S. Ahmed

MODIS and MERIS detection of dinoflagellates blooms using the RBD technique

Harmful Algal Blooms (HABs) can lead to severe economical and ecological impacts particularly in the coastal areas and can threaten human and marine health. About three-quarter of these toxic blooms are caused by dinoflagellates species which are well known to migrate vertically. During the day, they migrate up to the surface for photosynthesis, and consequently, their dense aggregations produce strong bio-optical signals that are detectable by space borne optical satellite sensors. In this study we use our recently developed low backscattering bloom detection technique, the Red Band Difference (RBD), to detect various dinoflagellates blooms using both MODIS (Moderate Resolution Imaging Spectroradiometer) and MERIS (Medium Resolution Imaging Spectrometer) data and present the results which confirm the potentials of the RBD technique. Here we present examples of bloom detection in waters off Gulf of Mexico, Monterey Bay, South Africa, and East China Sea.

Validation of the VIIRS Ocean Color

The Joint Polar Satellite System (JPSS) launched the Suomi National Polar-Orbiting Partnership (NPP) satellite including the Visible Infrared Imager Radiometer Suite (VIIRS) on October 28, 2011 which has the capability to monitor ocean color properties. Four months after launch, we present an initial assessment of the VIIRS ocean color products including inter-comparisons with satellite and in situ observations. Satellite ocean color is used to characterize water quality properties, however, this requires that the sensor is well characterized and calibrated, and that processing addresses atmospheric correction to derive radiometric water leaving radiance (nLw ). These radiometric properties are used to retrieve products such as chlorophyll, optical backscattering and absorption. The JPSS ocean calibration and validation program for VIIRS establishes methods and procedures to insure the accuracy of the retrieved ocean satellite products and to provide methods to improve algorithms and characterize the product uncertainty. A global monitoring network was established to integrate in situ data collection with satellite retrieved water leaving radiance values from ocean color satellites including Moderate Resolution Imaging Spectroradiometer (MODIS), MEdium Resolution Imaging Spectrometer (MERIS) and VIIRS. The global network provides a monitoring capability to evaluate the quality of the VIIRS nLw in different areas around the world and enables an evaluation and validation of the products using in situ data and other satellites. Monitoring of ocean color satellite retrievals is performed by tracking the gain at the Top of the Atmosphere (TOA) and then performing a vicarious adjustment fo reach site. VIIRS ocean color products are compared with MODIS and MERIS retrieved nLw and chlorophyll, and have been shown to provide similar quality. We believe that VIIRS can provide a follow-on to MODIS and MERIS equivalent ocean color products for operational monitoring of water quality. Additional research, including an assessment of stability, a full characterization of the sensor and algorithm comparisons is underway. Weekly sensor calibration tables (look up tables) are produced by JPSS and an evaluation of their impact on ocean color products is ongoing.

Evaluation of atmospheric correction procedures for ocean color data processing using hyper- and multi-spectral radiometric measurements from the Long Island Sound Coastal Observatory

In Ocean Color (OC) data processing one of the most critical steps is the atmospheric correction procedure used to separate the water leaving radiance, which contains information on water constituents, from the total radiance measured by space borne sensors, which contains atmospheric contributions. To ensure reliability of retrieved water leaving radiance values, and OC information derived from them, the quality of the atmospheric correction procedures applied needs to be assessed and validated. In this regard, the Long Island Sound Coastal Observatory (LISCO), jointly established by the City College of New York and the Naval Research Laboratory is becoming one of the key elements for OC sensors validation efforts, in part because of its capabilities for co-located hyper and multi-spectral measurements using HyperSAS and SeaPRISM radiometers respectively, with the latter being part of the NASA AERONET - OC network. Accordingly, the impact of the procedures used for atmospheric correction on the retrieval of remote sensing reflectance (Rrs) data can then be evaluated based on satellite OC data acquired from the LISCO site over the last two years. From this, the qualities of atmospheric correction procedures are assessed by performing matchup comparisons between the satellites retrieved atmospheric data and that of LISCO.

Measuring underwater polarization field from above-water hyperspectral instrumentation for water composition retrieval

Increasing efforts are devoted by the Ocean Color Radiometry community to explore the polarization features of the underwater light field in order to enhance possibilities for retrieving inherent optical properties (IOPs) of coastal waters. New instrumentations and data inversion algorithms are being developed to take into account the supplementary information contained in polarization data. However, estimating the Stokes vector components of the polarized water radiance from above water measurements is a challenging task, mainly because of their small magnitude and the strong contamination by the polarized sky light reflected from the sea surface. In this study, above-water measurements are used to assess the feasibility of such retrievals and their utility for retrieving IOPs. The Long Island Sound Coastal Observational platform (LISCO) near Northport, NY, was established in October 2009 to support satellite data validation. In June 2010, three customized hyperspectral HyperSAS systems (HyperSAS-POL) were added to LISCO platform enabling polarization measurements. A data processing algorithm, which includes vector radiative transfer computations, was developed and used to remove the polarization signal due to sky light reflected from the sea surface (sky glint) and derive the underwater polarization field. The spectral shape of the retrieved underwater degree of polarization was then evaluated against theoretical radiative transfer computations and in situ underwater measurements. The results confirmed the validity of the polarization measurements by the LISCO site, thus validating a continuous time series starting from the beginning of June 2010 to the present which can be used for retrievals of IOPs from polarization measurements.

