COVELIA

H Scientific Ltd, in collaboration with Chelsea Technologies Group, have developed the Correlation Velocity Log COVELIA for accurate absolute velocity measurement for AUVs.

COVELIA is a new velocity sensor which measures the speed of a vehicle relative to the sea bed. COVELIA is based upon the principles of Acoustic Correlation. COVELIA measures the displacement of the transducer, over the time interval between two acoustic pulses. Two short pulses of acoustic energy are transmitted towards the sea bed. The pulses are subsequently received on an array of receiver elements. The characteristic shape of the echo from each pulse, on each receiver element, depends upon the position of the transmitter at the time of transmission, the topology and nature of the seabed, and the position of the receiver at the time of reception. Each echo may therefore be regarded as a signature of the seabed as seen from a particular vantage point. By comparing the signals received from the two transmit pulses, the distance and direction travelled, between the two events, can be calculated.

COVELIA incorporates a number of novel features which allow this process to be optimised, particularly in the low- and near-zero velocity operating regimes which are of particular interest for AUV operation. In addition, its compact sensor and electronics packages are designed to use an internal power supply. The efficient design, for example using "sleep" modes to shut down parts of the system in the intervals between pulses, allows it to operate for 120 hours.

COVELIA offers the following advantages over existing methods of measuring vehicle velocity:

Unlike Doppler systems, the measurement is independent of the speed of sound in water.
The acoustic signal is transmitted to the sea bed in a single, relatively broad beam which is directed vertically downwards. This means that the beam is less likely to "lose" the seabed when the vehicle pitches or rolls.
The broad beam allows the use of relatively low frequencies which propagate further than the higher frequencies needed for Doppler measurements, so that the instrument has a greater range of operation.
The low frequency signals are transmitted in very short bursts, so that the average power consumption is significantly lower than for a comparable Doppler system.
Fundamentally, the sensor measures the displacement of a "signature" signal from the sea bed between two or more pulses. This means that the system remains accurate down to the lowest velocities. This is a regime of importance to AUVs, but one which poses difficult problems for Doppler systems.

To maximise the effectiveness of the COVELIA system, an INS interface has been developed. This interface not only stabilises the INS but it also uses the INS signals to stabilise the COVELIA measurement cycle. This protects COVELIA against transients such as noise bursts, temporary loss of signal during high-attitude manoeuvres. It also provides a means for locking COVELIA onto a velocity solution, allowing it to operate in its most accurate "high-resolution" mode on an almost permanent basis. Together, the INS and COVELIA are able to combine the best features of the two methods of measurement, achieving a true synergy and the best possible accuracy overall.

The following plots compare the position reported by GPS with that obtained from COVELIA during a recent sea trial. The overall distance covered during this manoeuvre was 23km.

COVELIA was developed during a collaborative program, part funded under the LINK Seasense and SMART programs by the UK Department of Trade and Industry (DTI), involving H Scientific, the Department of Mechanical Engineering at the University of Southampton, and Chelsea Technologies Group (formerly Marine Acoustics Ltd).

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