Description of the prior act AUVs are small, unmanned vehicles. Some of them use a pressure housing (a dry housing) and some use a so-called flooded housing. Most AUVs have some natural buoyancy which means that they float when they are still. These AUVs dive by speeding up on the surface and pointing their depth rudders down in order to overcome the buoyancy. Therefore a slight downward force is required all the time while the AUV is underwater to overcome the buoyancy.

Existing AUVs are frequently claimed to be modular, i. e. that it is possible to add new instruments, sensors and equipment into the vehicles. This normally only applies up to a certain point when the space inside the vehicle is exhausted. After this, the vehicle has to be literally cut a part and a new section added to house additional equipment.

These vehicles must be kept large enough so that new equipment and sensors can be inserted into them without major modifications. This means that their hulls must be kept large so that extra space is available. If no extra equipment will be used, the vehicles are unnecessarily big and heavy making them difficult to handle and operate.

In the same way modifications of the software, to reflect changes in hardware or functionality, is normally not an easy task and cannot be performed by the owner or operator.

Some other vehicles are claimed to be modular if it is possible to insert a hull section at a certain insertion point. Normally this requires major work in order to connect the new section electrically to the other parts of the vehicle and reroute existing interconnections.

In other cases vehicles are claimed to be modular if their hull can be elongated by adding tubular sleeves into the hull or in between existing hull sections. The new sleeve has the purpose of covering newly added electronics and account for the increase in length of the hull. The term modular in this case only applies to the tubular sleeve which is a simple metallic tube with sealing surfaces at its ends.

None of the above cases tackles modularity as a whole, for example, an electrical bus system, carrying control signals, power and data between modules or software modularity. Therefore drastic changes may have to be made to the vehicles when new equipment or functions are added.

General description of the Invention Gavia is a new and technically advanced AUV (autonomous underwater vehicle) that is truly modular. It tackles modularity as a whole, i. e. it deals with: 1. Electrical interconnection of the modules, i. e. a bus system 2. Mechanical interconnection of the modules including fastening the modules together 3. Software accompanying the modules.

The true modular construction of Gavia means that modules can be removed or added without change to the vehicle, even in the field. The modules will function irrespective of where they are inserted in Gavia's hull. No new connections have to be made nor any other changes to the vehicle.

Integration of new hardware and software modules are also possible with minimum effort and can be performed by the owner or operator. A Gavia module can contain sensors, various kinds of equipment or parts, which are necessary for the functioning of the module.

Figure 1: Gavia's modular construction with a typical set of modules. Here Gavia has been opened at the intersection of two modules, the Acoustic communication module and the Power module. A Gavia module bus connector is visible.

The module bus connectors, which are located at the centre of the end panels of each module, carry the Gavia Module Bus, which runs through the vehicle. The Gavia module bus carries 1. Power to the modules from the Power Module 2. Control signals and 3. Data In addition to this, opticai transmitters and receivers can be mounted on the end panels of the modules, carrying high-speed data or specialized data. The optical transmitters and receivers line up when the modules have been snapped together.

An Intelligent Artificial Crewmember accompanies most modules. This is a software module that integrates the module into Gavia's system. The customer may write his own crewmember to accompany a custom module.

Gavia is available with different depth ratings, currently up to 200 bar, equalling a depth of approximately 2000 metres. The same modular system is used in all Gavia vehicles, irrespective of their depth ratings. The modules are also identical in size and shape irrespective of their depth rating.

The true modular construction of Gavia has some more important advantage.

1. Faulty modules can be replaced with new ones or removed in minutes, insuring minimum"down-time"of the vehicle.

2. Gavia's modules can be tested as stand-alone units from an ordinary PC-computer, using a special interface unit and software.

Detailed description Gavia is small and lightweight and can in some cases be carried and operated by a single person. Measurement tasks can therefore be carried out with Gavia from the shore, using a small inflatable or from a research vessel of opportunity.

