Drawing of the prototype MOBY (courtesy of D. Clark).
In support of the Earth Observing System (EOS), a Marine Optical Buoy (MOBY) was deployed off the coast of Lanai, Hawaii on Feb. 21, 1994. Funded jointly by NASA and NOAA, MOBY was developed and deployed under the direction of Dennis Clark, a member of both the MODIS and SeaWiFS science teams. The buoy's primary purpose is to measure visible and near-infrared radiation entering and emanating from the ocean. It is the variations of the visible region-reflected radiation that is referred to as ocean color from which other quantities can be derived, such as the abundance of microscopic marine plants (phytoplankton).
Phytoplankton is critically important to the marine food chain, and it influences the global balance of carbon dioxide within the Earth's atmosphere. It is believed that the abundance of carbon in the atmosphere causes a general increase in global temperature and affects the ocean's productivity. Scientists have learned that the ocean stores vast volumes of carbon from the atmosphere, providing sustenance for phytoplankton. However, researchers don't know whether there will be a corresponding increase in the phytoplankton population to absorb the additional carbon as the volume of carbon dioxide steadily increases in the Earth's atmosphere.
In its role of measuring ocean color, MOBY provides a time-series database for bio-optical algorithm development. Because the buoy will acquire data 3-5 times per day at the same site, oceanographers can now monitor the daily fluctuations in biomass concentrations at that site and fine tune their algorithms accordingly. Clark also points out that in addition to the oceanographic applications, MOBY will serve another important function as a calibration reference station for satellite instruments--such as SeaWiFS, EOS COLOR, MODIS, MERIS, and ADEOS OCTS--to assist in maintaining those instruments' accuracies.
Drawing of the prototype MOBY (courtesy of D. Clark).
"The potential is there for not only calibrating ocean bands, but other low end radiances," Clark explains. "MOBY has science and calibration applications for the atmospheric and land sciences." So MOBY could, for example, support atmospheric correction programs, or radiation budget applications.
"The goal is to produce a site that has high accuracy and high precision radiometry throughout the spectral range from 380 to 900 nm at approximately 2-4 nm spectral resolution," Clark continues. "MOBY serves as one calibration point at specific levels of ocean radiances. Those spectra can then be convolved to match spectral responses of a variety of visible and near-infrared spaceborne sensors." In short, MOBY will provide data for checking the calibration of different sensors at that particular scene and radiance level for any or all EOS optical instruments.
In addition to the deployment, the Team also collected data at the MOBY site for the Marine Optical Characterization Experiment, or MOCE. Also under the direction of Clark, MOCE is a series of field experiments that has been in operation since 1992 parallel to the MOBY campaign. MOCE involves conducting experiments at sea at specific sites to obtain a comprehensive set of bio-optical measurements such as radiometry, pigment analysis, total suspended matter, beam transmittance, and physical properties.
MOBY and MOCE are pre- and post-launch campaigns. Prior to launch, these campaigns are being conducted to help in algorithm development. Post launch, these campaigns will serve as a validation and calibration data source. For MOCE, during the post-launch operations, the shipboard operations are timed to be coincident with satellite overpasses, with the ship moving into different oceanic optical regimes. These data are used to check the calibration of satellite-borne ocean color sensors, as well as validate their derived products.
Illustration of the Calibration and Validation Field Program.
The optical system uses two spectrographs with a dichroic ("water") mirror to measure radiometric properties with high spectral resolution and stray light rejection. This "water" mirror is designed to transmit the red (630 to 900 nm) and reflect the blue (380 to 600 nm) portions of the spectrum, making the transition from reflectance to transmittance between 590 and 650 nm. Thus potential for stray light is greatly reduced by splitting the visible spectrum at the beginning of the water absorption region since most of the short wavelength energy is diverted from the entrance slit of the long wavelength spectrograph. The splitting also allows the spectrographs (free spectral range and integration times) to be optimized for the two distinctive spectral domains. Internal calibration and ancillary sensors (temperature, inclination, pressure, etc.) are included.
Approximately 50 feet long, MOBY is the world's largest marine optical device. In the ocean, only its antennae, solar panels, strobe light, and surface buoy (which houses the computer and cellular phone for data transmission) are visible, standing about 7 feet above the waterline. A fiberglass mast extends more than 40 feet directly down beneath the buoy to the instrument bay. At depths of about 6, 16, and 28 feet respectively, 9-foot long booms or "arms" extend outward perpendicular to the mast.
