Autonomous Underwater
Vehicle
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.
Introduction ofAutonomous Underwater Vehicle
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 Navagation:
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 waterpenetrating 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.
