Per Espen Hagen

Making AUVs Truly Autonomous

Autonomous underwater vehicles are gaining acceptance in a number of applications and countries, as a safe, cost-effective and reliable alternative to manned or remotely controlled systems. However, the actual autonomy of these vehicles is limited in many ways, restricting their potential uses. Further advances in AUV autonomy will enable new operations, such as very long endurance missions (weeks), and operations in unknown areas. While some experimentation is already taking place with e.g. under ice operations, the chance of failure is unacceptably high for many potential users. De-risking of long- endurance autonomous operations in unknown areas is thus an important goal for the AUV community. This paper gives an overview of the AUV research being carried out towards this end at the Norwegian Defence Research Establishment.

The HUGIN AUV "Plug and play" payload system

A flexible and extensible payload system has been developed for the HUGIN family of autonomous underwater vehicles (AUVs). An important feature of the system is that it allows the HUGIN developers, the payload developers, or a third party, to develop interface software for each payload. The software is integrated in such a way that the vehicle operators can then mix and match payloads integrated by different parties. The HUGIN payload system has been used extensively in commercial missions over the last years, and the number of supported payloads is steadily growing. This paper explains the design of the subsea and topside segments of the payload system, and describes some of the important aspects of payload system integration in an AUV.

Applications of AUVs with SAS

Autonomous underwater vehicles with high-resolution synthetic aperture sonar (SAS) are now becoming available as products. The main benefit of such a system is very high and range-independent resolution (typically 2-5 cm both along- and across-track) across a very wide swath (typically 100-200 m). High-end SAS systems can also deliver additional products such as high resolution bathymetry. The technology was originally developed for naval mine countermeasures due to its requirements for very high resolution imaging and high area coverage rates. However, SAS clearly also has other uses. This article discusses and illustrates some of the potential applications for this technology, in particular in the civilian sector.

High fidelity synthetic aperture sonar products for target analysis

Synthetic aperture sonar (SAS) can produce images with centimetre-level resolution and area coverage of better than one square kilometer per hour. This makes SAS an ideal sensor for detection and classification of small targets over large areas. Fully automated target analysis allows improved autonomy when using autonomous underwater vehicles (AUVs) and saves a tedious manual analysis in post-mission analysis. Recognition of small targets in sonar imagery is, however, a difficult task. SAS imagery preserves wavenumber information. This gives the possibility for extra products in addition to high resolution imagery. We propose a two-stage processing where regions of interest are generated from reduced resolution SAS imagery and subsequently post processed images are used to generate relevant target analysis information. In this paper, we concentrate on the types of information available and their significance rather than the choice of intermediate resolution and initial detection methods. The extra processing products discussed in this paper are target-enhanced images by autofocus, shadow-enhanced images by fixed focusing, multi-aspect images, frequency-selective information and 3D shape from interferometry. We show examples of each of the additional products using data collected by the HISAS 1030 interferometric SAS carried by the HUGIN 1000-MR vehicle.

Horizontal mapping accuracy in hydrographic AUV surveys

Underwater vehicles are used in a wide range of tasks in various sectors. Cost-effective and accurate seabed surveying and mapping using autonomous underwater vehicles (AUVs) have been carried out for years in the offshore oil and gas sector. Much of the experience gained is now being benefited upon in new and challenging applications. One of the emerging AUV applications is hydrographic surveying (e.g. for creating nautical charts), particularly in waters shallower than 100 m. Key factors for mission-success include obtainable accuracy and resolution of the final digital terrain model (DTM), as well as the feature detection capability of the integrated system. This paper gives an in-depth discussion and analysis of the horizontal mapping accuracy achievable by integrated, state-of-the-art hydrographic AUV systems. The HUGIN 1000 AUV fitted with interferometric synthetic aperture sonar (SAS) is used as a case study, and the system accuracy as assessed in detail – starting from surface navigation, going all the way down to the acoustic seabed footprint. A discussion is given at the end on the feasibility of AUVs in terms of the minimum standards proposed by the International Hydrographic Organization (IHO).

Gap filler sensors for SAS

Like traditional side scan sonars, synthetic aperture sonar (SAS) systems have a coverage gap in an area directly underneath the sensor. Range resolution projected onto the seafloor deteriorates near the vertical, and objects do not cast shadows. Furthermore, to limit multipath in shallow water, some SAS systems do not transmit or receive at all near the vertical. It is difficult to come up with a sensor that can match the performance of a good SAS for the area around nadir - at least without increasing the cost and complexity of the system substantially, or imposing strict limits on operation. This paper investigates four possible technologies as gap fillers for SAS; acoustic as well as optical systems.

Interferometric synthetic aperture sonar in pipeline inspection

Accurate and reliable inspection of subsea pipelines traditionally requires the use of multiple sensors, which again have different requirements for range and geometry. This makes pipeline inspection a time consuming and costly process. The use of high resolution synthetic aperture sonar (SAS) can potentially change this substantially. Current state of the art interferometric SAS systems offer very high resolution imagery and bathymetry over a very wide swath. Mounted on an autonomous underwater vehicle (AUV), such a system can provide highly detailed data sets of a pipeline with just one or two passes - either fully autonomously, or with operator supervision as desired. In June 2009, a Kongsberg Maritime HUGIN 1000-MR AUV equipped with a HISAS 1030 interferometric SAS performed such a pipeline inspection mission in Western Norway. The paper will show examples of the data recorded during the above mission.