UNDERWATER GLIDERS – FORCE MULTIPLIERS FOR NAVAL ROLES
2. TECHNOLOGICAL DEVELOPMENTS 1 CONCEPT
The vision of small, low cost, long endurance and networked autonomous ‘Gliders’ is attributed to the oceanographer Henry Stommel of Woods Hole Oceanographic Institute (WHOI) and Douglas C. Webb [2]. The apparent simplicity of the concept makes it extremely attractive. A heavier-than-water vehicle (encapsulating requisite instrumentation and controls) will dive without propulsion. Due to its small size (length about 2 metres), the pressure-resistant enclosure (for internal electronics and controls) within its outer form can be made strong enough to withstand hydrostatic pressure of the order of 1000 metres. During its dive, the vehicle covers a distance forward, depending upon the lift generated due to its hydrodynamic form (body and wings) and its angle of attack. The trim angle of the vehicle as well as its buoyancy is controlled by internal mechanisms, using autonomous controllers programmed as a function of depth.
At the requisite depth, the buoyancy ‘engine’ is used to make the vehicle lighter (usually by operating a hydraulic pump, enlarging an inflatable chamber within the outer envelope of the vehicle). The vehicle then glides upwards, till the ocean surface, or till a pre- designated depth. Thus, the underwater glider describes a saw-tooth trajectory across the ocean depths (Figure 1).
During each glide, its on-board sensors record data, typically on salinity, temperature and density of sea water. Periodically the vehicle surfaces, raises its satellite antenna and transmits its recorded data via satellite to a shore monitoring station, during which it can also receive information, as well as update its position. The primary vehicle navigation system uses an on-board GPS receiver
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects coupled with an attitude sensor, depth sensor, and
altimeter to provide dead-reckoned navigation.
Figure 1. Method of Travel of an Underwater Glider [3]
2.2 ‘LEGACY’ GLIDERS
Today’s three most widely used designs of operational, commercially available, gliders are products of the Autonomous Ocean Sampling Network program of the Office of Naval Research (ONR), US Navy. These are termed here as ‘legacy’ gliders and their main characteristics are summarized in Table 1. The Slocum Electric Glider (Figure 2) developed by Teledyne Webb Research [4]; the Seaglider (Figure 3), developed by the University of Washington [5]; and the Spray (Figure 4), developed by Scripps Institute of Oceanography (SIO) and Woods Hole Oceanographic Institution(WHOI) [6].
Approximately 160 commercially available gliders of these three types were in operation in 2009 [8].
Figure 2. A Slocum glider on the surface [23]
The low-power propulsion system of underwater gliders enables deployments over distances greater than 1500 km, with durations longer than thirty days and diving depths of minimum 200 m. This characteristic
distinguishes them distinct from other AUVs, which require constant energy from batteries for propulsion, thus resulting in an endurance of a day or two at most.
Thus, the glider is an ideal autonomous remote sensing platform. The glider can transfer data and receive new missions, through Iridium communication link. It can carry physical, optical and acoustic sensor packages to measure various ocean environmental parameters. [9]
Figure 3. University of Washington’s Seaglider in its handling cradle [23]
Figure 4. Spray schematics [3]
Table 1 Characteristics of Legacy Gliders [7]
Property Slocum Electric
Seaglider Spray
Weight 52 kg 52 kg 51 kg
Length 1.5 m 1.8 m 2.0 m
Max. Depth 200/ 1000 m 1000 m 1500 m Avg. Speed 0.35 m/s 0.25 m/s 0.30 m/s Max. Range 1500 km 4600 km 4700 km Endurance 20 days 6 months
Developed by Webb Research
Univ. of Washington
SIO &
WHOI Produced by Teledyne iRobot Bluefin
Robotics
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects The low cost, long endurance and ease of handling of these Gliders offered significant advantages over typical oceanographic profilers, particularly since a mother ship is not required for expensive cruises to deploy/ recover them. Gliders are relatively inexpensive: about $100,000 each, as compared to $30,000 a day for a data-gathering mission using a ship [10, 11]. From another perspective, gliders can collect several multi-variable oceanographic profiles (e.g. temperature, salinity, velocity, oxygen, fluorescence, optical backscatter, etc.) for the cost of a single expendable bathythermograph (XBT) probe [9].
