Future Directions in Bioluminescence Research

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ABSTRACT

Remote sensing from the air and space

Tommy D. Dickey

Ocean Physics Laboratory, University of California Santa Barbara, CA 93117

Present and near future capabilities:
Mankind’s views, perceptions, and understanding of the Earth and its oceans have been dramatically affected first through images obtained from aircraft and more recently from space. The thinness of the atmosphere and the vastness of the oceans as well as the beauty and complexity of the atmosphere and oceans are only a few of the impressions provided in photographs by early space explorers. While the visual impacts of space-based images are profound and in themselves informative, the next difficult step has been to extract quantitative information. Satellite-based sensors are now capable of providing nearly global, and in some cases, "snapshot" or synoptic views, and importantly data, over much larger areas of the oceans (and ice) than possible from any other platforms. The data are typically empirical inferences of surface signals (e.g., either passive or active electromagnetic radiation) and are often based on groundtruth data sets obtained from ocean-based platforms. Because electromagnetic radiation only penetrates to very shallow ocean depths, satellite information must be complemented with in situ observations to characterize important subsurface structures in the various ocean properties. Considerable research effort is being devoted to the extraction of subsurface data using remote sensing, in situ data sets, and models. Some of today’s oceanographically important remotely sensed variables include: solar radiation, wind stress and direction, rainfall, surface heat fluxes and wind stress, sea surface temperature, ocean color (e.g., pigment concentrations), and sea surface height. A few of the interesting applications of remote sensing have included: studies of mesoscale features, seasonal evolution of temperature and phytoplankton (via ocean color), El Nino and La Nina, equatorial waves, planetary scale waves (Kelvin and Rossby waves), wakes of ships, hurricanes and typhoons, coastal upwelling, storm runoff, surface and internal gravity waves, bottom topography, island wakes, and ice age, extent, thickness, and motion.

Toward the future:
Airplane-based systems will be able to provide very high spatial resolution for altimetry, color, salinity, and other variables. Aircraft-borne lidar has been used to detect mixed layer thickness as well as optical properties from backscatter profiles. Autonomous aircraft technologies are progressing and operating ranges of 2500 km with diverse sensor packages (e.g., for meteorology, sea surface temperature and color, altimetry for tides and currents, etc.) are projected.

Maximal utility of particular satellite platforms or sensor suite observations can be derived by using data sets for several different measurements. An especially important complementary use of data will likely involve concurrent gravity (e.g., for accurate, high spatial resolution measurements of the geoid to within 1 cm over 100 km length scales) and altimetry measurements, which will enable computation of absolute currents (e.g., opposed to relative currents) and reduce errors in estimates of water transport throughout the water column (especially important for straits and throughflow areas). Improved capabilities of remote sensing systems will likely be realized in temporal and spatial resolution for many other parameters. Multi-satellite missions are already collecting sea surface temperature data capable of resolving the diurnal cycle (sampling at hourly time scales) on cloud free days. The major problem of cloud obscuration for temperature and color measurements will remain, however use of more satellites to decrease time gaps can be effective in "filling in" data sets.

For altimetry measurements, which are essentially unaffected by clouds concurrent and coordinated sampling with multiple satellite missions and new wide swath systems will enable collection of essentially two-dimensional data (present altimeters have good resolution in a narrow beam along directly under the satellite’s flight path). Another important application of altimetry is for determining sea level time series, which are determined in conjunction with traditional tide gauge network data sets and benchmarked using GPS data. It is interesting to note that this methodology has led to observations suggesting that the globally averaged mean sea level increased by over 10 cm during the 1997-1998 ENSO event. Planned missions for color and temperature expect to realize spatial resolutions of 10’s of meters or less (likely over limited selected areas) as well as increased optical spectral resolution (down to a few nanometers and less). Work is progressing in remote sensing of other important oceanic variables. In particular, studies are underway to measure salinity from satellites and some promising results have already been obtained using airplane-based sensors in areas with strong salinity gradients.

Other likely variables will include dissolved organic materials and signatures of different phytoplankton groups including those associated with "red tides" or harmful algal blooms (HABs). Remote sensing of various chemical and biological variables (e.g., zooplankton and fish) remains as a major research challenge. Although direct observations of organisms from satellite platforms is not presently feasible, the use of ocean color, temperature, and current data can be valuable for identifying features (e.g., fronts, eddies, upwelling areas, red tide blooms, etc.) where high biological activity may be located. Further, extremely high resolution imagery may eventually be capable of sensing surfacing mammals and large schools of fish. Also, studies of larger organisms such as marine mammals have utilized satellite radiotracking, but require initial tagging. In some cases, tagging instrument packages have included sensors for temperature and depth as well as positioning. Possibilities for event triggered sampling using sensors placed on special satellite platforms (steerable instruments in geostationary orbit; likely color sensors initially) are being considered. This approach is most attractive for responses to disasters, directing field and other remote sensing observations to key locales, and providing data, which would otherwise be unattainable. Advanced analytical and modeling activities will be required to optimize utilization of present and future remote sensing data sets. Examples include removal of tides from altimeter data and incorporation of remote sensing and in situ data into models for three-dimensional spatial descriptions and predictions.

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