- Email: [email protected]
Underwater Vehicle Networks for Acoustic and Oceanographic Measurements in the Littoral Ocean
Copyright@ IFAC Manoeuvring and Control of Marine Craft, Aalborg, Denmark, 2000
UNDERWATER VEHICLE NETWORKS FOR ACOUSTIC AND OCEANOGRAPHIC MEASUREMENTS IN THE LITTORAL OCEAN Henrik Schrnidt' Edoardo Bovio"
• Massachusetts Institute of Technology, Cambridge, MA 02139 •• SACLANT Undersea Research Center, 1-19100 La Spezia, Italy
Abstract: Recent progress in underwater robotics and acoustic communication has led to the development of a new paradigm in ocean science and technology, the Autonomous Ocean Sampling Network (AOSN). AOSN consists of a network of fixed moorings and/or autonomous underwater vehicles (AUV) , tied together by state-ofthe-art acoustic communication technology. The GOATS2000 (Generic Oceanographic Array Technology Systems) Joint Research Program is aimed toward the development of environmentally adaptive AOSN technology specifically directed toward Rapid Environmental Assessment in coastal environments. The research program combines theory and modeling of the 3-D environmental acoustics with experiments involving AOSN and sensor technology . To explore the potential of providing a reliable environmental forecasts of the coastal ocean, the observational capabilities are coupled with a nested ocean modeling and assimilation framework, consistently coupling basin scale oceanography with small scale ocean processes. The central element of the the program is a prototype underwater vehicle network managed remotely by a surface platform , including moored sources and arrays , AUVs equipped with a variety of acoustic and environmental sensors, and reliable navigation and communication system. Copyright©2000 IFAC Keywords: Autonomous vehicles, Adaptive systems, Sensor systems
1. INTRODL'CTIO:\'
tal Assessment (REA) in coastal environments. The JRP combines theory and modeling of the 3-D environmental acoustics with experiments involving AOSN and sensor technology. The central element of the JRP is a prototype underwater vehicle network operated remotely from the SACLAl\TCEI\ oceanographic vessel R/V Alliance, including moored sources and arrays , AUVs equipped with a variety of sensors, and reliable navigation and communication systems. The major environmental acoustics objective of the JRP is to use an acoustic array mounted on an A UV to characterize the spatial and temporal characteristics of the 3-D scattering from seabed targets and the associated reverberation,
Recent progress in underwater robotics and acoustic communication has led to the development of a new paradigm in ocean science and technology, the Autonomous Ocean Sampling ~et work (AOS!\) (Curtin et al. (1993)). AOS!\ consists of a network of fixed moorings and/or autonomous underwater vehicles (AUV) tied together by state-of-the-art acoustic communication technology. MIT and SACLANTCE); have initiated a Joint Research Program (JRP), called GOATS'2000 (Generic Oceanographic Array Technology Systems), aimed towards the development of environmentally adaptive AOSN technology specifically directed towards Rapid En\'ironmen-
\05
including the effects of multipaths. This effort is aimed at establishing the environmental acoustics foundation for future multi-static sonar concepts exploring 3-D acoustic signatures for combined detection and classification in very shallow water . The REA component of the JRP addresses the potential of the AOS:'-l' for collecting environmental information of significance to mine counter measure (:vIC:'vl) operations. The JRP investigates the performance of a variety of REA sensors. and develops survey strategies that optimally ch~rac terize the littoral environment. A major potential advantage of the AOS;\ concept is its adaptive sampling capabilities. The network can be designed to change its behavior dependent on the sensor responses. Thus, for example, REA surveys are optimized by adaptively concentrating the measurements in regions where rapid environmental changes are detected, and ADYs carrying MC;'v1 sonars can be programmed to change their survey patterns to optimize the classification of detected targets. By implementing the REA sensors directly into the MCM sonar platforms , or in the same network, the development of new environmentally adaptive sonar concepts become a realistic possibility. To explore the potential for enhancing the REA with a coastal forecasting component, the REA data from the various observational resources deployed during the experiment will be assimilated in quasi-real time into a nested ocean modeling framework coupling basin- scale phenomena with the smallscale coastal oceanography of the experimental site. The GOATS effort will incrementally address the scientific and technological issues associated with the development of new AOSN-based MCM and REA concepts, including reliable communication and navigation capabilities. Building on the results of the GOATS '9S sea trial, the next experiment is scheduled for September 2000.
"," Odyssey contigurll lion for GOATS 98
Fig.
1. Configuration of Odyssey II ACV 'Xanthos' for acoustic measurements in GOATS'9S. The ADV control electronics and batteries are located in two 17" Benthos glass spheres. An S-element array is mounted in a 'swordfish' configuration, and connect.ed to a dedicated acquisition system in the centre ,,,-ell of the vehicle.