All three ‘legacy’ gliders are typically launched either directly from a small boat, or by a sling using a shipboard winch or crane. They can be recovered either directly into a small boat, or by bridle (or lasso) using a crane or winch from the deck of a larger vessel.
In addition, gliders are extremely stealthy. They are quiet, with very low self-noise, small acoustic cross section and leave a practically invisible wake. They are scalable in design (from small to large). They also offer energy recovery capability: from ocean temperature gradients, from ocean currents, from waves (surface gravity & internal).
2.3 SCIENTIFIC DEPLOYMENTS
Glider missions so far have mainly focused on data- collection for biophysical and physical oceanography, contributing to studies on ecosystem dynamics, red tides, ocean circulation and climate-related research [1].
In the 15-odd years since the first glider first went to sea, the ‘legacy’ gliders have been used extensively across world oceans to provide online data on oceanographic parameters. They have been used to measure ocean currents, salinity and temperature. They are also being used to monitor pollution levels, plankton blooms and, monitor marine animals. They have been used in the Atlantic, Pacific and Indian oceans, and even for polar ice missions, in Arctic as well as Antarctic seas. They have been used in coordinated surveys for oceanographic applications. Their applications include surveys near oil rigs, marine biology studies. Gliders were used to monitor the quality of water in the Gulf of Mexico after the oil spillage in summer of 2010 [12].
Oceanographic departments in universities worldwide are increasingly using gliders for scientific research. For example, gliders of Rutgers University (New Jersey, USA) have logged 40,000 undersea miles on 166 missions [10]. There are currently 4 gliders deployed in Antarctica, which are used to gather and transmit data about penguin habitat to researchers in Rutgers University, 25,000 miles away, via satellite connections [13]. In the past five years, a fleet of 32 gliders operated by Oregon State University (OSU)’s College of Oceanic and Atmospheric Sciences has covered more than 43,000 kilometers in the Pacific Ocean. In those five years, the
gliders have recorded more than 156,000 oceanic profiles, almost 40 times that provided by six decades of shipboard studies [14].
Gliders have crossed the Atlantic and operated in hurricanes. As examples of maximal deployment endurance, one Seaglider mission covered 3,200 km in six months, while another covered 3,750 km in seven months [1]. A Slocum Glider completed a 5,700 km voyage over 160 days by in 2008. Implementation of different mission planning algorithms to gather spatio- temporally variable data in a given region has been demonstrated at sea on several occasions [15].
2.4 ‘FLYING WING’ GLIDERS
The first Underwater Glider purely for defence application was the ‘X-Ray’ or ‘Liberdade Flying Wing’
(Figure 5), developed by the Scripps Institution of Oceanography and the University of Washington [16].
Unlike the body-wing combination used for the initial
‘legacy’ Gliders, the ‘X-Ray’ consists of a ‘blended wing body’. This shape was motivated by the purpose of functioning as a passive sonar array, with hydrophones arranged on the leading edges of the wings, as well as to maximise the horizontal distance covered in each glide.
Thus, the X-Ray has been designed for an anti-submarine warfare role. The glider can surface to transmit data to a satellite, or stay submerged to send acoustic communications.
The X-Ray has a 6.1 m wing span and is 20 times larger by volume than the legacy gliders (weight: 850 kg). Its lift-to-drag ratio is 17/1 at a horizontal speed of about 1.8 m/s. The payload includes a low-power, 32-element hydrophone array placed along the leading edge of the wing (for large physical aperture at frequencies above 1 kHz), and a 4-component vector sensor [17]. Its endurance at nominal load is 200 hours, while endurance on hotel load is about 6 months [18]. It can be programmed to monitor large areas of the ocean (maximum ranges exceeding 1000 km with on-board energy supplies). The glider is very quiet, making it hard to detect using passive acoustic sensing.
Figure 5. X-Ray glider during at-sea testing [16]
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects Trials of the X-Ray were carried out over 2006-2008.
Initial sea trials were conducted in March 2004 to validate hydrodynamic design, in which glide trajectory at a prescribed net buoyancy was observed to confirm that the wing was flying as designed. Subsequent phases of trials were for integrating the sensors and proving endurance and range. During trials in August 2007, the glider was deployed using the glider launch and recovery system [19, 20, 21].
