HAWAII MR1 : Technical Description
Download a copy of the technical paper describing MR1 by Mark Rognstad, "Hawaii MR1: A New Underwater mapping Tool", as a PDF.
Towfish Instrumentation The MR1 transducer arrays were custom built by Reson, Inc. to HMRG specifications. The arrays use an air based tonpilz design that features two rows of elements per array, 32 individual elements per row, and rows spaced 1/2 wavelength apart. Pitch and roll are measured using a Lucas Accustar II dual-axis clinometer that has a range of plus/minus 20 degrees and a resolution of 0.01 degrees. Heading is measured using a KVH C100 microprocessor-controlled fluxgate compass that has a resolution 0.1 degree and an accuracy of plus/minus 0.5 degree. Depth is measured using a Micron Instruments MP405-500-G pressure sensor that can be used to 500 PSI (~=3400 kPa or 340 m) with 0.25% error. The towfish is attached to the ship by a steel armored coaxial cable. Telemetry is accomplished using a half-duplex, frequency-shifted keyed datalink with a data rate of 422 kilobaud.
Additional new capabilities will include an upgrade of the system's programmable eletronics to permit operation with the swept-frequency (chirp) transmit signals for improved data quality, and faster pulse repetition rates for increased resolution in shallow water. This capability should be available by January 1, 1997.
Sonar Transducers The sonar transducers in the HAWAII MR1 were acquired in 1988 for use with SeaMARC II. They have been thoroughly tested and calibrated (near field) at the Navy's test facility in San Diego, and both beam pattern and phase response are well-determined. These transducers are rated to allow transmitted pulses of up to 10 kW per row (40 kW total power) in contrast to approxiamately 5 kW/row in the older SeaMARC II system. HMRG plans to upgrade HAWAII MR1 with new transducer arrays and reprogrammed eletronics, to improve data quality, during late 1996. The new arrays will provide enhanced bathymetry resolution by featuring wider row spacing, which improves the system's ability to measure the phase of returning acoustic signals. The modular design of the new arrays will also simplify system maintenance.
Electronics HAWAII MR1 is designed around a network of up to 16 digital signal processors (DSPs). At present, three are used in the system - two located in the subsurface vehicle and one in the topside data acquisition workstation. These DSPs process the acoustic signals digitally. Earlier sonar designs performed much of the signal processing with analog electronics, subject to noise and component drift that would introduce artifacts into the data. Digital computations are not subject to drift, and the 24-bit word size of the present system allows a dynamic range of 144 dB. The software that implememts this signal processing is remotely loaded from the surface, and can be reloaded in a few seconds without recovering the tow vehicle.
Subsurface Electronics
This drawing illustrates one side only; electronics are identical on port
and starboard sides. Each block represents a single printed circuit board
The acoustic receivers are made up of three subsystems: the preamplifier, the digitizer, and the and the DSP. Data are digitized immediately after the preamp and transferred to the DSP. The digital data pass through a quadrature detection and base-banding algorithm, then are decimated with a filter matched to the transmitted waveform. Data are packetized for transmission to the surface and telemetered using half-duplex FSK modems operating at 422 kbaud. Error detection and correction are carried out by the acquisition software, which was developed at the Hawaii Institute of Geophysics and Planetology.
Surface Electronics
The MR1 system sails with redundant topside systems that can be
interchanged on-the-fly to minimize data loss due to computer failure.
Surface electronics for the system consist of a DC power supply for subsurface electronics, a topside telemetry system and redundant Sun SPARCStation computers equiped with digital signal processors in one SBus slot. Operator controls are provided by mouse-activated buttons on the console screen, which simultaneouly provides a scrolling display of swath bathymetry and side-scan sonar. Hard copies of bathymetry and side-scan data are also provided.