Field programmable gate array processing of eye-safe all-fiber coherent wind Doppler lidar return signals

A field deployable all-fiber eye-safe Coherent Doppler LIDAR is being developed at the Optical Remote Sensing Lab at the City College of New York (CCNY) and is designed to monitor wind fields autonomously and continuously in urban settings. Data acquisition is accomplished by sampling lidar return signals at 400 MHz and performing onboard processing using field programmable gate arrays (FPGAs). The FPGA is programmed to accumulate signal information that is used to calculate the power spectrum of the atmospherically back scattered signal. The advantage of using FPGA is that signal processing will be performed at the hardware level, reducing the load on the host computer and allowing for 100% return signal processing. An experimental setup measured wind speeds at ranges of up to 3 km.

The impact of algal fluorescence on the underwater polarized light field

Multiangular, hyperspectral measurements of the underwater polarization light field, as well as comprehensive measurements of IOPs were collected in several cruise campaigns in the Chesapeake/Virginia area and New York Harbor/Hudson River areas. The waters examined were mostly eutropic water with Chlorophyll a concentration up to approximately 57 μg/L. It is found that Chlorophyll a fluorescence markedly impacts (reduces) the underwater degree of polarization (DOP) in the 650 - 700 nm spectral region. By taking note of the unpolarized nature of algal fluorescence and the partially polarized properties of elastic scattering, particularly by non-algal particles, we were able to separate the Chlorophyll a fluorescence signal from the total radiance. The analysis is based on comparisons of the underwater multiangular, hyperspectral polarization measurements which include fluorescence, compared with adding - doubling polarized radiative transfer simulations of elastic scattering which use measured IOPs as input, and which do not include fluorescence. The difference between the two shows the impact of fluorescence. These relationships are examined in detail, and the efficacy of using DOP measurements for underwater fluorescence retrieval is evaluated for different scattering geometries and water conditions.

Aerosol retrieval over urban areas using spatial regression between V/NIR and MIR Hyperion channels

Determination of aerosol optical depth from satellite remote sensing measurements is extremely complex due to the large variability of aerosol optical properties. Significant simplification occurs when measurements are taken over water since the ocean reflection signal can be taken as negligible in the NIR. Unfortunately, over land, most of the signal can be attributed to ground reflectance. While conventional approaches look for dark pixels in an image to isolate aerosols, these pixels are subjected to increased noise. In this paper, we explore the feasibility of a regression approach utilizing correlations between the V/NIR and MIR channels to extract the aerosol reflection signal over urban areas. This approach is applied to hyperspectral high resolution Hyperion data where the aerosol reflectance signal is shown to agree very well with coincident Aeronet derived reflectance spectra. Comparisons between the regression technique and dark pixel thresholding clearly exhibit the improvement using regression methods. Finally, practical spatial resolution concerns are explored and specifications of the GOES-R imager are assessed.

The challenges of implementing and testing two signal processing algorithms for high rep-rate coherent Doppler lidar for wind sensing

In this paper, we present two signal processing algorithms implemented using the FPGA. The first algorithm involves explicate time gating of received signals that correspond to a desired spatial resolution, performing a Fast Fourier Transform (FFT) calculation on each individual time gate, taking the square modulus of the FFT to form a power spectrum and then accumulating these power spectra for 10k return signals. The second algorithm involves calculating the autocorrelation of the backscattered signals and then accumulating the autocorrelation for 10k pulses. Efficient implementation of each of these two signal processing algorithms on an FPGA is challenging because it requires there to be tradeoffs between retaining the full data word width, managing the amount of on chip memory used and respecting the constraints imposed by the data width of the FPGA. A description of the approach used to manage these tradeoffs for each of the two signal processing algorithms are presented and explained in this article. Results of atmospheric measurements obtained through these two embedded programming techniques are also presented.

Ocean Color products from Visible Infared Imager Radiometer Suite (VIIRS)

The Ocean Color CAL/VAL team is evaluating the VIIRS bio-optical products for real-time operations. VIIRS ocean data are being processed using standard government algorithms, and channel calibration and product validation evaluation activities are ongoing. A network of 27 global “Golden Regions” has been established to evaluate and validate bio-optical products. Satellite inter-comparison for data consistency with current ocean color products, and real time vicarious adjustment calculation are performed using in situ water leaving radiance propagated to Top of Atmosphere in coastal and open ocean regions. In addition, routine matchups with VIIRS and MODIS-Aqua are done with in situ data collection from ships and real time coastal AERONET-OC sites. The above activities, product evaluation and tracking of channel stability, are being contributed to the JPSS Team to evaluate the overall mission, including calibration and inter-satellite product consistency. Initial NPP VIIRS ocean bio-optical products are demonstrated with other ocean color satellites.

Estimation of the polarized water leaving radiance from above water measurements

An acquisition system was developed to measure the above water polarized radiance. This system consists of one irradiance sensor for downwelling irradiance, one radiance sensor oriented at 40° from the zenith to measure sky radiance and three radiance sensors looking down at 40° from the nadir to measure above water radiance. In order to obtain the polarized radiance, polarizers with orientation of 0°, 90° and 45° respectively were placed in front of the three radiance sensors. The whole system was installed on the bow of the boat for continuous observations of above water polarized radiance along the ships track during a recent cruise in the NY Bight area. Water optical properties were measured by an optical package towed from a small R/V. In order to obtain the degree of polarization (DOP) of the water body, the contribution of the sky radiance must be first removed and this process has to be done for all components of the Stokes vector. Using a model employing the polarized Fresnel coefficients of the interface the polarized component of reflection is estimated from the direct measurement of sky radiance and downwelling irradiance data. These components are then subtracted from the measured values to obtain the water contribution. The DOP of the ocean body is then related to the in - water IOPs.