Ice Profiling Sonar for an AUV : An approach for obtaining SCICEX quality ice draft data

The United States Navy announced plans to eliminate the Sturgeon class submarine fleet some years ago. Since then a discussion has continued about the availability of other Navy assets to continue the SCICEX cruises used for measuring and monitoring Arctic oceanographic properties including ice draft. This is of particular importance at this time, since recent studies indicate the ice cap may be disappearing at an alarming rate, and its important relationship with global climate is becoming more and more evident. The Navy is expecting to ice harden two submarines of another class, but scientific access is still an unresolved issue. Addressing this problem for science requires a platform with proven capability that can provide comparable data. Ultimately the goal is to provide the services science wants, and to replace the lost usage of the U.S. submarine assets. With the development of proper navigation and attitude reference systems capable of accurate performance in the Arctic, this should be possible using an autonomous underwater vehicle (AUV). This paper will address proposed efforts at MBARI looking to integrate an ice sonar into an AUV and using it to perform SCICEX quality ice draft measurements in the Arctic Ocean. The equipment necessary to integrate an ice profiling sonar into an AUV system is not extensive because one can take advantage of the many sensors already incorporated into a basic AUV that are used primarily for navigation and basic science (such as the CTD). The most important aspects of the integration are first, the physical limitations in size and weight of the system, and second, proper assessment and coalescence of the necessary physical measurements which, unlike in a moored application, are spread throughout the vehicle. The discussion will further outline the expected system requirements and equipment tolerances that will be needed to be successful. It will additionally describe the experiment planned to test the system against its functional requirements in the Arctic. INTRODUCTION The physical oceanography of the Arctic Basin and its influence on and response to climate change have become very important topics in the last decade (Steele and Boyd, 1998; Dickson, 1999). Studies have been carried out from ice camps, icebreakers, satellites and aircraft, moored and drifting buoys, and naval submarines. The submarines provided a much more flexible platform than previously available for scientific observation under the ice cover. These submarine assets made available to the science community are from numerous countries, and those offered through the United States Navy in the 1990’s became part of project known as the SCICEX Program. The data resulting from this collaboration between the academic institutions and the Navy has been substantial (Rothrock et al., 1998). Unfortunately, now that the SCICEX Program has ended, the US submarines are no longer available to the community, and prospects for their future use are slim. Long tracklength measurements of ice draft are one of the unique capabilities the submarine platforms provided. Except for the SCICEX cruises, submarine ice draft measurements have had limited spatial and temporal resolution and have been difficult to obtain at all, in many circumstances, due to the sensitive nature of submarine missions in the Arctic Ocean (Bourke and Garrett, 1987). It has been determined, however, that continued, large-scale monitoring of the ice draft over the Arctic basin is vital to improving our understanding of the role that the Arctic plays in the larger picture of global climate (McLaren et al., 1992; Wadhams, 1990). A new tool being developed at the Monterey Bay Aquarium Research Institute in collaboration with a number of other institutions and industry partners will be capable of long duration autonomous under ice scientific missions (Bellingham et al., 2000). As part of the Atlantic Layer Tracking Experiment (ALTEX), this AUV is scheduled for its first Arctic engineering test in Fall of 2001. Given the ability of the vehicle to remain under the ice for up to ten days and traverse up to 1500 kilometers underwater, it is an ideal platform candidate for continuing the measurements and monitoring of ice draft. It has been decided, therefore, to outfit this platform with an ice profiling sonar capable of scientific quality ice draft measurements. Adding this tool to the ALTEX vehicle sensor package is beneficial for another reason, as well. It can be used to locate appropriate launch locations for the data transfer buoys used to transmit vehicle data via ARGOS. The objectives of this project are to demonstrate the functional ability of a long duration Arctic-capable AUV to measure ice draft, and to further assess its usefulness as a platform for performing unique Arctic science. This work is based on using the equipment and vehicle platform of the ALTEX program. Specifically, we would like to 1. determine how accurately an AUV can obtain ice draft measurements, where ice draft is defined as the distance from the bottom of the ice to the mean surface water level 2. to collect up to four 50 km data sets of ice draft measurements 3. to compare the quality of the results of these measurements with those historically collected by other available platforms – specifically those collected by submarines as part of the SCICEX cruises This paper will give a brief background on under-ice measurement platforms and the autonomous underwater vehicle (AUV) development occurring at MBARI. It will then present the system requirements and explain the selection of the system that is to be used. It will also describe both the physical and systematic integration plans for the system, a description of the Arctic test plan, and finally it will conclude with a number of suggested science applications.