Figure 2: Gavia AUV seen from the front. Here Gavia has been opened at the intersection of two modules making the module bus connector visible.

Figure 3: A typical Gavia module.

Figure 3 s hows a module bus connector on the end panel of a Gavia module. A similar connector is at the other end of the module. Electronics, equipment and other parts are located inside the module.

Each module has a set of double 0-rings at one end and a sealing surface at the other, where the next module joins it. The customer has the choice of standard hull modules each with a different function but he also has the option of making custom modules for his specific application.

The modules are closed at the ends using aluminium panels that are sealed using 0-rings.

Thus the modules themselves are splash and spray proof and Gavia can be opened in the field or on the deck of a ship without danger of damage to the electronics or parts inside the module.

Figure 4: A cut-view of two Gava-long modules joined together, showing also the module frames and end panels with the Gavia module bus connectors. The electronics of the modules, equipment and parts may be assembled into the module frames.

Each module is designed to be neutrally buoyant in fresh water, i. e. it will neither float, nor sink when placed in fresh water. This is necessary in order to make the modules interchangeable without altering the balance of Gavia. The so-called payload modules have reserve buoyancy in fresh water, allowing equipment and sensors to be added to them.

The module bus connector sits at the centre of each panel. One end of each module has a male connector and the other has a female connector. These connectors carry signals to the modules and between modules. More specifically they carry: 1. Power 2. Control signals 3. Data The electronics contain intelligent processing devices, electronics or some other parts depending on the purpose and functioning of each module.

The modules are fixed together using a type of bayonet system. When two modules are joined, they are: 1. Pushed together; steering pins on one module mate with steering holes on the other so that the module connectors mate correctly.

2. The mating module is turned clockwise approximately 15 degrees to lock the modules in place. The mating module bus connector rotates along with the module, up to approximately 15 degrees from the neutral position where the modules are fully joined together.

This system is both easy to use and secure. In addition, it joins Gavia's modules to form a whole unit that has a rigidity that approaches the rigidity of a hull, which is made as a single unit. Thus Gavia has high immunity against bending forces, which is an important

factor in some applications, for example, in mine counter measures (MCM) where a submerged bomb may explode in the vicinity of Gavia, exposing her hull to high stresses.

This modular system also facilitates easy assembly and disassembly of the modules.

Gavia can be opened up anywhere for easy insertion or removal of modules. For example, the power module can be rapidly replaced with a new one which is fuliy<BR> charged. Therefore the"down time"of Gavia is significantly lower than for a non-modular<BR> vehicle. Modules with various battery technologies can be inserted in Gavia's hull as a result of this type of modularity.

A pair of hand tools, grippers can be used to assist in the assembly or disassem, uly of the modules, especially for rotating the mating module the approximately 15 degrees necessary to lock the modules together.

Figure 5 Figure 4 shows the assembly of two modules. The 0-rings of the module on the left slide under the sealing surface of the module to the right. The module connectors are steered<BR> together using a pair of steering pins on the module to the right and a corresponding pair of steering holes on the module to the left.

The 0-rings are ordinary hydraulic neoprene 0-rings, shore 70 or higher. Other types of 0-rings may be used.

Double 0-rings are used to reduce the risk of leaks in case the 0-rings get damaged or O- ring groves are scratched.

The modules are made of anodised aluminium alloy.

An Intelligent Artificial Crewmember accompanies some modules. This is a software module that handles the functions of the module and integrates the new module into Gavia's software system. The customer may write his own crewmember to accompany the module provided he adheres to certain rules.

Each Gavia module has a so-called Module Interface Units or MIU, which is a bus node of the Gavia Module Bus. The MIU is an interface between the Gavia Module Bus and this particular module. It also controls the power and monitors the voltages and current drawn by the electronic units and circuitry of this module.

The Module Interface Unit contains the following: 1. Interface connectors to the Gavia Module-bus.