Optical collectors (irradiance and radiance) have been placed at the ends of the arms, as well as on top of the buoy above the surface to collect light coming into the ocean, and then the light reflected back out of the ocean. The reflected portion of the light has been modified by particles such as phytoplankton suspended at various depths, that modifies the signal available to the satellite sensors. Lenses within these collectors focus the light onto fiber optic cables which then transmit the light to a fiber optic multiplexer housed in the instrument bay. The multiplexer relays the light into a dual spectrograph with detectors that measure the spectral radiant energy. These signals are then digitized and relayed by microprocessors and transmitted up to a main computer housed in the surface buoy. This information is then stored on a disk drive, which may be accessed via cellular phone and downloaded for processing back at the MOBY Team facility.
Much of the buoy's hardware and acquisition software was engineered by San Jose State University's Moss Landing Marine Laboratory (MLML) personnel. Mark Yarbrough of MLML was responsible for the final integration of MOBY's components. He also oversees the daily operation of the MOBY Lab sites in Monterey and Honolulu. The acquisition and control software was written by Richard Reaves.
A formal MOBY project was originally funded by MODIS to help calibrate and validate the satellite sensor. Fabrication of the buoy was accelerated when in 1991 MOBY received additional funding from SeaWiFS to help in its calibration and validation effort. Charles McClain approached Clark with the idea of using MOBY for two purposes:
McClain based the first purpose on his experience with the Nimbus-7 Coastal Zone Color Scanner (CZCS). During its first 18 months in space, the CZCS orbital degradation was well-characterized by the Nimbus experiment team and confirmed by measurements taken aboard ships. After that period, there were no more ship measurements and, subsequently, there was no way of knowing how much error was contained in CZCS data. MOBY's second purpose--providing a time-series data set--will help scientists remove temporal biases from SeaWiFS observations. "Additionally," Clark adds, "MOBY is not constrained by when it takes measurements, so we can observe the response of phytoplankton to different light conditions." Whereas satellites can only view certain sites at certain times, MOBY continually takes data at its site, enabling scientists to interpolate between satellite observations.
MOCE is totally a MODIS-funded endeavor.
Also, MOBY contains an internal calibration system. Light from two onboard LED (Light Emitting Diode) lamps (green and red) illuminate a Labsphere Spectralon target that absorbs light at certain wavelengths and reflects all others. The Spectralon target reflects onto an internal mirror (when it is in the calibration position), and the light is then reflected into the relay optics of the dual spectrograph. This allows for calibration checks on the spectral and absolute intensity of the system.
Additionally, every month, divers from MLML and the University of Hawaii will dive at the site with submersible calibration lamps that will be coupled with the collectors in order to monitor system performance.
In order to implement this operational plan, two advanced versions of the MOBY prototype are now being built. Over the course of 5 years these buoys will be swapped out for general maintenance and calibration testing on a quarterly basis. But, before the prototype is removed from the water, both buoys will operate together at the same site for several measurement cycles to establish a crossover calibration between the two instruments to characterize offsets and biases between the two. During this time, both MOBYs will gather concurrent sets of observations to allow the different systems' offsets and biases to be characterized. These observations will allow the Team to characterize or observe the magnitude of biofouling that may be affecting the measurements. (Presently, the biofouling problem is being addressed by coating the collectors with an optically transparent organo-tin compound.)
According to Stanford Hooker, a member of NASA's SeaWiFS Project, MOBY's data will help scientists better understand the effects of atmospheric CO2 on oceanic biomass concentrations. Hooker says there has been a global increase in atmospheric CO2, resulting from an increase in fossil fuel burning over the last century. Some of that CO2 is absorbed into the oceans; however, there is a limit to how much the oceans can store. Scientists don't know what the oceans' saturation limit is, nor do they know how long it will take to reach it.
"Plants consume CO2 as part of their metabolism," he explains. "But, if you change ocean conditions, will phytoplankton take up more or less CO2? Some scientists argue that the plants are at their saturation level and cannot adapt." One theory holds that if the marine plant community cannot help absorb the additional CO2, then the increase of the gas in the atmosphere will accelerate global warming.
MOCE and MOBY operations include taking measurements in three broad categories: optical, biological, and physical. According to Hooker, MOBY's data products include water-leaving radiance and absorption and attenuation parameters that enable the Team to derive the following products: phytoplankton pigments, phytoplankton concentration, total suspended matter, chlorophyll a, and fluorescence quantum efficiency.