Odyssey II to support a wide range of payload systems that include CTD, ADCP IDYL, ADV , side-scan sonar, USBL tracking systems, OBS , and several video systems. The core vehicle has a depth rating of 6,000 m, weighs 120 kg, and measures 2.2 m in length and 0.6 m in diameter. It cruises at approximately l.5 mls with endurance in the range of 3-12 hours, depending on the battery installed and the load. Included in the core vehicle are the guidance and navigation sensors necessary to support autonomous control: attitude and heading , pressure, altimeter, and LBL acoustic navigation. The Odyssey is controlled using a behavior-based layered architecture. At the highest level, behaviors seek to achieve mission goals such as following a track line to a way-point. The layered controller generates high level commands that specify the desired speed, depth, and heading of the vehicle. Taking these commands as inputs , a PlO dynamic controller generates the low-level commands that feed directly to the thruster and actuators. The layered control architecture allows the Odyssey to perform complicated adaptive missions such as bottom following surveys, or missions tracking isothermal contours, for example, while maintaining overriding safety procedures such as keeping a minimum distance from shore or avoiding obstacles. Two Odvssevs are used in the GOATS experiments, the ac·ous~ic ATV and the REA ADV.
2. COr-.IPOT\E;\TS OF THE GOATS AOSN The GOATS prototype observational network is based on the Odyssey class vehicles developed by MIT , on the Ocean Explorer developed by Florida Atlantic Lniversity, and on the SEPTR profilers developed by SACLA~TCEN.
2.1 The Odyssey AUVs The Acoustic A UV The acoustic ATV features an acoustic payload , developed at SACL-\;\TCEl\' , consisting of a line array, mounted in the vehicle 's nose, in a 'swordfish' configuration , and an autonomous data acquisition system , installed in a watertight canister in the vehicle 's payload bay shown in Fig. 1
The Odyssey II class autonomous underwater vehicles were chosen as the mobile sensor platform for the GOATS JRP because of their flexible architecture and proven performance. These ADVs have logged many hundreds of dives in 16 field deployments. A substantial fraction of the vehicle is dedicated to wet volume , which enables the
106
Tow Adt .,..
RF aueon , 5.1'0 •• ligh.
Fig. 2. The Florida Atlantic University Ocean Explorer: the vehicle consists of an aft propulsion control navigation section, a payload section and a forward nose cone
The REA A UV The REA ACV is equipped with a Datasonics combined sidescan and sub-bottom profiler system based on the Datasonics Chirp technology. The sidescan system features a 200 kHz and a 400 kHz component while the subbottom profiler is operating at 8 kHz, providing penetration of order one meter, adequate for seabed characterization of relevance to shallow water I\·fCM . The REA AUV also features a CTD probe for water column temperature and salinity measurements.
The payload section contains the mission sensors and is attached to the aft portion by a bayonet mechanism that allows for quick payload changes. Elements in the payload are simply additional nodes on the LO!\Talk network and are integrated with the tail section with a power conductor and a control line. The nose section contains a ,·ideo camera, a flashing strobe, the emergency drop weight and the vehicle recovery line device. The highly modular design allows to readily switch pay loads on a rear section or switch rear sections on a payload several times a day. The depth rating of the vehicle is 300 m and its endurance with standard batteries is 50 km at 3 kn. During operations OEX can be tracked acoustically via USBL and communicate via the acoustic modem. Launching and recovery can be achieved up to seastate 4 and does not require diver intervention. The vehicle specifications are summarized in Table 1. Table 1. Specifications for Ocean Explorer AUV AU\, Parameter Vehicle Specification Shape Gertler Series 58 Model 5154E Diameter 0.533 m (21 inches) 2.66 m - 3.91 m Length Weight 550 kg max. Speed 1 m/s - 2.4 m/s Depth 300 m Materials Fiberglass , AI , Various Plastics 0.2 m 3 - 0.3 m 3 Payload Volume Actuation Type 4 independent stepped motors Propulsion Type Direct Drive DC motor Battery Capacity Ni-MH , 3000 watt-hrs, 500 cycles Operating System QNX (PC- 104) Dy namic Characteristics at cruise speed Yaw 1.020 R~1S 3 Hz Roll 0.500 RMS 3 Hz Pitch 0.360 R:v1S 3 Hz Turning radius < 10 m
2.2 The Ocean Explorer A UV In 99 SACLANTCEN has specified and procured through international competitive bidding an Autonomous Underwater Vehicle to conduct experiments in support of REA and MCM studies. The selected ACV is a modified Ocean Explorer (OEX), manufactured by the Florida Atlantic University. The OEX is a highly modular vehicle, featuring a PC-104 Pentium executing the supervisory control structure, interfaced to a distributed communication and control network based on the open LO~Talk network protocol , a modular power system and a modular payload capability. The modular design results in a field reconfigurable vehicle, well suited to the experimental work in support of the scientific program of work. The vehicle consists of an aft propulsion/control/navigation section , a payload section and a forward nose cone (Fig. 2). Located in the aft section are the propulsion engine, the control surface motors, the navigation sensors, the control computer, the RF and acoustic communication systems, and the I\ickel ~1etal Hydride (:\"i-~1H) battery packs. The navigation relies on a standard suite of sensors (tri-axial flux gate compass , precision rate gyros and accelerometers, Doppler Velocity Logger) that provide both relative and geodetic positional information. The modular design facilitates the inclusion of a variety of geodetic navigational instruments including a DGPS, long baseline (LBL), and ultra-short base line (CSBL) acoustic tracking systems.