2. Control-bus node 3. DC-DC converters which supplies power of the appropriate voltage and power rating to the module 4. Processing unit 5. Power switches for turning on and off power to individual units of the module 6. 1/0-interface connectors to individual units of the module 7. A 100Base_T Ethernet connector for enabling a PC-computer, which can be installed in the module, to be LAN connected to the Gavia main PC.

The Gavia module bus runs through the modules via the module connectors on each end of the modules. The module connectors are sturdy military-type connectors with gold plated contacts. There are 26 pins in the connectors and 26 leads used for the bus.

The bus is divided into three parts: control, signal and power.

1. The control part consists of 2 leads, the serial CAN-bus (Controller Area Network).

2. The signal part consists of: a) 7 leads, and is used for device communication, RS233/RS485 b) 100Base_T Ethernet bus (4 leads) 3. The power part consists of 13 leads, which are used to carry power to the modules and devices.

There is no preferred length of the vehicle; rather this is determined from the number of modules installed at each time. The Gavia modularity is an effort to make life easier for the customer and an attempt to standardize AUV construction and sensor installation.

Sensors can be easily integrated into Hafmynd's modules as shown in the figure below.

Figure 6 Figure 5 shows a typical mounting hole for a sensor in one of the modules.

Hafmynd has, for example, integrated a transparent window into modules for a digital camera and a strobe light. The diameter of the windows is around 90 mm. Various types of sensors can be installed in the modules in this way. Battery modules are also available with different battery technologies and different sizes of batteries. The modules can be assembled almost in an arbitrary way. The Nosecone Module must though be located at the front and the Motor Module at the stern.

The base configuration is as follows : 1. Nosecone Payload Module 2. DVL (Doppler Velocity Log) Module with optional ADCP capability 3. Control Module (with conning antenna tower) 4. Power Module 5. Servo Modules (depth and heading).

6. Motor Module Other optional modules available : 1. Camera Module 2. Strobe Module 3. Scanning Sonar/Drop Weight Module 4. Acoustic Modem Module with optional LBL capability 5. Payload Module (short or long) 6. INS (Inertial Navigation System) Module

7. Dynamic Buoyancy Control system housed in a separate module (under development) Artificial crewmembers A typical artificial crew is composed of the following members: 1. Captain 2. Mission Leader 3. Pilots-several different pilots may be used on board Gavia 4. Engineer 5. Scientists-several scientists are possible The Captain oversees the vehicle and is responsible for the bringing it back home.

The Mission Leader is responsible for carrying out the mission plan in the most suitable way at the same time adhering to the Captains instructions.

There may be several pilots : 1. One for tracking objects such as pipes 2. One for steering past obstacles in a controlled manner 3. One for steering to a fixed point 4. One for steering along a straight line The Engineer monitors the well being of the AUV hardware components, such as motor, servos etc. and warns should anything go wrong.

The Scientists carry out tasks in a similar way as on board a research vessel. Thus one scientist may read off a sensor and interpret its meaning. He may then report to the mission leader who takes action accordingly.

The current availability of Gavia modules Nosecone payload module Figure 7 Various sensors and cameras can be mounted in the Nosecone Payload Module. Different mounting arrangements are possible as the pictures show. A digital video recorder and floodlight can, for example, be mounted in the nosecone of Gavia, either looking front or down. A strobe located in a Strobe Module provides subject illumination for the camera if needed.

Figure 8: A frame from a video recorder mounted in the Nosecone, looking forward.

Doppler velocity log module (DVL) Figure 9

A doppler velocity log (DVL) is standard equipment on Gavia. The DVL is mounted in a separate hull module, a so-called Gavia-short module. The DVL uses a frequency of 1200 kHz. It can track the bottom from less than a meter up to a height of 30 m depending on conditions.

