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DART II System Components



Overview

A DART II system, shown inside the dashed lines of Figure 3, consists of two physical components: a tsunameter on the ocean floor and a surface buoy with satellite telecommunications capability. The DART II systems have bi-directional communication links and are thus able to send and receive data from the Tsunami Warning Center and others via the Internet. The web site for the DART data is supported by the National Data Buoy Center and can be seen at: http://www.ndbc.noaa.gov/dart.shtml.

Tsunameter
The block diagram below shows how the components of a tsunameter function together. The computer reads pressure readings, runs a tsunami detection algorithm, and sends and receives commands and data to and from the buoy via an acoustic modem.



    1 Pressure Sensor
The DART II pressure sensor is a 0-10,000 psi model 410K Digiquartz unit manufactured by Paroscientific, Inc.9 The transducers use a very thin quartz crystal beam, electrically induced to vibrate at its lowest resonant mode. The oscillator is attached to a Bourdon tube that is open on one end to the ocean environment10. The pressure sensor outputs two frequency-modulated square waves, proportional to the ambient pressure and temperature. The temperature data is used to compensate for the thermal effects on the pressure-sensing element.
    2 Reciprocal Counter
The high resolution precision reciprocal counting circuit continuously measures the pressure and temperature signals simultaneously, integrating them over the entire sampling window, nominally set to 15 seconds. There is no dead period between the sampling windows. The circuit has a sub-millimeter pressure and sub-millidegree temperature least-count resolution. The reference frequency for the reciprocal counter is derived from a low power, very stable, 2.097152 MHz, temperature-compensated crystal oscillator. A real time calendar-clock in the computer also uses this reference for a time base. At the end of each sampling window, the computer reads the pressure and temperature data and stores the data in a flash memory card. A 15-second sampling period generates about 18 megabytes of data per year.
    3 Computer
The embedded computer system in both the buoy and the tsunameter was designed around the 32-bit, 3.3 volt Motorola 68332 microcontroller, and was programmed in C. It was built to be energy efficient for long-term battery powered deployment. The computer has 4 Mb of flash memory, a 12-bit A/D converter with 8 input channels, two RS232 channels, a hardware watchdog timer, a real-time clock, and 512 bytes of RAM. The embedded computer implements and regulates the primary functions of the surface and seafloor units: transmitting data communications, running the tsunami detection algorithm, storing and retrieving water column heights, generating checksums, and conducting automatic mode switching.
    4 Acoustic Modem and Transducer
A Benthos11 ATM-880 Telesonar acoustic modem with an AT-421LF directional transducer has a 40 conical beam which is used to transmit data between the tsunameter and the surface buoy. Modems transmit digital data via MFSK modulated sound signals with options for redundancy and convolutional coding. Transducers are baffled to minimize ambient noise from entering the receiver.
    5 Tilt Sensor
Each tsunameter has a Geometrics 900-45 tilt sensor mounted in the base of one of the housings. This is used to determine the orientation of the acoustic transducer when the system has settled on the seafloor. If the tilt is greater than 10 degrees the tsunameter can be recovered and redeployed. The watch circle of the surface buoy could carry it out of the acoustic projection cone from the tsunameter if the angle from the vertical is too great.
    6 Batteries
The tsunameter computer and pressure measurement system uses an Alkaline D-Cell battery pack with a capacity of 1560 watt-hours. The acoustic modem in the tsunameter is powered by similar battery packs that can deliver over 2,000 watt-hours of energy. These batteries are designed to last for four years on the seafloor; however, this is based on assumptions about the number of events that may occur and the volume of data request from the shore. Battery monitoring is required to maximize the life of the system.
    7 Tsunami Detection Algorithm
Each DART II tsunameter is designed to detect and report tsunamis autonomously12. The Tsunami Detection Algorithm works by first estimating the amplitudes of the pressure fluctuations within the tsunami frequency band, and then testing these amplitudes against a threshold value. The amplitudes are computed by subtracting predicted pressures from the observations, in which the predictions closely match the tides and lower frequency fluctuations. If the amplitudes exceed the threshold, the tsunameter goes into Event Mode to provide detailed information about the tsunami.
    8 Reporting Modes
Tsunameters operate in one of two data reporting modes: A low power, scheduled transmission mode called Standard Mode and a triggered event mode simply called Event Mode. Standard Mode reports once every six hours. Information reported includes the average water column height, battery voltages, status indicator, and a time stamp. These continuous measurements provide assurance that the system is working correctly. Event Mode reports events such as earthquakes and /or tsunamis when a detection threshold is exceeded. The Tsunami Detection Algorithm triggers when measured and predicted values differ by more than the threshold value. Waveform data are transmitted immediately (less than a three-minute delay). Tsunami waveform data continue to be transmitted every hour until the Tsunami Detection Algorithm is in a non-triggered status. At this point the system returns to the Standard Mode.



***Information From : http://www.ndbc.noaa.gov/