2.3 The SEPTR profiler A Shallow-water Environmental Profiler in Trawlsafe, Real-time configuration (SEPTR) (Tyce et al. (1999)) is being developed by SACLA:\T L'ndersea Research Centre in support of oceanographic modeling and rapid environment assessment programs. It is intended for extended duration deployments in areas where water column instruments are at risk from fishing trawlers, but with real-time data return and control via twowa:, cellular or satellite communication. It consists of a trawl-safe bottom platform which houses an Acoustic Doppler Current Profiler (ADCP) , wave/tide gage, ambient noise sensor array, and water column profiler buoy system. The profiler buoy system and associated winch are designed for autonomous vertical profiling of CTD and optical properties of the water column in depths down to lOOm. The profiler buoy includes DGPS
107
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.
,-............. '
-
I
/
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.....
----
Fig, 3, SEPTR operational scenario navigation capability along with two-way communication capability while on the surface, It also includes acceleration and magnetic field sensors capable of surface wave buoy measurements. The bottom platform includes an extended duration battery package in the recoverable barnacleshaped housing. Recovery of the entire system is accomplished through normal communication with the profiler in order to release a messenger buoy. The system is designed for 360 profiles during a 3-month deployment in lOOm water depth , with current, wave/tide and noise measurements everv hour. Two way communication of data, position and control allows profile results to be returned in real time , and operational commands, profile schedules , and DGPS correctors to be sent to multiple profiler instruments. Complete profiles will also be stored on board the recoverable units. The operational scenario for SEPTR is detailed in Fig. 3. The platform is lowered to the bottom bv a surface ship, and released via acoustic comm~nd after monitoring depth and tilt of the platform . At least once an hour , measurements are made of Doppler current profiles, tides and waves, and ambient noise. Every 4-6 hours a profile of water column properties is made by the profiler buoys, stopping at the surface to communicate data ashore, and to collect any new instructions, via GS!\1 cellular or ORBCOI\1M satellite communication .
between which the AUV would navigate using the long baseline (LBL) acoustic navigation system , at selected depths and speeds. The mission file was downloaded via the computer network to the vehicle computer. At the same time the acoustic acquisition system was programmed for the mission. T /B Manning deployed the TOPAS sound source facilitv and the fixed arrays near the target area. A sh;re laboratory controlled the TOPAS sound source and recorded data from a 16-element vertical arrav a 12S-element horizontal line array, and a buri~d hydrophone array. Alliance anchored offshore was used as a platform for the AUV operation and data processing centre. A typical mission took the A DV from the Alliance to the target area at a speed of 3 kn. Once reached the target area, the AUV made several passes at less than 2 kn over the targets. When the survey pattern was complete, the ACV transited back to the Alliance. After surfacing approximately lOOm from the Alliance, the AUV was towed back with the work-boat and lifted onto the deck, On deck, the ACV was connected to the computer network, and data was uploaded from the navigation computer and the acoustic acquisition system. The ACV navigation data were then processed in order to verify the completion of the mission plan, and for parsing of the reduced navigati?~ data with the acoustic data. Thirty-nine At:\ missions were launched and completed , totaling 14 hours of submerged time. The performance of the ATV was completely reliable, and no serious hardware problems were encountered . The control algorithms developed at 11IT were capable of repeatedly performing the desired survey patterns in the target area with a navigation accuracy of a few metres using the LBL acoustic net deployed in the area.
3. AC\' OPERATIONS DCRING THE GOATS'9S EXPERl}.IE:\T During GOATS '9S the Odyssey ATV was operated from the Alliance, anchored approximately 600 m from the target area (Fig.4) (Schmidt et al. (199S)). :\Iission design and implementation comprised several steps. First, the mission track \\'as designed using a graphical tool on a workstation onboard Alliance. The mission plan was then translated into a series of way-points
108
Alliance
~ -~
.
Honzonta~rn;",~ ~
~~·1. . ./7
~
Targel field
../
Odyssey
N
Marclana
'.
Hartlour
Fig . 4. Schematic (not to scale) of GOATS'98 operational scenario at Marciana :\1arina. 4. GOATS '2000 SCIENCE PLAI\
validation of numerical models of mono- and bistatic seabed reverberation. The experiment will demonstrate the adaptive detection and sampling capabilities of AlIVs: the results of the measurements will be used in real time for changing the AUV behavior in order to optimize the information collected. The MIT Acoustic AUV will enter the target field insonified by parametric or conventional sources on SLC tower. Initially only proud targets will be insonified. Using real-time beamforming in the acoustic acquisition system, the target scattering lobe of the proud target will be detected and the vehicle will initiate a series of 360 deg circles at different depths centered at estimated target position , sampling full 3-D scattering. Following this relatively simple configuration, the adaptive sampling of the bistatic acoustic field will be measured with the insonification being provided by the sub bottom profiler on the MIT REA AUV.
