A European military contractor has developed a system for mine hunting based on a powerful ROV. The ROV runs ahead of the ship at a distance of a few hundred meters, tracking targets as it goes. In mine disposal configuration it is equipped with a relocation sonar and mine disposal charge.
To measure the speed and direction, the vehicle is fitted with a new generation Nortek Aquadopp current meter developed specifically to get accurate readings at 1-sec intervals. Traditionally, speed through water is measured using mechanical or electro-magnetic single-point current meters. These sensors have major disadvantages because they are protruding and they measure the velocity of the water right where the probe is located.
A protruding probe was considered to be a disadvantage, both because the sensor will easily get damaged if the ROV hits the bottom or a solid object, and also because it was important for the overall drag coefficient that the ROV should be as streamlined as possible. The issue of the measurement area was even more of a problem because there is a flow field generated in the vicinity of a fast moving ROV. Conventional current meters will thus underestimate the actual ROV speed unless they are placed on long protruding arms. The Aquadopp current meter from Nortek AS eliminates these problems.
Similar to other Doppler Velocity Logs (DVL) the Aquadopp uses the Doppler principle to measure the velocity of the water. Ultrasonic pulses at 2 MHz frequency are transmitted from the sensor along three narrow acoustic beams (figure). When these pulses hit small particles in the water, a portion of the energy is reflected back to the sensor, and the echo is frequency-shifted (Doppler shift). Combining the measured Doppler-shift along three beams with the sensor geometry gives the 3D ROV velocity through.The Aquadopp in ROV configuration uses a right-angle sensor head and it can be mounted flush with the ROV without any protruding parts.
The sensor is buoyant in water so it does not “steal” from the ROV ballast. The Aquadopp measuring volume can be set as far as 5m away from the ROV, which means that the measured velocity is unaffected by ROV itself. Finally, the Aquadopp price tag is only about 1/3 of the price of comparable bottom tracking DVLs so it is an easy choice when the primary motivation is to measure the ROV speed and direction.
The Aquadopp calculates the along and cross ship axis velocities, in addition to the vertical velocity. The data output is serial or analogue, which made integration with existing ROV control systems quite simple.
More about the Aquadopp can be found here