Feature ArticlesOnboard Acoustic Remote Sensing System For Air-Sea
By Salvatore Aronica
Istituto per l'Ambiente Marino
Costiero del Consiglio
Nazionale delle Ricerche
Capo Granitola, Italy
Acoustic remote-sensing technologies for monitoring the atmospheric boundary layer (ABL) are well established. They have been applied in several contexts, including in applications such as the study of the microclimate in the urban environment, wind shear in or close to airports and wind turbulence.
However, despite the great potential that these technologies could offer in the study of air-sea interactions and their effects on biological resources, such research is hampered by challenges related to onboard installation, navigational movements and positioning in environmental marine applications.
This article will describe an onboard modular acoustic remote-sensing system for air-sea interaction and the first results of data collected in the open marine atmosphere of the central Mediterranean Sea, in which the Istituto per l'Ambiente Marino Costiero del Consiglio Nazionale delle Ricerche (IAMC-CNR) group conducts interdisciplinary cruises on board a research vessel every year.
Components of the MASD System
The technology has been named sound detection and ranging (SODAR). It emits acoustic signals to explore the ABL and to estimate useful variables in the study of meteorological phenomena and air-mass dynamics. The equipment records backscattered echoes, analyzing the air-density dishomogeneities. Discontinuities in the lower atmosphere, mainly due to air thermal differences and turbulences, allow the system to evaluate atmospheric phenomena.
Using the SODAR technology, the Marine Atmosphere Sound Detector (MASD) was developed. The system can be mounted on a fixed or mobile marine platform, and it is interconnected with external devices to take vessel movements (pitch and roll) into account and to correct the acoustic beam angle. Georeferenced data is obtained, linking the system to other onboard instruments like the global positioning system (GPS) and compass, while concurrent meteorological data (wind strength and direction, air pressure and temperature, humidity, etc.) are obtained by the vessel's weather station.
The MASD system was installed and tested on the CNR research vessel Urania. Because the equipment is monoaxial, the only detectable movement components are along the vertical axis. From a qualitative point of view, it is possible to follow convective plumes, the evolution of inversion layers and gravitational waves. With a quantitative approach, it is possible to estimate the w wind speed (where w is the wind vertical component), the related turbulent kinetic energy and the momentum flux.
The MASD system antenna has a monostatic configuration, since it transmits a signal at a frequency of 2,000 hertz with a pulse duration of 100 milliseconds and receives a backscattered signal filtered within the 160-hertz bandwidth centered at the transmitted frequency. Simultaneously, the low atmosphere vertical structure and its dynamics are estimated by means of one scan per five seconds. The air-mass speed is obtained by measuring the frequency difference between the emitted and received signal (i.e., the Doppler effect).
An exploratory study was conducted to find the optimal emitting frequency so as to obtain the maximum scattering efficiency, since air-mass structures, such as eddies, can be singled out only when the signal wavelength is comparable to their dimension. The height of the investigated low atmosphere (900 meters) depends on the power of the transmitted acoustic signal and the acquisition time.
Hardware and Software
The MASD system structure, installed on the Urania, is composed of the following: a central PC with an Austin, Texas-based National Instruments (NI) MIO 6713 signals generation board and a data acquisition board (NI DAQ 6252), a control unit with a power amplifier for signal transmission (80 volts peak to peak) and a filter to improve echo-signal reception, a preamplifier with a module able to switch from the transmission to the acquisition phase, a motion reference unit (MRU) system to single out the pitch and roll vessel motions, a navigation PC to acquire all navigational data and environmental parameters, a cable system, and a parabolic antenna with an acoustic driver and a lateral soundproofing shield.
The software that runs on the central PC was developed with NI LabVIEW 7.0 Express. It is able to generate and transmit a signal; acquire an echo signal; acquire information useful for data elaboration (such as geographical coordinates, compass direction, navigational speed, and pitch and roll movements); acquire meteorological data; process echo-signal samples and produce the facsimile output format (where x is the time, y is the height and z is the echo intensity); estimate the vertical wind speed at various heights (where x is the time, y is the height and z is the vertical wind component); plot data both in a facsimile format and as wind speed vertical profiles; and record the raw and processed data.