Control and communication module Figure 10 This module holds the control and communication electronics and equipment and optionally the side scan sonar electronics. If side scan sonar is used, the transducers are mounted on the outside of this module. The transducer signals are fed out through watertight plugs on each side of the module. The plugs are located on the caps shown in the figure (9).

The conning antenna tower is mounted in a small mounting hole on top of the module.

Thus a separate antenna module is not required. Besides, this has the advantage that the antenna cables go directly to their corresponding mating connectors inside the control module and do not need to be fed through connectors on their way.

The Control and communication module, as most other Gavia modules, can be mounted anywhere into Gavia's hull. A good place is behind the DVL Module or behind the Scanning Sonar/Drop Weight Module if this is installed.

The following equipment is housed in the Control and communication module.

1. PC computer. The PC computer is equipped with wireless LAN which is typically used for uploading large volumes of data, such as pictures and sonar images to the operators (laptop) PC on land or on board a ship or boat. Gavia can also be remotely controlled through the wireless LAN up to a distance of several hundred meters or more depending on sea state.

2. Conning antenna tower. The antenna sticks out of the water by 15-20 cm when Gavia is on the surface. The antenna housing at the top of the conning tower contains the communications and GPS antennas. It also contains an emergency strobe beacon so that Gavia may be spotted in the dark.

3. Iridium satellite telephone. The Iridium system is the only one that covers the whole globe, including both polar areas. Typically the phone is used for instruction uploading and for locating Gavia by instructing her to indicate her GPS fix. Once this has been done she can be readily found using a GPS receiver.

Gavia can also be instructed to go to a predetermined location where she can be picked up. By providing this communication link Gavia can be called up from anywhere in the world. Hafmynd's specialists can, for example, call any Gavia anywhere in the world and monitor their operation. This is a comfort to the operator as he can ask for assistance from Hafmynd no matter where he is located. The Iridium phone can be used for remotely controlling Gavia on the

surface. Data can also be downloaded through the Iridium phone though the bandwidth is limited.

4. High speed bi-directional data and command link, WLAN (Wireless Local Area Network). This high-speed data link is available to the operator of Gavia while she is on the surface and within line of sight. This link is especially useful for downloading large amounts of data from Gavia when she has surfaced after completing a mission. The WLAN can be used for remotely controlling Gavia on the surface much like a model boat. A high gain antenna and amplifier is included with Gavia.

5. Magnetic compass. A high quality electronic magneto-inductive 3-axis compass is used in Gavia. The compass also measures tilt and roll and operates at angles up to 45°. The compass is used in conjunction with the rate gyro. A compass calibration program is included with Gavia.

6. Yaw rate gyro. This is a silicon vibrating structure gyroscope (Si-VSG), a solid- state single axis rate sensor. The new concept ring-shaped micro-machined resonator shows distinguished resistance against external shocks and vibration over a wide range of temperature. The rate gyro is used in conjunction with the electronic compass.

7. GPS receiver. The GPS receiver is a low power advanced embedded type receiver using a high gain active antenna. The antenna is located in the antenna housing at the top of the conning tower.

8. Pressure sensor. An electronic piezo-resistive pressure sensor is used for measuring and controlling the depth of Gavia, a 20 bar type for Gavia coastal and a 200 bar for Gavia offshore ; sealed gauge. A delta-sigma A/D-converter measures the pressure signal with a resolution of 1 cm. Pressure sensor calibration can be easily performed using the optional hand pressure pump. For Gavia coastal a 20 bar pump using air pressure is used. For Gavia offshore a hand pump using water pressure and a small water reservoir is used. The pump is screwed into the pressure sensor valve (over the pressure sensor) and Gavia is put into calibration mode, which gives instructions on how to proceed. A hand pump made by Keller GmbH is used as a calibration tool. For calibration, the plastic ring that holds the pressure sensor in place is removed. The fitting for the hand pump is screwed on. A reading is acquired from Gavia using the User operator interface and at the same time read off the digital manometer of the Keller pump.

