The small AUV - a tool for detecting marine life
Marine Instrumentation Division
National Institute of Oceanography
Dona Paula, Goa 403 004, INDIA
Remote sensing of the oceans by satellite sensors is perhaps the only tool by which one can study large-scale oceanic processes both in space and time. For example, basin scale maps of chlorophyll distributions due to phytoplankton can be derived from ocean color spectro-radiometers fitted on sun synchronous satellite platforms like the OCM monitor on IRS-P4, the MOS sensor ( Modular Opto-electronic scanner ) on IRS-P3, and the well known SeaWiFs sensor on the Seastar satellite. However, satellites provide integrated information of the surface layer of the ocean only. In contrast, conventional ships provide the opportunity to study the vertical structure of the water column, but offer only point sampling of the sea surface. The ship approach suffers from the problem of extreme under-sampling and inadequate spatial resolution.
An extended arm of the ship - the small AUV
Between these two sampling approaches, and one that has not been sufficiently exploited, is the possibility of sampling the environmental "volume" around a research vessel in great detail. These notes address the notion of using small Autonomous Underwater Vehicles [AUV] as extended arms of the ship to support research at sea. The AUV approach overcomes the inherent drawbacks of satellite and ship platforms, as they could be used to explore the surface properties of a wide area ( several kms. ) around a ship, and by enabling repeated dives of the vehicle, detailed knowledge on water column properties can be obtained.
A small AUV for use in the coastal zone would offer significant advantages-
Monitoring marine life with a small AUV - some problems to solve
Past developments in AUV's have tended to confront underwater technological challenges of long mission ranges of several tens of kms, and deep depth dives to 600 meters. The application area has been in bathymetric surveys and pipe line inspection. Consequently, these developments have resulted in a class of large heavy AUV's with buoyancy of 1300 kgs , body lengths of ~ 5 meters or more, and high costs ~ $200,000 thereby rendering them beyond the budgets of most oceanographic applications. Present developments of small AUV's are being seen in developments of REMUS ( WHOI), Fetch ( Silas Patterson Inc.), Ocean Explorer ( FAU), Cetus ( LMC ). Most of these have been tested as "prototypes' from several research institutions and universities in the US , Europe and Japan.
In the small AUV approach, we focus on science problems that will shape the specifications of the AUV. In particular we chose the following marine life problems -
The Myctophid AUV
The resolution of these three inter-linked problems require further detailed investigations with a suitably equipped AUV having -
Other suitable sensors ( oxygen, temperature) could be added. A well planned mission programme would have to be formulated for the AUV to execute.
The Optical AUV for Satellite Validation Missions
An optics equipped AUV can be made to roam around the ship sampling the water column over several satellite pixels around the ship. The Indian satellite OCM which was launched recently has a pixel size of 360 meters compared to a pixel size of 1100 meters for the SeaWiFs sensor. If the range of the AUV is ~ 3kms then optical data averaged over 16 pixels of OCM sensor would be statistically more meaningful that having only 1 pixel coverage from the ship as would be obtained from conventional ship cruises.
The present way of validating satellite ocean color sensors is by taking the ship along a defined cruise track. At each station, the light fields of irradiance [Ed] and radiance [Lu] are measured at the surface and vertically along the water column upto a depth of ~ 60 meters. At each meter, the spectral signatures of Ed and Lu are measured in real time using a cable connected to the instrument. Soon after an optical cast, the biologist collects samples of sea water at fixed depths which then are analyzed to give the 'chlorophyll ' profile.
The optical AUV will have the following characteristics
Trichodesmium blooms are of great scientific interest now in the Satellite Remote Sensing Community. The phytoplankton in this bloom fixes nitrogen gas (N2 ) under fully aerobic conditions while photosynthetically evolving oxygen It is now known to occur throughout the oligotrophic and sub tropical oceans. The (N2 ) fixation property of Trichodesmium is likely to be a major input to the marine and global nitrogen cycle.
Although Trichodesmium is seen easily in the tropical seas, it has been difficult to detect it remotely by satellite. A robust detection protocol needs to be developed, but this is hampered by the fact that the spectral signature of Trichodesmium is strikingly identical to that of sediment plumes. High resolution optical spectra at sea using hyper-spectral radiometers would resolve this problem. Present day commercial radiometers used routinely by oceanographers are equipped with a few wavelengths that correspond to the satellite wavebands. These low-resolution radiometers miss the spectral structure from the phycoerythrobilin [PEB] and phycourobilin [PUB] pigments in Trichodesmium. With a hyperspectral sensor, the diel variations in the ratio of [PUB/PEB] can be studied in detail throughout the day, and used in a new protocol.
There is a further logistic problem of measuring Trichodesmium spectra from a ship, as the act of lowering the radiometer into a Trichodesmium patch tends to separate and break the patches. This is because Trichodesmium is highly buoyant, and tends to be driven apart by the smallest disturbance on the sea surface
Sallow water AUV in a process study of Trichodesmium
An AUV could solve the problem of recording high-resolution Trichodesmium spectra, and the way would be to:
Scientific sensor payloads for AUV's
A small AUV would require small sensor payloads to be accommodated within the limited space permitted. Fortunately, off-the-shelf sensors of small size are now available for most oceanographic parameters. They have built in data loggers with serial outputs, low power requirements, and low costs. Some examples are:
Other sensors rate gyros, doppler logs , GPS and radio modems for control and navigation purposes are also available in small dimensions.
General Specifications for a small AUV
The target audience for the small AUV application is the oceanographic community, and in particular its use to study and monitor marine life in the coastal zone, as these present the most challenging problems to solve. Some general specifications can be drawn -
The era of the small AUV is clearly at hand. There have been major advances in the technology of miniaturizing scientific sensors, and in the control and navigation algorithms of underwater vehicles over the last decade. Battery technology on high energy density cells is emerging, and could be usefully employed to power the small AUV for long periods. Similar advances in thruster motors with built in motor controllers, rate gyros, fin controllers. doppler logs, and miniature Ultra Short Baseline (USBL) systems are in the market, and can be used in the small AUV. Miniature GPS systems, and high-speed radio modems are other breakthroughs that benefit the small AUV development. On the software side, embedded micro-controller cards with real time operating systems running high level languages make it sufficiently easy to program the AUV in a well structured modular approach that is quickly amenable to change, and to fault detection analysis.
Research cruise ships carrying a variety of mission specific small AUV's could address a wide variety of investigations that would encompass the major disciplines in oceanography - biology, chemistry and the physical oceanography of the coastal zone.
Finally, we may consider the possibility of networking several AUV's in coastal areas to search and coordinate an intelligently coordinated data collection exercise at sea by reporting to the mother ship.
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