The GOATS 2000 experiment will have three broad objectives, REA , yIC:\-1, MEAl\S.
4.1 Rapid Environmental Assessment (REA ) The scope of this experiment is to demonstrate the capabilities of AUVs as REA platforms for MCM in very shallow water. The information collected includes bathymetry, density of mine like objects, seabed composition, backscattering strength and water mass properties like current, visibility, salinity, density and temperature. In GOATS'2000 two ACVs , the SLC AUV with the Marine Sonic dual frequency SSS and the MIT REA ACV with a combined side-scan/sub-bottom profiler system, will be launched from R/V Alliance, and transect to one or more of the bays on the East side of Procchio. The AUVs will acquire side scan sonar and sub-bottom profiler data and water mass properties like current, visibility, salinity, density and temperature, to be used by nested now-cast and forecast oceanographyc models. Emphasis will not be put on fully covert operation, and both LBL based naYigation and DGPS navigation will be utilized as part of the experiment. The ability of long range naYigation and operation will howeyer be demonstrated. For this purpose R/V Alliance willleaye the Procchio Bay, and launch SLC ADV from a distance of several miles.
4.3 Multiscale Environmental Assessment Network Studies (MEANS) A nested modeling and assimilation capability will be provided by Harvard University and SACLA:"iTCEI\ . A local model at 20 km scale, covering the North shore of Elba and the Procchio Bay, will be nested within a 200 km scale model covering the Ligurian sea. At the basin scale these models will be nested within the :'-1AVO Mediterranean sea model. Environmental data collected by two SEPTR buoys and AUV s in Procchio Bay will be assimilated into the local model providing a now-casting and forecasting capability for the experimental area. On the regional scale, the environmental sampling will be performed by R/ V Alliance at night. The basin-scale :\ AVO :'-.1editerranean Sea model is constrained by satellite remote sensing data, including surface temperature and altimetry.
4.2 Mine Counter Measures (MCM) The rippled field at the main test site in Biodola Bay will be insonified by the TOPAS parametric sound source mounted on the SLC rail at a yariety of incident and aspect angles. The acoustic :\UT AU\-, will sample 3-D reyerberation using survey patterns similar to those used in GOATS '98. These measurements will provide a data set for
\09
Fig. 5. Modular design coupled with plug and play SW will provide just-in-time n:anufacture of s~ecial purpose AUVs that will be the intelligent nodes of distributed ocean samplIng networks (FIgure courtesy of Thomas B. Curtin, Office of Naval Research) ACKNOWLEDGMENT
5. CONCLUSION
The MIT component of this work is supported by the Office of Naval Research. The GOATS JRP provides a series of coordinated, incremental implementations of the Autonomous Ocean Sampling Network concept for coastal REA and MCM as envisioned by Curtin et al. (Curtin et al. (1993)) , Fig. 5. Thus, the GOATS'2000 experiment will include a number of different AUVs operating in shallow and very shallow coastal environments with a variety of sensors for water column and seabed characterization. The AUVs will be communicating with the control center onboard R/V Alliance via acoustic communication while submerged, and radio modems while surfaced. These communication capabilities will be utilized for providing vehicle navigation and control data to the control center, and for remote control of sampling patterns. Subsequent experiments are envisioned to incorporate nextgeneration small AUVs with enhanced control, navigation and communication capabilities, and new sensor technology. Several vehicles are currently being designed by academia, government laboratories and industry. For example, it is expected that feature-based navigation will become particularly important for coastal MCM and REA applications of this new technology. Also, future deployments of the AOSN may utilize cellular acoustic communication networks connected to shore or surface ships through RF and satellite communication links with the potential for the development of entirely new operational paradigms for coastal oceanography and littoral warfare. In this regard, however , the biggest obstacle will probably not be of technical nature, but rather be associated with the breaking down of barriers of tradition among the 'customers', i.e. the scientific and operational communities.
References T . Curtin, J.G . Bellingham , J . Catipovic, and D. Webb. Autonomous oceanographic sampling networks. Oceanography, 6(3) :86-94, 1993. H. Schmidt, A. Maguer, E. Bovio, W.L. Fox, K. LePage, N.G. Pace, R. Hollett, P. Guerrini, P.A. Sletner, E. Michelozzi, B. Moran , and R. Grieve. GOA TS'98 - Bistatic measurements of target scattering using autonomous underwater vehicles. SR 302, SACLANT Undersea Research Centre, La Spezia, Italy, 1998. R. Tyce, F. de Strobel, V. Grandi, and L. Gualdesi. Shallow-water expendable and trawler safe environmental profilers. In Proceedings of Oceans '99, pages 1229-1233, Seattle, WA, 1999. MTS/IEEE.
