Isurus is a REMUS class AUV, acquired to the Woods Hole Oceanographic Institution, MA, USA, in 1997 (Fig. 1). These vehicles are low cost, lightweight AUVs specially designed for coastal waters monitoring (Alt et al., 1994). The reduced weight and dimensions make them extremely easy to handle, requiring no special equipment for launching and recovery. Isurus has a diameter of 20 cm and is about 1.5 m long, weighting about 35 kg in air. The maximum forward speed of the vehicle is 2 m/s, however the best energy efficiency is achieved at about 1 m/ s. At this velocity, the energy provided by a set of rechargeable lithium-ion batteries may last for over 20 hours.
Although small in size, this vehicle can accommodate a wide range of oceanographic sensors, according to mission objectives. For the field experiment described here two specific sensors were integrated: a conductivity, temperature, depth (CTD), OS200 model from Ocean Sensors, Inc., USA, and an altimeter to measure the height from the sea bottom, from Imagenex, Inc., Canada.
Since this type of vehicle operates without external control to perform the pre-defined mission transects, they have to be able to locate themselves in the field. The navigation system of Isurus AUV estimates in real time its position based on data provided from several devices and systems. The vertical coordinate is obtained from a pressure cell installed on the vehicle. To estimate the horizontal position, the navigation system fuses together dead reckoning data with absolute positioning data (Matos et al., 1999).
Typically, a mission starts with the vehicle programming, by editing a mission file in the laptop computer, with the tasks that need to be successively accomplished and the exact field location of the navigation transponders. After the final diagnostic, where the status of the various subsystems is checked, the mission file is transferred to the vehicle and the cable is disconnected. The vehicle is then launched in the water, from the coast or from a small boat. At this stage, the navigation and control programs are running and dictate the vehicle manoeuvres throughout the mission. The mission ends with the recovery of the vehicle in the expected final position. The connection with the laptop is then re-established and all the stored information is downloaded.
There are two data sets collected during a typical mission. The first set is related to the vehicle internal data, which are only analysed in the laboratory. These allow to evaluate the performance of various subsystems (navigation, control, power consumption, etc) and provide useful information for potential improvements. The other data set comprises all the information from the oceanographic sensors.
Usually, before processing, it is necessary to locate the collected data in time and space. While time location is obtained directly from the vehicle real-time clock, the spatial location is based on the vehicle position estimate provided by the navigation system (horizontal coordinates) and the depth cell (vertical coordinate).
If the spatial location of oceanographic data gathered with an AUV is known in a relatively precise manner, the same cannot be said for towyo platforms. In this case, even if the research vessel location is perfectly known, the natural instability of the cable, very difficult to control in a precise way specially with increased depth, can lead to positioning errors of the towed sensor package which can be critical for small scale phenomena observation.
Fig. 1. Isurus Autonomous Underwater Vehicle.
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