Figure 11 9. Emergency strobe beacon. A high power Xenon strobe is located inside the antenna housing at the top of the conning tower. Gavia may be easily spotted in the dark after the strobe has been activated. The Strobe beacon electronics is housed in the Control and communication module. It is a stand-alone unit

powered by its own batteries. Operation of the strobe beacon is set by a magnet with a visual feedback given by a LED.

10. Side scan sonar. The side scan sonar is available in several frequencies ranging from 150 kHz to 1200 kHz. This is a low power design that is based on the principle that less power emitted generates less unwanted noise in the received signal. Images are very sharp and resolution is high. The transducers are mounted as shown in the photo (black rods on the side of Gavia's hull). A side scan sonar image is shown here. The scale is 30 meters on each side. Gavia's altitude is 2 metres.

Figure 12 Figure 13 CTD meter. A CTD (conductivity, temperature and depth) recorder can be mounted on Gavia for continuous profile measurements of the salinity and temperature. The standard scientific CT meter offered is the SeaBird MicroCat-SI37. It is mounted piggyback on Gavia behind the antenna tower and underneath the streamlined cover.

Figure 14 Power module. This module holds the batteries and power management unit. Currently there is a choice of three different battery technologies.

1. NiMH rechargeable batteries with a capacity of 500 Whours 2. LiIon (Li+) rechargeable batteries with a capacity of 850 Whours to 1000 Whours.

3. Lithium primary batteries with a capacity in excess of 2000 Whours Battery capacity can be increased by adding battery modules to Gavia.

Battery recharge takes less than 4 hours. Power is supplied to the on-board battery chargers from an external DC power supply.

Figure 15 Servo module. The Servo Module contains servomotor mechanisms, one for each rudder.

This module contains the servomotors and magnetic couplings. A separate servomotor controls each rudder. This enables Gavia to be controlled in a unique way.

Figure 16 Motor module. This module contains the propulsion motor and drive mechanism for the propeller. The motor shaft is brought out through a magnetic coupling requiring no dynamic sealing. Dept rating is up to 200 metres.

Figure 17 Scanning sonar and drop weight module. The optional scanning sonar/drop weight module is located towards the front of the vehicle. The sonar scans 360 degrees and has a variable frequency range and beam width. This sonar is used for obstacle avoidance and object tracking depending on type of software modules.

When the drop weight is released, Gavia will point its nose up speeding the ascent. The drop weight is released automatically in a settable time after being activated by a magnet. The drop weight will also be released if its batteries become exhausted. An LED shows when the drop weight has been activated and when it will be released. The drop weight is a stand-alone unit and is powered by separate batteries.

Figure 18 Acoustic communication and LBL/USBL positioning module. The optional acoustic modem has a working range up to 3000 meters and data rate is up to 4000 bits/s depending on conditions. The modem comes standard with an accurate ranging function based on round-trip timing. This means that a submerged stationary vehicle can be located from the surface by measuring the range to the vehicle from different locations (by cruising on the surface). The accuracy can be within a few metres provided there is some knowledge of the water temperature distribution and the surface locations are well distributed. A software application is available for doing this. A LBL (long baseline) option is available for this module.

Figure 19 Digital camera module. The Camera Module is either a Gavia-short or a Gava-long module depending on the type of camera used. A view port is provided for the camera.

Normally the camera"looks"down onto the seabed, but other arrangements are possible.

A digital camera or video recorder can also be mounted in the nosecone.

Figure 20 The camera works in TTL mode (through the lens measuring system), meaning focusing and exposure metering through the lens to ensure optimum image quality. Most camera settings are available through the software, including zoom. The standard camera can shoot 3 photos in succession in less than a second. Time between shots can be set from 3 seconds and up.

It is advantageous to keep some distance between the camera and strobe to minimize illuminating particles in the sea between the camera and the subject. See information on the Strobe module.