In particular, the generated signal is converted into an analogical signal and sent to the power amplifier, which, through the cable system, pilots the antenna driver. This driver is situated in correspondence to the antenna throat section. The acquired echo signal, via the antenna, is amplified by the pre-amplifier and multiplied with a ramp signal to intensify the signal at high heights, and then it is filtered by the high-quality band-pass filter system situated in the unit control. The filtered analogical signal is sampled and converted into digital format (raw data), elaborated and plotted on the screen—in both facsimile and vertical wind speed formats. The data and the navigational information are also recorded as raw data and wind data files.
Instruments for Mobile Analysis
The MASD-system-acquired data must have a well-known reference point both in time and spatial localization (geographical position and pitch and roll angles of the platform) to allow for correct analysis. These parameters are used to establish the correct normal direction of the antenna on the horizontal plane and to position it.
To collect all of this information, the system includes external devices—in particular the MRU and the navigation PC. Both the MRU and the navigation PC transmit with National Marine Electronics Association 0183 protocol and are linked to the central PC by means of two standard RS-232 serial ports. The system software is provided with a subVI (virtual instruments) function able to acquire and elaborate on this information.
The Kongsberg Maritime (Kongsberg, Norway) Seatex MRU 3 device is mounted on the central part of the platform of the research vessel, and it is able to determine the pitch and roll angles even in high-velocity conditions. The device is linked through an opportune cable to a junction box, which constitutes both its power supply and its configurable interface.
The navigation PC collects all the navigational data transmitted by the other devices (GPS, compass system, weather station) and, by means of Communication Technology's (Cesena, Italy) NavPRO 5.5 software, transmits them to the central PC. The central PC uses them to correct the acoustic beams directly, calculate the wind speed vectors and record the raw and wind data files for post-processing.
The MASD system was installed and tested during an oceanographic cruise, Bansic '06, carried out in the summer of 2006 on board the research vessel Urania, with the system's antenna oriented vertically with respect to the prow of the ship. On this cruise, in addition to the MASD data, a multidisciplinary set of data was acquired, including water temperature, pressure, conductivity, salinity, air temperature, atmospheric pressure, wind speed and direction, and atmospheric net radiation. The study area included a station grid of four square nautical miles in sea zones closer to the coasts, and a 12-square-nautical-mile grid was adopted for the offshore areas. Ichthyioplankton samples were also collected.
The MASD system was demonstrated to be stable with a reliable and fast correction of the pitch and roll deviations from the vertical axes. The system showed that it could capably characterize the lower layer of the marine atmosphere, both from a qualitative and quantitative point of view. An on-land calibration test showed that the system is able to make measurements in the first 900 meters of the atmosphere, which is the stratum that features the most interesting meteorological and dynamic phenomena because it is the layer biological organisms inhabit.
The marine ABL typically has a reduced height compared to the terrestrial one, and it varies slowly because of the intense mixing of sea surface water and its high thermal capacity. These phenomena produce a thermostat effect on the neighboring atmosphere.
Generally, marine ABL changes are produced by mesoscale synoptic processes that mix air masses with different characteristics. Both the marine and land ABLs are characterized by turbulent transport, the main mechanism responsible for their dynamic and thermodynamic properties.
In particular, the MASD system measurements show that a complex vertical wind pattern only exists in the first 450 meters of atmosphere over the sea. The system software can plot the intensity of the echoes as they are backscattered by atmospheric dishomogeneities (mainly thermal) versus height (facsimile) and also create a vertical wind diagram that associates a quantitative Doppler wind estimation every 25 meters from 25 to 500 meters above sea level. In most observations it was possible to detect strong convective activities (mainly at night) and the velocity of vertical winds from a few centimeters to a few meters per second.
This instrumentation would be useful in the monitoring and study of dynamic metrological-climatic processes of the marine atmosphere at micro- and mesoscales and in the study and evaluation of air-sea interactions.
For a full list of references, please contact Salvatore Aronica at firstname.lastname@example.org.
Salvatore Aronica is an electronic engineer with a Ph.D. in energetics. He currently has a contract with the Istituto per l'Ambiente Marino Costiero del Consiglio Nazionale delle Ricerche.
Giuseppa Buscaino holds a Ph.D. in environmental science and is an Istituto per l'Ambiente Marino Costiero del Consiglio Nazionale delle Ricerche researcher.s
Angelo Bonanno, a researcher at the Istituto per l'Ambiente Marino Costiero del Consiglio Nazionale delle Ricerche, has a Ph.D. in environmental technical physics.
Salvatore Mazzola is a physicist involved in the interdisciplinary application of acoustics, mainly underwater. He is the director of the Istituto per l'Ambiente Marino Costiero del Consiglio Nazionale delle Ricerche.