110
UNDERWATER VEHICLE NETWORKS FOR ACOUSTIC AND OCEANOGRAPHIC MEASUREMENTS IN THE LITTORAL OCEAN Henrik Schrnidt' Edoardo Bovio"
• Massachusetts Institute of Technology, Cambridge, MA 02139 •• SACLANT Undersea Research Center, 1-19100 La Spezia, Italy
Abstract: Recent progress in underwater robotics and acoustic communication has led to the development of a new paradigm in ocean science and technology, the Autonomous Ocean Sampling Network (AOSN). AOSN consists of a network of fixed moorings and/or autonomous underwater vehicles (AUV) , tied together by state-ofthe-art acoustic communication technology. The GOATS2000 (Generic Oceanographic Array Technology Systems) Joint Research Program is aimed toward the development of environmentally adaptive AOSN technology specifically directed toward Rapid Environmental Assessment in coastal environments. The research program combines theory and modeling of the 3-D environmental acoustics with experiments involving AOSN and sensor technology . To explore the potential of providing a reliable environmental forecasts of the coastal ocean, the observational capabilities are coupled with a nested ocean modeling and assimilation framework, consistently coupling basin scale oceanography with small scale ocean processes. The central element of the the program is a prototype underwater vehicle network managed remotely by a surface platform , including moored sources and arrays , AUVs equipped with a variety of acoustic and environmental sensors, and reliable navigation and communication system. Copyright©2000 IFAC Keywords: Autonomous vehicles, Adaptive systems, Sensor systems
1. INTRODL'CTIO:\'
tal Assessment (REA) in coastal environments. The JRP combines theory and modeling of the 3-D environmental acoustics with experiments involving AOSN and sensor technology. The central element of the JRP is a prototype underwater vehicle network operated remotely from the SACLAl\TCEI\ oceanographic vessel R/V Alliance, including moored sources and arrays , AUVs equipped with a variety of sensors, and reliable navigation and communication systems. The major environmental acoustics objective of the JRP is to use an acoustic array mounted on an A UV to characterize the spatial and temporal characteristics of the 3-D scattering from seabed targets and the associated reverberation,
Recent progress in underwater robotics and acoustic communication has led to the development of a new paradigm in ocean science and technology, the Autonomous Ocean Sampling ~et work (AOS!\) (Curtin et al. (1993)). AOS!\ consists of a network of fixed moorings and/or autonomous underwater vehicles (AUV) tied together by state-of-the-art acoustic communication technology. MIT and SACLANTCE); have initiated a Joint Research Program (JRP), called GOATS'2000 (Generic Oceanographic Array Technology Systems), aimed towards the development of environmentally adaptive AOSN technology specifically directed towards Rapid En\'ironmen-
\05
including the effects of multipaths. This effort is aimed at establishing the environmental acoustics foundation for future multi-static sonar concepts exploring 3-D acoustic signatures for combined detection and classification in very shallow water . The REA component of the JRP addresses the potential of the AOS:'-l' for collecting environmental information of significance to mine counter measure (:vIC:'vl) operations. The JRP investigates the performance of a variety of REA sensors. and develops survey strategies that optimally ch~rac terize the littoral environment. A major potential advantage of the AOS;\ concept is its adaptive sampling capabilities. The network can be designed to change its behavior dependent on the sensor responses. Thus, for example, REA surveys are optimized by adaptively concentrating the measurements in regions where rapid environmental changes are detected, and ADYs carrying MC;'v1 sonars can be programmed to change their survey patterns to optimize the classification of detected targets. By implementing the REA sensors directly into the MCM sonar platforms , or in the same network, the development of new environmentally adaptive sonar concepts become a realistic possibility. To explore the potential for enhancing the REA with a coastal forecasting component, the REA data from the various observational resources deployed during the experiment will be assimilated in quasi-real time into a nested ocean modeling framework coupling basin- scale phenomena with the smallscale coastal oceanography of the experimental site. The GOATS effort will incrementally address the scientific and technological issues associated with the development of new AOSN-based MCM and REA concepts, including reliable communication and navigation capabilities. Building on the results of the GOATS '9S sea trial, the next experiment is scheduled for September 2000.
"," Odyssey contigurll lion for GOATS 98
Fig.
1. Configuration of Odyssey II ACV 'Xanthos' for acoustic measurements in GOATS'9S. The ADV control electronics and batteries are located in two 17" Benthos glass spheres. An S-element array is mounted in a 'swordfish' configuration, and connect.ed to a dedicated acquisition system in the centre ,,,-ell of the vehicle.
