Underwater Glider Reliability and Implications for Survey Design

It has been 20 years since the concept of the Autonomous Ocean Sampling Network (AOSN) was first introduced. This vision has been brought closer to reality with the introduction of underwater gliders. While in terms of functionality the underwater glider has shown to be capable of meeting the AOSN vision, in terms of reliability there is no communitywide hard evidence on whether persistent presence is currently being achieved. This paper studies the reliability of underwater gliders in order to assess the feasibility of using these platforms for future AOSN. The data used are taken from nonunderwater glider developers, which consisted of 205 glider deployments by 12 European laboratories between 2008 and 2012. Risk profiles were calculated for two makes of deep underwater gliders; there is no statistically significant difference between them. Regardless of the make, the probability of a deep underwater glider surviving a 90-day mission without a premature mission end is approximately 0.5. The probability of a shallow underwater glider surviving a 30-day mission without a premature mission end is 0.59. This implies that to date factors other than the energy available are preventing underwater gliders from achieving their maximum capability. This reliability information was used to quantify the likelihood of two reported underwater glider surveys meeting the observation needs for a period of 6 months and to quantify the level of redundancy needed in order to increase the likelihood of meeting the observation needs.