The demand for a more sophisticated underwater robotic technology that minimizes the cost and eliminates the need for human operator and is therefore capable of operating autonomously becomes apparent. These requirements led to the development of Autonomous Underwater Vehicles (AUVs). A key problem with autonomous underwater vehicles is being able to navigate in a generally unknown environment. The available underwater sensor suites have a limited capability to cope with such a navigation problem. In practice, no single sensor in the underwater environment can provide the level of accuracy, reliability and the coverage of information necessary to perform underwater navigation to cent percent safety.
In order to navigate accurately an AUV needs to employ a navigation sensor with a high level of accuracy and reliability. It is therefore necessary to use a number of sensors and combine their information to provide the necessary navigation capability. To achieve this, a multisensor data fusion (MSDF) approach, which combines data from multiple sensors and related information from associated databases, can be used. The aim of this paper is to survey previous work and recent development in AUV navigation and to introduce MSDF techniques as a means of improving the AUV's navigation capability.
Dead reckoning is a mathematical means to determine position estimates when the vehicle starts from a known point and moves at known velocities, the present position is equal to the time integral of the velocity. Measurement of the vector velocity components of the vehicle is usually accomplished with a compass (to obtain direction) and a water speed sensor (to obtain magnitude), The principal problem is that the presence of an ocean current can add a velocity component to the vehicle, which is not detected by the speed sensor.
An Inertial Navigation System (INS) is a dead reckoning technique that obtains position estimates by integrating the signal from an accelerometer, which measures the vehicle's acceleration. The vehicle position is obtained by double integration of the acceleration. The orientation of the accelerometer is governed by means of a gyroscope, which maintains either a fixed or turning position as prescribed by some steering function. The orientation may also, in principle, be determined by integration of the angular rates of the gyroscope. Both the accelerometer and the gyroscope depend on inertia for their operation.
A dead reckoning navigation system is attractive mainly because it uses sensors that are able to provide fast dynamic measurements. Unfortunately in practice, this integration leads to unbounded growth in position error with time due to the noise associated with the measurement and the nonlinearity of the sensors, and there is no built-in method for reducing this error.
Two types of dead reckoning sensors have been widely employed in AUVs: Inertial Measurements Units (IMUs) and Doppler velocity sonar (DVS). DVS sensors provide measurement of a velocity vector with respect to the sea floor. However, these results can only be achieved when the speed of sound in the AUV's area of operation does not vary significantly as a result of changes in the salinity, temperature and density of the water.
Radio Navigation :
Radio navigation systems mainly use the Global Positioning System (GPS). The GPS is a satellite-based navigational system that provides the most accurate open ocean navigation available. GPS consists of a constellation of 24 satellites that orbit the earth in 12 hours.
The GPS based navigation system is used extensively in surface vessels as these vehicles can directly receive signals radiated by the GPS. Unfortunately, these signals have a limited water-penetrating capability. Therefore to receive the signals, an antenna associated with an AUV employing a GPS system must be clear and free of water. There are three possible antenna configurations to meet this requirement. These are fixed, retractable, or expendable antennas. A fixed antenna is a non-moving antenna placed on the outside of the AUV.
The AUV has to surface to expose this antenna and stay surfaced until the required information has been received and processed adequately. A retractable antenna is one that the AUV would deploy while still submerged. When the required information is received, the antenna is retracted back to the AUV .The expendable antenna works along the same principle as the retractable antenna, except that it is used once and discarded. When required, another antenna would be deployed.