Odyssey II to support a wide range of payload systems that include CTD, ADCP IDYL, ADV , side-scan sonar, USBL tracking systems, OBS , and several video systems. The core vehicle has a depth rating of 6,000 m, weighs 120 kg, and measures 2.2 m in length and 0.6 m in diameter. It cruises at approximately l.5 mls with endurance in the range of 3-12 hours, depending on the battery installed and the load. Included in the core vehicle are the guidance and navigation sensors necessary to support autonomous control: attitude and heading , pressure, altimeter, and LBL acoustic navigation. The Odyssey is controlled using a behavior-based layered architecture. At the highest level, behaviors seek to achieve mission goals such as following a track line to a way-point. The layered controller generates high level commands that specify the desired speed, depth, and heading of the vehicle. Taking these commands as inputs , a PlO dynamic controller generates the low-level commands that feed directly to the thruster and actuators. The layered control architecture allows the Odyssey to perform complicated adaptive missions such as bottom following surveys, or missions tracking isothermal contours, for example, while maintaining overriding safety procedures such as keeping a minimum distance from shore or avoiding obstacles. Two Odvssevs are used in the GOATS experiments, the ac·ous~ic ATV and the REA ADV.
2. COr-.IPOT\E;\TS OF THE GOATS AOSN The GOATS prototype observational network is based on the Odyssey class vehicles developed by MIT , on the Ocean Explorer developed by Florida Atlantic Lniversity, and on the SEPTR profilers developed by SACLA~TCEN.
2.1 The Odyssey AUVs The Acoustic A UV The acoustic ATV features an acoustic payload , developed at SACL-\;\TCEl\' , consisting of a line array, mounted in the vehicle 's nose, in a 'swordfish' configuration , and an autonomous data acquisition system , installed in a watertight canister in the vehicle 's payload bay shown in Fig. 1
The Odyssey II class autonomous underwater vehicles were chosen as the mobile sensor platform for the GOATS JRP because of their flexible architecture and proven performance. These ADVs have logged many hundreds of dives in 16 field deployments. A substantial fraction of the vehicle is dedicated to wet volume , which enables the
106
Tow Adt .,..
RF aueon , 5.1'0 •• ligh.
Fig. 2. The Florida Atlantic University Ocean Explorer: the vehicle consists of an aft propulsion control navigation section, a payload section and a forward nose cone
The REA A UV The REA ACV is equipped with a Datasonics combined sidescan and sub-bottom profiler system based on the Datasonics Chirp technology. The sidescan system features a 200 kHz and a 400 kHz component while the subbottom profiler is operating at 8 kHz, providing penetration of order one meter, adequate for seabed characterization of relevance to shallow water I\·fCM . The REA AUV also features a CTD probe for water column temperature and salinity measurements.
The payload section contains the mission sensors and is attached to the aft portion by a bayonet mechanism that allows for quick payload changes. Elements in the payload are simply additional nodes on the LO!\Talk network and are integrated with the tail section with a power conductor and a control line. The nose section contains a ,·ideo camera, a flashing strobe, the emergency drop weight and the vehicle recovery line device. The highly modular design allows to readily switch pay loads on a rear section or switch rear sections on a payload several times a day. The depth rating of the vehicle is 300 m and its endurance with standard batteries is 50 km at 3 kn. During operations OEX can be tracked acoustically via USBL and communicate via the acoustic modem. Launching and recovery can be achieved up to seastate 4 and does not require diver intervention. The vehicle specifications are summarized in Table 1. Table 1. Specifications for Ocean Explorer AUV AU\, Parameter Vehicle Specification Shape Gertler Series 58 Model 5154E Diameter 0.533 m (21 inches) 2.66 m - 3.91 m Length Weight 550 kg max. Speed 1 m/s - 2.4 m/s Depth 300 m Materials Fiberglass , AI , Various Plastics 0.2 m 3 - 0.3 m 3 Payload Volume Actuation Type 4 independent stepped motors Propulsion Type Direct Drive DC motor Battery Capacity Ni-MH , 3000 watt-hrs, 500 cycles Operating System QNX (PC- 104) Dy namic Characteristics at cruise speed Yaw 1.020 R~1S 3 Hz Roll 0.500 RMS 3 Hz Pitch 0.360 R:v1S 3 Hz Turning radius < 10 m
2.2 The Ocean Explorer A UV In 99 SACLANTCEN has specified and procured through international competitive bidding an Autonomous Underwater Vehicle to conduct experiments in support of REA and MCM studies. The selected ACV is a modified Ocean Explorer (OEX), manufactured by the Florida Atlantic University. The OEX is a highly modular vehicle, featuring a PC-104 Pentium executing the supervisory control structure, interfaced to a distributed communication and control network based on the open LO~Talk network protocol , a modular power system and a modular payload capability. The modular design results in a field reconfigurable vehicle, well suited to the experimental work in support of the scientific program of work. The vehicle consists of an aft propulsion/control/navigation section , a payload section and a forward nose cone (Fig. 2). Located in the aft section are the propulsion engine, the control surface motors, the navigation sensors, the control computer, the RF and acoustic communication systems, and the I\ickel ~1etal Hydride (:\"i-~1H) battery packs. The navigation relies on a standard suite of sensors (tri-axial flux gate compass , precision rate gyros and accelerometers, Doppler Velocity Logger) that provide both relative and geodetic positional information. The modular design facilitates the inclusion of a variety of geodetic navigational instruments including a DGPS, long baseline (LBL), and ultra-short base line (CSBL) acoustic tracking systems.