The Team has also recently deployed another device for collecting in situ data to complement MOCE and MOBY observations in order to provide higher quality data sets. A device resembling a delta wing--called a paravan--is lowered to different depths of the ocean and towed alongside the ship. The paravane has been modified to house a pumping system (which collects relatively uncontaminated sea water and pumps it via a hose back up to the ship) and an in situ fluorometer. The pumping system delivers sea water to instruments onboard the ship which measure fluorescence, transmissivity, and particle size distribution. These data will be compared to SeaWiFS/MODIS-derived products for validation.
Initially, MOBY data will be processed by MLML and stored by the SeaWiFS Project, an operation that will ultimately be taken over by EOSDIS. The data processing system software--adapted from CZCS processing software--was engineered by Michael Feinholz, of MLML. MOBY data will be cooperatively analyzed by Clark, MOBY Chief Scientist, and Professor William Broenkow, oceanographer at MLML. These data will eventually be made available to researchers worldwide.
Already other investigators are becoming involved with the project. For example, researchers at the University of Hawaii will use MOBY data to aid their efforts. Bob Bedigary is developing oceanic primary productivity models, Tony Clark will use MOBY to study atmospheric aerosols, and Dave Karl is monitoring CO2 fluxes in the area.
The new, permanent deployment site of MOBY is 20 degree 49.0 sec N Latitude, 157 degree 11.5 sec W Longitude--or about 11 nautical miles west of Lanai, Hawaii. This site was chosen primarily because it meets the MODIS Ocean Team's calibration/validation criteria. Also, the neighboring islands--Molokai, Lanai, and Maui--provide a lee from the wind so conditions there remain relatively calm. Of course, should typhoon conditions threaten, MLML may quickly retrieve the buoy and then re-deploy it after the storm. Finally, the MOBY site is within range of cellular phone coverage, rendering its data easily accessible.
Clark readily admits that there are many ways the buoy can fail. "More buoys have been lost due to mooring failure than probably any other reason," he states. "However, nothing is gained unless you push technology to its limits, and that in itself is a high risk." In an age of dwindling funding, pushing technological limits is a tall order. Because funds were not available in 1986 for beginning and developing a complete system, the incremental development approach was used in which the total system concept was developed and then the individual components were built as funds became available. These components were bought in the mid-80s, so many of the components of the MOBY prototype--such as the linear array detectors--consist of old technology, but the new MOBYs will be state-of-the-art. "Since the beginning of this project, most of our resources have gone into hardware," Clark explains, "forcing us to use a small development team (three people). Given the funding profile of the 1980s, this project could never have come to fruition without using an incremental development approach."
Yet, the integration and testing process greatly accelerated with the infusion of SeaWiFS funds in 1991. "I've never tried to push any system into use as fast as the MOBY system," Yarbrough states. "For economic reasons you want to get things done as fast as possible. Yet for scientific purposes, the longer you use a system, testing it and gaining experience with it, the more reliable it is. It's been a continuous balancing act." The compromise is evident in the Team's work schedule; 14-and 16-hour work days are not uncommon, even on weekends.
Developing and deploying MOBY in a timely manner probably couldn't have been done without a highly resourceful Team dedicated to the project's success. Each MOBY Team member has had to wear many hats, in addition to their individual areas of expertise. Yet, resourcefulness is simply par for the course to Yarbrough. For example, in 1982 he invented a modified CTD (Conductivity, Temperature, and Depth) device that had an immediate impact on the way oceanography is conducted. His resourcefulness has paid off for the MOBY Project too--he has been responsible for the final integration of all the components in the MOBY system.
Although there have been setbacks along the way, the MOBY/MOCE Teams now seem poised to begin enjoying the fruits of their labors. "We're making an effort to provide accurate data and analyses of the biological state of the oceans from an optical standpoint," Clark concludes. "If that database isn't there, you can never formulate analyses which accurately evaluate bio-optical processes or the impact of man's intervention on those processes."
The following is an alphabetical list of the MOBY/MOCE field campaign principal contributors:
James Brown University of Miami
Charles Trees RSMAS San Diego State University Center for Hydro-Optics and Remote Sensing
Dennis Clark NOAA NESDIS
Ken Voss University of Miami
Michael Feinholz MLML
Mark Yarbrough MLML
Stanford Hooker NASA GSFC
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