Our research programs are oriented towards providing an effective marine research capability for Singapore to fulfill its needs with respect to managing its marine resources. We also support affiliated marine research laboratories in their work where acoustic tools and research inputs are required.

There are four broad focus areas of research (outlined below) at the ARL, although the research is heavily cross-linked with plenty of interaction across research areas. In addition, there are many other smaller research topics that we undertake, as evident from some of our publications.

• ABC
• ANI
• Biosonar
• DSAAV
• PANDA
• STARFISH
• TLA
• UNA
• UNET

Cross-links:

• Publications

Novel sensing and signal processing

High frequency ambient noise in warm shallow waters is dominated by snapping shrimp. These noisy creatures produce impulsive noise that adversely affects most systems based on signal processing techniques using Gaussian noise assumptions. Appropriate signal processing with non-Gaussian noise assumptions, however, enables us to get good detection and communication performance in these waters (Chitre et al. 2006, Chitre et al. 2007).

Although the snapping shrimp noise is a nuisance to traditional communication and sonar systems, they can be treated as stochastic sources of opportunity in other applications. For example, the concept of ambient noise imaging allows us to image passive targets underwater using a completely passive "acoustic camera" that we built at ARL. This acoustic camera, known as ROMANIS, won the defence technology prize in 2004. Another example of opportunistic use of snapping shrimp as sources is a geoacoustic inversion application where one can tell something about the seabed by simply listening to the ambient noise in the ocean (Chitre et al. 2003).

In order to keep the flow noise away from the sensors, traditional towed arrays are large and heavy. These arrays are rather difficult to use from small platforms such as sail boats, autonomous underwater vehicles (AUV) and unmanned surface vehicles (USV). For several years, we have explored the possibility of making thin towed arrays for use from such platforms. We have shown that such arrays are not flow noise limited at speeds of interest, and developed the technology to squeeze in the electronics necessary to make these arrays work reliably into flexible tubes which are only about 10 mm in diameter!

Cooperative autonomous platforms

We believe that the future of marine sensing lies in cooperative autonomous assets working together to provide a coherent picture of the marine environment. In line with this, we work with fixed and mobile platforms, both underwater and on the water surface, for sensing applications.

Under the STARFISH program, we have developed our own team of cooperative modular AUVs. We are working with GraalTech, a company that developed the hybrid glider-like Folaga AUV, to develop an enhanced eFolaga that is modular and can cooperate with our STARFISH AUVs. In collaboration with MIT, under the CENSAM program, we are exploring cooperative environmental monitoring using our STARFISH AUVs, MIT's surface kayaks and other fixed sensing platforms. In other projects, we explore the use of teams of AUVs in off-shore surveying and port security applications.

We also have developed fixed underwater and surface platforms for sensing for ambient noise monitoring and real-time water quality monitoring. The PANDA is an underwater fixed buoy that allows easy diver-free deployment and recovery. We have used PANDAs for ambient noise data collection and cooperative target tracking so far, and expect to use them as navigational beacons and for environmental sensing in the near future.

DSAAV is a distributed software platform originally developed for the STARFISH AUV, but now being made available to researchers interested in implementing distributed components in autonomous marine systems.

Underwater acoustic communications and networking

In order to have underwater assets cooperate, we need them to communicate. As EM waves are not effective for medium-long range communication underwater, we use acoustics for this application. The snapping shrimp noise dominated environment in Singapore waters limits the performance of many commercial communication systems. We have therefore developed our own acoustic communication system that is robust to non-Gaussian noise, and works well in both warm and cold shallow waters.

Once we have a good point-to-point communication link, the next natural step is to explore how multiple modems coordinate their access to the communication channel. On this front, we have an active medium access control (MAC) research initiative. We also contribute to standardization efforts such as NURC's JANUS initiative and UNA, a collaborative effort between MIT, WHOI and us. We are also exploring some network and transport layer problems for underwater networks.

Bioacoustics

Dolphins are able to investigate their environment through echolocation in conditions where vision is limited (e.g. turbid water or at night). Cross-modal matching-to-sample studies have shown that object shape can be directly perceived through echolocation in a holistic manner, similar to the direct shape perception of shape through vision. No man-made sonar of comparable bandwidth can perform this feat. We are currently investigating the dolphin's ability to recognize shape through echolocation with an aim to understand the underlying signal processing that the animal employs.

We have been investigating the vocalizations of humpback whales that come to the wintering grounds around the four-island region of Maui, Molokai, Lanai and Kaho'olawe each year between December and April. Humpback whales produce "songs" that typically last about 10-12 minutes. The function of humpback whale song remains unclear although several explanations have been offered over the years. We have developed instrumentation that allows us to make field measurements on the source level, directionality and source location of the sounds produced by overlaying "acoustic contours" on video images of these animals (Potter et al. 2003).

On a very different front, we have developed a real-time acoustic bandwidth compression algorithm that enables acoustic signals with large bandwidth to be compressed into a bandwidth that humans can hear. The application of this algorithm is to enhance the diver experience, by enabling him or her to appreciate the rich soundscape the underwater world has to offer. We have demonstrated that a stereo implementation of the algorithm enables the diver to localize the sound source. This further enhances the experience and may potentially improve dive safety.