2.3 The SEPTR profiler A Shallow-water Environmental Profiler in Trawlsafe, Real-time configuration (SEPTR) (Tyce et al. (1999)) is being developed by SACLA:\T L'ndersea Research Centre in support of oceanographic modeling and rapid environment assessment programs. It is intended for extended duration deployments in areas where water column instruments are at risk from fishing trawlers, but with real-time data return and control via twowa:, cellular or satellite communication. It consists of a trawl-safe bottom platform which houses an Acoustic Doppler Current Profiler (ADCP) , wave/tide gage, ambient noise sensor array, and water column profiler buoy system. The profiler buoy system and associated winch are designed for autonomous vertical profiling of CTD and optical properties of the water column in depths down to lOOm. The profiler buoy includes DGPS
107
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.
,-............. '
-
I
/
-1
.....
----
Fig, 3, SEPTR operational scenario navigation capability along with two-way communication capability while on the surface, It also includes acceleration and magnetic field sensors capable of surface wave buoy measurements. The bottom platform includes an extended duration battery package in the recoverable barnacleshaped housing. Recovery of the entire system is accomplished through normal communication with the profiler in order to release a messenger buoy. The system is designed for 360 profiles during a 3-month deployment in lOOm water depth , with current, wave/tide and noise measurements everv hour. Two way communication of data, position and control allows profile results to be returned in real time , and operational commands, profile schedules , and DGPS correctors to be sent to multiple profiler instruments. Complete profiles will also be stored on board the recoverable units. The operational scenario for SEPTR is detailed in Fig. 3. The platform is lowered to the bottom bv a surface ship, and released via acoustic comm~nd after monitoring depth and tilt of the platform . At least once an hour , measurements are made of Doppler current profiles, tides and waves, and ambient noise. Every 4-6 hours a profile of water column properties is made by the profiler buoys, stopping at the surface to communicate data ashore, and to collect any new instructions, via GS!\1 cellular or ORBCOI\1M satellite communication .
between which the AUV would navigate using the long baseline (LBL) acoustic navigation system , at selected depths and speeds. The mission file was downloaded via the computer network to the vehicle computer. At the same time the acoustic acquisition system was programmed for the mission. T /B Manning deployed the TOPAS sound source facilitv and the fixed arrays near the target area. A sh;re laboratory controlled the TOPAS sound source and recorded data from a 16-element vertical arrav a 12S-element horizontal line array, and a buri~d hydrophone array. Alliance anchored offshore was used as a platform for the AUV operation and data processing centre. A typical mission took the A DV from the Alliance to the target area at a speed of 3 kn. Once reached the target area, the AUV made several passes at less than 2 kn over the targets. When the survey pattern was complete, the ACV transited back to the Alliance. After surfacing approximately lOOm from the Alliance, the AUV was towed back with the work-boat and lifted onto the deck, On deck, the ACV was connected to the computer network, and data was uploaded from the navigation computer and the acoustic acquisition system. The ACV navigation data were then processed in order to verify the completion of the mission plan, and for parsing of the reduced navigati?~ data with the acoustic data. Thirty-nine At:\ missions were launched and completed , totaling 14 hours of submerged time. The performance of the ATV was completely reliable, and no serious hardware problems were encountered . The control algorithms developed at 11IT were capable of repeatedly performing the desired survey patterns in the target area with a navigation accuracy of a few metres using the LBL acoustic net deployed in the area.
3. AC\' OPERATIONS DCRING THE GOATS'9S EXPERl}.IE:\T During GOATS '9S the Odyssey ATV was operated from the Alliance, anchored approximately 600 m from the target area (Fig.4) (Schmidt et al. (199S)). :\Iission design and implementation comprised several steps. First, the mission track \\'as designed using a graphical tool on a workstation onboard Alliance. The mission plan was then translated into a series of way-points
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Fig . 4. Schematic (not to scale) of GOATS'98 operational scenario at Marciana :\1arina. 4. GOATS '2000 SCIENCE PLAI\
validation of numerical models of mono- and bistatic seabed reverberation. The experiment will demonstrate the adaptive detection and sampling capabilities of AlIVs: the results of the measurements will be used in real time for changing the AUV behavior in order to optimize the information collected. The MIT Acoustic AUV will enter the target field insonified by parametric or conventional sources on SLC tower. Initially only proud targets will be insonified. Using real-time beamforming in the acoustic acquisition system, the target scattering lobe of the proud target will be detected and the vehicle will initiate a series of 360 deg circles at different depths centered at estimated target position , sampling full 3-D scattering. Following this relatively simple configuration, the adaptive sampling of the bistatic acoustic field will be measured with the insonification being provided by the sub bottom profiler on the MIT REA AUV.
