William A. Snyder

Ocean PHILLS hyperspectral imager: design, characterization, and calibration

The Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS) is a hyperspectral imager specifically designed for imaging the coastal ocean. It uses a thinned, backsideilluminated CCD for high sensitivity and an all-reflective spectrograph with a convex grating in an Offner configuration to produce a nearly distortionfree image. The sensor, which was constructed entirely from commercially available components, has been successfully deployed during several oceanographic experiments in 1999-2001. Here we describe the instrument design and present the results of laboratory characterization and calibration. We also present examples of remote-sensing reflectance data obtained from the LEO-15 site in New Jersey that agrees well with ground-truth measurements.

Optical scattering and backscattering by organic and inorganic particulates in U.S. coastal waters

We present the results of a study of optical scattering and backscattering of particulates for three coastal sites that represent a wide range of optical properties that are found in U.S. near-shore waters. The 6000 scattering and backscattering spectra collected for this study can be well approximated by a power-law function of wavelength. The power-law exponent for particulate scattering changes dramatically from site to site (and within each site) compared with particulate backscattering where all the spectra, except possibly the very clearest waters, cluster around a single wavelength power-law exponent of -0.94. The particulate backscattering-to-scattering ratio (the backscattering ratio) displays a wide range in wavelength dependence. This result is not consistent with scattering models that describe the bulk composition of water as a uniform mix of homogeneous spherical particles with a Junge-like power-law distribution over all particle sizes. Simultaneous particulate organic matter (POM) and particulate inorganic matter (PIM) measurements are available for some of our optical measurements, and site-averaged POM and PIM mass-specific cross sections for scattering and backscattering can be derived. Cross sections for organic and inorganic material differ at each site, and the relative contribution of organic and inorganic material to scattering and backscattering depends differently at each site on the relative amount of material that is present.

The Hyperspectral Imager for the Coastal Ocean (HICO) on the International Space Station

The HICO (Hyperspectral Imager for the Coastal Ocean) program is the first demonstration of environmental characterization of the coastal zone using a spaceborne maritime hyperspectral imager. HICO is sponsored by the Office of Naval Research as an Innovative Naval Prototype (INP), and will demonstrate coastal products including water clarity, bottom types, bathymetry and on-shore vegetation maps. As an INP, HICO will also demonstrate innovative ways to reduce the cost and schedule of this space mission by adapting proven aircraft imager architecture and using Commercial Off-The-Shelf (COTS) components where possible.

The Hyperspectral Imager for the Coastal Ocean (HICO)- design and early results

The design and early results of the Hyperspectral Imager for the Coastal Ocean (HICO) are presented. The performance requirements imposed on the sensor to measure the low signals and to differentiate the optically complex spectra of the coastal ocean are discussed. It is shown the as-built sensor meets or exceeds the design parameters. Further, environmental products from early retrievals of the HICO imagery are presented.

The Hyperspectral Imager for the Coastal Ocean (HICO™) environmental littoral imaging from the International Space Station

The Hyperspectral Imager for the Coastal Ocean (HICO™), launched to the International Space Station in September 2009, is the first spaceborne hyperspectral imager optimized for environmental characterization of the coastal ocean. Building on the heritage of airborne hyperspectral imagers, HICO™ combines high signal-to-noise ratio, contiguous 10 nm wide spectral channels over the range 400 to 900 nm, and a scene size of 42 × 190 km to capture the scale of coastal dynamics. HICO™ image data is being exploited to produce maps of coastal ocean properties including bathymetry, in-water suspended and dissolved matter, and bottom characteristics, offering a new remote sensing capability for coastal environments worldwide. In this paper we discuss the development and performance characteristics of the HICO™ imager, and present example HICO™ data products.

Patch recognition of algal blooms and macroalgae

Fraunhofer lines and atmospheric absorption bands interfere with the spectral location of absorption bands of photosynthetic pigments in plankton. Hyperspectral data were used to address this interference on identifying absorption bands by applying derivative analysis of radiance spectra. Algal blooms show elevated radiance data even at longer wavelengths compared to oligotrophic water and may reach radiance values of around 800 W/m2/micrometer/sr at a wavelength of about 0.8 μm. Therefore, the use of a spectral range beyond 0.55 μm is useful to describe bloom characteristics. In particular, the slope between 0.55 μm to 0.80 μm shows an advantage to depict gradients in plankton blooms. Radiance spectra in the region from 0.4 to 0.8 μm for oligotrophic water and near coastal water show similar location of absorption bands when analyzed with derivative analysis but with different amplitudes. For this reason, radiance spectra were also analyzed without atmospheric correction, and various approaches to interpret radiance data over plankton blooms were investigated. Cluster analysis and ratio techniques at longer wavelengths were found to assist in the separation of ocean color gradients and distinguish bio-geochemical provinces in near-coastal waters. Furthermore, using the slope of spectra from plankton blooms, in connection with scatter diagrams at various wavelengths, shows that details can be revealed that would not be recognized in single channels at lower wavelength.