Camera strobe (flash) module. The Strobe Module is a standard Gavia-short module with an opening for mounting the strobe lamp holder, It has a face, which is tilted forwards for optimum illumination for the camera when Gavia is at an altitude of 1 to 2 meters above the seabed. An extra Strobe Module can be installed in the hull of Gavia for subject illumination from a different angle. Together with this powerful strobe the camera can shoot at a high shutter speed ensuring sharp images.

Figure 21 Short payload module. This is a standard Gavia-short module which can be installed anywhere in Gavia's hull. The module is available with different mounting-hole arrangements. One is shown here, i. e. one large mounting hole.

The short payload module comes with the Gavia module bus and associated electronics, including power conditioning for the payload (DC-DC converters). An Ethernet connector is also provided for a local PC-computer inside the module so that it can communicate with the Gavia main PC-computer.

Figure 22 Long payload module. This is a standard Gava-long module which can be installed anywhere in Gavia's hull. The module is available with different mounting-hole arrangements.

The Payload Module comes with the Gavia module bus and associated electronics, including power conditioning for the payload (DC-DC converters). An Ethernet connector is also provided for a local PC-computer so that it can communicate with the Gavia main PC-computer.

Figure 23 Inertial navigation system (INS) module. The INS is housed in a Gava-long module. The Aided Inertia Navigation system integrates INS measurements, combined GPS position data, DVL data, pressure transmitter data and compass heading in a mathematically optimal way.

Autonomous Control Software Figure 24 Overview of Gavia's artificial crew architecture showing the three components of the vehicle software: (1) the core vehicle crew and navigational instruments, (2) scientific personnel and payload sensors, and (3) the surface operator interface.

Software architecture. Gavia's control software is organized in a unique distributed architecture modelled on the division of responsibilities among the hands of a manually controlled vessel. The Intelligent Artificial Crew (IAC) comprises a full crew responsible for the safe navigation of the vessel together with scientific personnel responsible for meeting the goals of the mission.

Reliability. This has been assured through modular design and clearly defined responsibilities of the Intelligent Artificial Crew members that are common to all versions of Gavia irrespective of the number or types of modules installed. Members of the crew execute as separate independent processes on the networked PCs running the open source Linux@ operating system kernel, while time-critical functions are handled by the real-time microcomputer/micro-controller network.

Figure 25 Figure 25 shows a sample mission plan written in Gavia's XML based AUV scripting language.

Mission execution. Mission execution follows a mission plan expressed in Gavia's powerful XML based AUV Scripting Language (ASL). The mission plan can contain both fixed and dynamic paths. Fixed paths consist of waypoints and lines. Dynamic paths are determined in real-time by an on-board scientist analysing sensor data. In addition the ASL language allows dynamic switching between paths based on events flagged by scientists, such as a"found"event or a"lost track"event. Throughout the mission sensors can be turned on and off as required. A short mission plan is shown in the figure.

Exception handling. Reaction to exceptional or critical operational conditions is handled by the captain of the Intelligent Artificial Crew, who with the aid of his crew oversees the proper running of the vehicle. This relieves the planner of a mission of the task of foreseeing every possible situation and leaves him free to concentrate on the goals of his mission.

Extensibility. A variety of missions can be specified in the mission plan using Gavia's powerful ASL language. Further functionality can be added by incorporating additional specialist members into the crew capable of solving application specific tasks. Since crewmembers communicate using industrial standard CORBA interfaces, they can be written in any of a number of programming languages offering CORBA support. Similarly crewmembers can be accommodated on optional further PC computers to meet specific processing needs or operating system requirements.

Pre-mission testing. Gavia has a built-in simulation mode that enables the testing of complete missions prior to deployment. Simulated missions are executed by the same

crewmembers following the same mission plans as real missions. The simulator receives control output from the pilots and accurately models the dynamic response of the vehicle and its interaction with the environment and makes that information available to the crew via virtual sensors. The simulator can also be accessed from the Mathworks Matlabo/Simulinke environment for controller design and testing.