The GOATS 2000 experiment will have three broad objectives, REA , yIC:\-1, MEAl\S.
4.1 Rapid Environmental Assessment (REA ) The scope of this experiment is to demonstrate the capabilities of AUVs as REA platforms for MCM in very shallow water. The information collected includes bathymetry, density of mine like objects, seabed composition, backscattering strength and water mass properties like current, visibility, salinity, density and temperature. In GOATS'2000 two ACVs , the SLC AUV with the Marine Sonic dual frequency SSS and the MIT REA ACV with a combined side-scan/sub-bottom profiler system, will be launched from R/V Alliance, and transect to one or more of the bays on the East side of Procchio. The AUVs will acquire side scan sonar and sub-bottom profiler data and water mass properties like current, visibility, salinity, density and temperature, to be used by nested now-cast and forecast oceanographyc models. Emphasis will not be put on fully covert operation, and both LBL based naYigation and DGPS navigation will be utilized as part of the experiment. The ability of long range naYigation and operation will howeyer be demonstrated. For this purpose R/V Alliance willleaye the Procchio Bay, and launch SLC ADV from a distance of several miles.
4.3 Multiscale Environmental Assessment Network Studies (MEANS) A nested modeling and assimilation capability will be provided by Harvard University and SACLA:"iTCEI\ . A local model at 20 km scale, covering the North shore of Elba and the Procchio Bay, will be nested within a 200 km scale model covering the Ligurian sea. At the basin scale these models will be nested within the :'-1AVO Mediterranean sea model. Environmental data collected by two SEPTR buoys and AUV s in Procchio Bay will be assimilated into the local model providing a now-casting and forecasting capability for the experimental area. On the regional scale, the environmental sampling will be performed by R/ V Alliance at night. The basin-scale :\ AVO :'-.1editerranean Sea model is constrained by satellite remote sensing data, including surface temperature and altimetry.
4.2 Mine Counter Measures (MCM) The rippled field at the main test site in Biodola Bay will be insonified by the TOPAS parametric sound source mounted on the SLC rail at a yariety of incident and aspect angles. The acoustic :\UT AU\-, will sample 3-D reyerberation using survey patterns similar to those used in GOATS '98. These measurements will provide a data set for
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Fig. 5. Modular design coupled with plug and play SW will provide just-in-time n:anufacture of s~ecial purpose AUVs that will be the intelligent nodes of distributed ocean samplIng networks (FIgure courtesy of Thomas B. Curtin, Office of Naval Research) ACKNOWLEDGMENT
5. CONCLUSION
The MIT component of this work is supported by the Office of Naval Research. The GOATS JRP provides a series of coordinated, incremental implementations of the Autonomous Ocean Sampling Network concept for coastal REA and MCM as envisioned by Curtin et al. (Curtin et al. (1993)) , Fig. 5. Thus, the GOATS'2000 experiment will include a number of different AUVs operating in shallow and very shallow coastal environments with a variety of sensors for water column and seabed characterization. The AUVs will be communicating with the control center onboard R/V Alliance via acoustic communication while submerged, and radio modems while surfaced. These communication capabilities will be utilized for providing vehicle navigation and control data to the control center, and for remote control of sampling patterns. Subsequent experiments are envisioned to incorporate nextgeneration small AUVs with enhanced control, navigation and communication capabilities, and new sensor technology. Several vehicles are currently being designed by academia, government laboratories and industry. For example, it is expected that feature-based navigation will become particularly important for coastal MCM and REA applications of this new technology. Also, future deployments of the AOSN may utilize cellular acoustic communication networks connected to shore or surface ships through RF and satellite communication links with the potential for the development of entirely new operational paradigms for coastal oceanography and littoral warfare. In this regard, however , the biggest obstacle will probably not be of technical nature, but rather be associated with the breaking down of barriers of tradition among the 'customers', i.e. the scientific and operational communities.
References T . Curtin, J.G . Bellingham , J . Catipovic, and D. Webb. Autonomous oceanographic sampling networks. Oceanography, 6(3) :86-94, 1993. H. Schmidt, A. Maguer, E. Bovio, W.L. Fox, K. LePage, N.G. Pace, R. Hollett, P. Guerrini, P.A. Sletner, E. Michelozzi, B. Moran , and R. Grieve. GOA TS'98 - Bistatic measurements of target scattering using autonomous underwater vehicles. SR 302, SACLANT Undersea Research Centre, La Spezia, Italy, 1998. R. Tyce, F. de Strobel, V. Grandi, and L. Gualdesi. Shallow-water expendable and trawler safe environmental profilers. In Proceedings of Oceans '99, pages 1229-1233, Seattle, WA, 1999. MTS/IEEE.
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