Laboratory characterization of the Hyperspectral Imager for the Coastal Ocean (HICO)

The laboratory characterization of the optical and radiometric properties of the Hyperspectral Imager for the Coastal Ocean (HICO) is presented. It is shown the as-built sensor meets or exceeds the design parameters necessary to meet the stringent requirements imposed by maritime hyperspectral imaging. The results confirm that in general, the design parameters have been satisfied and the measured system response and signal to noise ratio is shown to match the sensor model. The results are discussed.

Hyperspectral imaging of an inter-coastal waterway

This paper demonstrates the characterization of the water properties, bathymetry, and bottom type of the Indian River Lagoon (IRL) on the eastern coast of Florida using hyperspectral imagery. Images of this region were collected from an aircraft in July 2004 using the Portable Hyperspectral Imager for Low Light Spectroscopy (PHILLS). PHILLS is a Visible Near InfraRed (VNIR) spectrometer that was operated at an altitude of 3000 m providing 4 m resolution with 128 bands from 400 to 1000 nm. The IRL is a well studied water body that receives fresh water drainage from the Florida Everglades and also tidal driven flushing of ocean water through several outlets in the barrier islands. Ground truth measurements of the bathymetry of IRL were acquired from recent sonar and LIDAR bathymetry maps as well as water quality studies concurrent to the hyperspectral data collections. From these measurements, bottom types are known to include sea grass, various algae, and a gray mud with water depths less than 6 m over most of the lagoon. Suspended sediments are significant (~35 mg/m3) with chlorophyll levels less than 10 mg/m3 while the absorption due to Colored Dissolved Organic Matter (CDOM) is less than 1 m-1 at 440 nm. Hyperspectral data were atmospherically corrected using an NRL software package called Tafkaa and then subjected to a Look-Up Table (LUT) approach which matches hyperspectral data to calculated spectra with known values for bathymetry, suspended sediments, chlorophyll, CDOM, and bottom type.

Naval EarthMap Observer (NEMO) science and naval products

A wide variety of applications of imaging spectrometry have been demonstrated using data from aircraft systems. Based on this experience the Navy is pursuing the Hyperspectral Remote Sensing Technology (HRST) Program to use hyperspectral imagery to characterize the littoral environment, for scientific and environmental studies and to meet Naval needs. To obtain the required space based hyperspectral imagery the Navy has joined in a partnership with industry to build and fly the Naval EarthMap Observer (NEMO). The NEMO spacecraft has the Coastal Ocean Imaging Spectrometer (COIS) a hyperspectral imager with adequate spectral and spatial resolution and a high signal-to- noise ratio to provide long term monitoring and real-time characterization of the coastal environment. It includes on- board processing for rapid data analysis and data compression, a large volume recorder, and high speed downlink to handle the required large volumes of data. This paper describes the algorithms for processing the COIS data to provide at-launch ocean data products and the research and modeling that are planned to use COIS data to advance our understanding of the dynamics of the coastal ocean.

Hyperspectral sensor characteristics needed for coastal ocean science

The complexity of the coastal ocean, particularly when the bottom is visible, necessitates the use of hyperspectral imagery for remote measurement of bathymetry, bottom type, and water properties. This is in contrast to the open ocean where water column properties alone are of interest and for which the use of multispectral imagery is normally sufficient. The Remote Sensing Division of the NRL has had a program to develop hyperspectral imagery for coastal studies for more than 10 years. The Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS), the most recent version of NRLs imaging spectrometer, is designed specifically for this application. It uses a thinned, backside-illuminated CCD for high sensitivity and an all-reflective spectrograph with a convex grating in an Offner configuration to produce a nearly distortion-free image. The sensors, which are constructed entirely from commercially available components, have been successfully deployed on numerous aircraft based experiments since 1999. After data collection is complete the data are geolocated, calibrated, atmospherically corrected and used for a variety of ocean and land products. In this presentation, we describe the instrument design, with emphasis on the specifications required for observing the coastal ocean. We also present examples of remote-sensing reflectance data obtained from the LEO-15 site in New Jersey that agree well with ground-truth measurements