Based upon the experience developed over three years of at-sea testing, the XRay has been followed by the ‘Z- Ray’ (Figure 6), an even larger version of the ‘flying wing’ type of Glider [22]. Sea trials were planned in Dec 2010/ Jan 2011. The Z-Ray has been designed and built by the Marine Physical Laboratory of Scripps Institution of Oceanography (MPL/SIO) and Applied Physics Laboratory of University of Washington (APL/UW). The outer shroud is made of plastic and is mounted to a titanium inner strength structure. The glider has a maximum design depth of 300 m and weighs about 680 kg in air. [23].
Figure 6. Photograph of Z-Ray, without its 3-ft antenna mast or wing tips [23]
Some of the design changes made in the Z-Ray as compared to the X-Ray were as follows [22]:-
x A new airfoil was chosen for the outer shape, specifically designed to operate in conjunction with camber-changing trailing-edge flaps. The wing has a larger aspect ratio and a swept-back angle of 30 deg, moving the center of pressure aft.
ZRay should achieve lift-to-drag ratios exceeding 35-to-1, over twice that of the XRay.
x The outer shroud is designed to be made of ABS plastic mounted to an inner strength cage made of titanium. The XRay used “monocoque”
construction of fiberglass and carbon-fiber composite materials for reinforcement, which has superior strength-to-weight ratio, but is difficult and expensive to modify.
x The pressure housings containing the glider flight electronics with an oil-filled housing have a shape conformal to the interior space (instead of spherical shape).
x Small water jets are incorporated for fine attitude control at or near neutral buoyancy, particularly important for orienting the leading-edge hydrophone array aperture in specific directions.
x To increase the passive sensing capability, mountings for four large sensors (e.g., low frequency acoustic vector sensors or very wideband (200+ kHz) hydrophones), are incorporated one each at each wingtip and in the tail, in addition to the one in the nose (for X-Ray).
(Figure 7)
Figure 7. CAD/CAM drawing of Z-Ray, showing the locations of passive acoustic sensor systems in the glider. [23]
2.5 FUTURE CONCEPTS
In the footsteps of the ‘legacy’ gliders, several other designs have been developed. These are listed in Table 2.
To further reduce dependence on stored electrical energy, concepts for harnessing thermal, solar and wave energy are being developed [24].
The concept of ‘hybrid’ gliders refers to a provision of AUV-type thruster in a glider, enabling it to follow a horizontal trajectory when necessary. Examples are the AUV-Glider, SeaExplorer and AutoSub LongRanger.
The thruster will also enable the glider to overcome strong ocean currents and maintain its course. It will also be able to use greater speed when on thrusters [7].
The maturity as well as the potential of glider technology can be gauged by the more than sixty papers presented in the 5th EGO Workshop and Glider School (European/Everyone's Gliding Observatories) on experiences related to glider technology and its multidisciplinary applications, held from 14-18 March 2011 at Gran Canaria, Spain [25]. Sensors are now being
Warship 2011: Naval Submarines and UUVs, 29 – 30 June, 2011, Bath, UK
© 2011: The Royal Institution of Naval Architects custom-produced for use on gliders. Technologies are being developed for coordinating operations of multiple gliders of different types, managing collaborative data- collection and operations.
Table 2 Summary of Other Futuristic Gliders
Name Main Features Developer Webb
Thermal Glider
Buoyancy change by using temperature variation with ocean depth; extremely long endurance
Webb Research/
Teledyne
Deep Seaglider
Greater depth than Seaglider
iRobot SeaExplorer Hybrid. Uses acoustic
fix for navigation.
ACSA Autosub
LongRanger
Hybrid.
Bionik Manta
Biomimetic form.
Endurance 24 hours.
Evo Logics USM Glider Prototype University
Sains Malaysia ALBAC Sea trials in 1992; one
glide cycle
University of Tokyo
AUV-Glider Hybrid. 6000 m depth. 2 knots speed.
Florida Inst. of Tech.
WaveGlider Solar & wave- powered.
Surface float tethered to sub-surface glider.
Liquid Robotics Sterne
Hybrid Glider
Hybrid. 3.5 kn on thruster, 2.5 kn gliding.
4.5 m long.
Ecole NSD’I, Brest, France