions of the beacons just have to be known thanks to GPS when deployed from helicopter or from ship. The INS drift is then automatically reset when the AUV comes into the range of a beacon (4000 m range).