Data logging. Sensor data and vehicle logs are stored on one or more internal hard disks.

Data quality is assured by run-time verification of sensor operation and data storing, functionality for time synchronization, time drift verification and accurate time tagging of sensor data. The geodetic position and attitude of the vehicle as estimated by Gavia's navigator using available navigational sensors is logged continuously to allow for geodetic referencing of sensor data.

Control system components. Gavia is controlled by a number of computers in the vehicle and on land or on board a vessel. The control software is organized as an artificial crew with a clear division of responsibilities between crewmembers.

The control hardware consists of the following main units: 1. Pentium PC computer or networked PC computers with navigation sensors 2. Emergency electronics and systems 3. Motor, rudder and stern plane drivers 4. Main batteries Gavia's onboard computer system is organized in a two-tier architecture: Upper level : One or more Pentium PC computers Lower level : Real-time micro controller network Figure 26 The Pentium PC computers are connected together in a TCP/IP network over a 100Mbit/s Ethernet, with the real-time micro controllers communicating on a dedicated bus.

External TCP/IP connections are available over llMbit/s Wireless Ethernet LAN, Iridium satellite link at 2.4 kbit/s and optionally an acoustic link at 2 kbit/s.

Gavia can be remotely field serviced worldwide through the Iridium satellite link. Through the satellite link Hafmynd's or the customer's technical support can access Gavia's onboard computers, sensors and processes to aid in troubleshooting.

Figure 27 Services provided by the Gavia AUV over all three communication links : WLAN RF link, Iridium satellite link, and the optional acoustic link.

Figure 28

Gavia can be remotely controlled through any of her communication channels: Wireless LAN, Iridium satellite link, and the optional acoustic modem. This is especially useful when Gavia is operated from the shore, when she can be made to surface a safe distance off the shore and then brought to the shore under remote control at a location where her operator can pick her up.

Navigation and positioning Gavia is equipped with a suite of navigation sensors: 1. Magneto-inductive 3-axis compass.

2. Yaw rate gyro 3. Doppler Velocity Log (DVL) 4. Depth pressure transducer 5. 360° degree horizontally scanning obstacle avoidance sonar 6. Global Positioning System (GPS) when on the surface 7. Inertia Navigation System (INS), optional All of the above navigational instruments are connected to Gavia's control system where they are made available to Gavia's artificial pilots.

The depth sensor allows Gavia to accurately maintain a set depth.

The GPS receiver accurately fixes Gavia's position on the surface, while the doppler velocity log (DVL) measures Gavia's true speed with respect to water as well as the true speed and height over bottom while submerged. When combined with the optional high- accuracy Inertia Navigation System (INS) these instruments allow Gavia to position herself accurately underwater at any time, with an accumulated error of only a few meters for each hour since the last GPS fix.

The sensors provide steering information for heading, depth and height control as well as detecting critical operational conditions for the error detection and the error handling systems. The global position of Gavia is established by the integration of data from the GPS, DVL and INS (if installed).

The Aided Inertia Navigation system integrates INS measurements, combined GPS position data, DVL data, pressure transmitter data and compass heading in a mathematically optimal way.

The inertia navigation system solution provides an accurate real-time position solution, allowing the vehicle mission plan to be based on waypoint navigation.

Acoustic link. An acoustic link is optional on Gavia. The acoustic modem and transducer is contained in the Acoustic Module. Full control of Gavia is possible through the acoustic link in the same way as through the satellite telephone.

Obstacle avoidance. Obstacle avoidance is provided by the 360-degree scanning sonar that is mounted in the Sonar/Drop weight Module towards the front of Gavia's hull. The on-board software can set the frequency and beam width of the sonar. The sonar provides input to the control system of Gavia.