Numerous techniques have been developed for remote sensing of the ocean. Some techniques that the team encountered during the trip included: a multipurpose airborne laser system Lidar, for oil spill detection and ocean thermocline research; radars for wind-wave research; and acoustic Doppler current profilers (ADCP) for ocean current measurement at various depths. (Additional information on the Lidar and wind-wave radars may be found in the site reports on the General Physics Institute and the Institute of Applied Physics in Appendix B.) The following section describes the ADCP capabilities noted during the site visits.
During the past ten years, acoustic Doppler current profilers have progressed from an alternative method to an established technique for measuring the ocean's horizontal flow field at various depths. ADCPs provide high quality data for requirements as diverse as oil production, fisheries development, and environmental monitoring. The ADCP is basically a multibeam (there are typically three or four beams) pulsed, range-gated sonar that employs the acoustic Doppler principle to remotely measure vertical profiles of horizontal water currents from a moving vessel, buoy, or the seafloor. In areas where the water depth is within the ADCP bottom tracking range, the sonar also measures the earth referenced vessel velocity.
The Institute of Applied Physics, Nizhny Novgorod, Russia, has developed a universal ADCP that can provide real-time measurements of three components of flow velocity at different depth cells. These measurements can be made from a stationary platform, from the sea bottom, from ice, or from a ship. When operating at depths of 300 m to 400 m, ship speed relative to the bottom can be measured. Table 2.4 summarizes the technical specifications for the institute's ADCP.
This ADCP system also includes an IBM-PC/AT with appropriate peripheral equipment and software. The mean values of the calculated velocity profiles are depicted on the computer monitor and as data files on the hard disk. Also incorporated in the design is a sound velocimeter that provides measurements of sound speed to allow corrections in the ADCP data. The shipboard version also includes a gyro sensor for pitch and roll measurement, and a digital adapter for the ship gyrocompass. Analysis of these specifications indicates that this narrow-beam version has capabilities comparable to a commercially available ADCP (e.g., RD Instruments, Inc.).
Institute of Applied Physics
Acoustic Doppler Current Profiler Specifications
A broadband version of the ADCP with enhanced capabilities was developed two years ago by the same company. The broadband design implements advanced signal processing technology known as pulse-to-pulse coherent sonar. On receipt of the broadband signal, it is amplified and digitized. This design provides a four-fold resolution increase, an order of magnitude faster updates, shallower operating depths, and highly accurate Doppler velocity log capabilities when compared to the narrowband ADCP version (Spain and Gordon 1992).