The Naval Research Laboratory (NRL) has established a Regional Coastal Oceanography with Nanosatellites (ReCON) project which will explore the ability of high-resolution nanosatellites to monitor coastal, estuarine, riverine, and other maritime environments in support of U.S. Navy operations. The project will initially focus on using data from the almost 150+ Planet “Dove” nanosatellites which fly in “flocks” acquiring remotely sensed data from sunlight reflecting off the earth surface. The usefulness of remotely sensed data within our research and operations is determined by the ability to accurately perform atmospheric correction and compute water leaving radiances (Lw), which are then normalized (nLw) and form the basis for the generation of remote sensing reflectance and other inherent and apparent optical property products. These nanosatellites have a single infrared band, although two such bands are typically required to automatically select an appropriate aerosol model during atmospheric correction, prior to estimating nLw. While early in the project, this initial study will assess nanosatellite capabilities to accurately retrieve nLw measurements by specifying the aerosol model selection during the atmospheric correction process. Here we present nLw retrievals for a variety of Planet nanosatellite imagery covering an entire year over a northern island of Venezuela, which covers coastal and open ocean type waters. The nLw retrievals from the nanosatellites using forced aerosol models are compared to coincident nLw retrievals from the Suomi-National Polar-orbiting Partnership (SNPP) Visible Infrared Imaging Radiometer Suite (VIIRS) to gauge the potential reliability and accuracy of using nanosatellite imagery as a competent data source for ocean color optics.
In September, 2018, Hurricane Florence made landfall in North Carolina as a Category 1 hurricane and inundated the eastern United States with significant rainfall. Precipitation from this slow moving storm event caused massive flooding. Outflow from this flooding carried suspended solids including sediments and other particulates as the rainwater worked its way through river and watershed systems toward the Atlantic Ocean. The Advanced Baseline Imager (ABI) on the NOAA Geostationary Operational Environmental Satellite - 16 (GOES-16) monitors the eastern United States. ABI data from GOES-16 is available every 5 minutes and provides a platform for studying the increased volume of river flow into the Atlantic Ocean. Data from the GOES-16 ABI covering the Atlantic waters off the eastern United States were downloaded after the Hurricane Florence event. Methodologies for atmospheric correction were used to generate water leaving radiance values from the GOES-16 ABI data sets. Using the multiple looks per day, the plumes of suspended solids were delineated and studied.
Boreal winter meteorological fronts manifest across the northern Gulf of Mexico as rapid 10-15° C drops in air temperature and accelerating northerly winds. The physical coastal ocean response across the Louisiana-Texas (LATEX) continental shelf system involves a complex interplay between coastal buoyancy, wind forcing, and intense thermal energy fluxes out of the ocean. Herein we combine numerical simulations, in situ optical surveys, and coincident satellite images derived from the Ocean and Land Colour Imager (OLCI) and other sensors to further unravel the mechanistic functioning and optical signatures of these complex events. The conspicuous optical gradients evident in color satellite images coincident with cold air outbreak (CAO) events appear to result from surface ventilation of sediment-laden bottom waters and wind/buoyancy-driven surface currents. The hyperspectral gradients associated with water mass types (sediment resuspension in marine waters versus freshwater effluent plumes) give rise to true color gradients that may be tracked with low spectral resolution color sensors at very high temporal resolution.
The SAtellite VAlidation Navy Tool (SAVANT) was developed by the Navy to help facilitate the assessment of the stability and accuracy of ocean color satellites using ground truth (insitu) platform and buoy stations positioned around the globe and support methods for match-up protocols. This automated, continuous monitoring system for satellite ocean color sensors employs a website interface to extract and graph coincident satellite and insitu data in near-real-time. Available satellite sensors include MODerate resolution Imaging Spectrometer (MODIS) onboard the Aqua satellite, Visible Infrared Imaging Radiometer Suite (VIIRS) onboard Suomi National Polar-orbitting Partnership (SNPP) & Joint Polar Satellite Sensor (JPSS), Ocean and Land Colour Instrument (OLCI) onboard the Sentinel 3A and Geostationary Ocean Color Imager (GOCI) onboard the Communication, Ocean and Meteorological Satellite (COMS). SAVANT houses an extensive match-up data set covering nineteen plus years (2000- 2019) of coincident global satellite and ground truth spectral Normalized Water Leaving Radiance (nLw) data allowing users to evaluate the accuracy of ocean color sensors spectral water leaving radiance at specific ground truth sites that provide continuous data. The tool permits changing different match-up constraints and evaluating the effects on the match-up uncertainty. Results include: a) the effects of spatial selection (using single satellite pixel versus 3x3 and 5x5 boxes, all centered around the insitu location), b) time difference between satellite overpass and ground truth observations, c) and satellite and solar zenith angles. Match-up uncertainty analyses was performed on VIIRS SNPP at the AErosol RObotic NETwork Ocean Color (AeroNET-OC) Wave Current surge Information System (WavCIS) site, maintained by NRL and the Louisiana State University (LSU) in the North Central Gulf of Mexico onboard the Chevron platform CSI-06. The VIIRS SNPP and AeroNET-OC assessment determined optimal satellite ocean color cal/val match-up protocols that reduced uncertainty in the derived satellite products.
The success of current ocean color satellite missions relies on the spectral quality, consistency, accuracy and precision of products (water leaving radiances, aerosols and clouds) derived from the satellite sensors. We propose leveraging available in situ data from various autonomous ocean color data collection sites to provide a near real time (NRT) spectral calibration for the Ocean Land Colour Imager (OLCI) by tuning the top of atmosphere (TOA) spectral radiances. Using the Naval Research Laboratory – Stennis Space Center (NRL-SSC) Automated Processing System (APS) software, NRT calibration of OLCI is demonstrated using in situ data from the MOBY and AERONET-OC collection sites. This calibration procedure has been used with other multi-spectral satellites to rapidly improve the data quality of emergent sensors so that they can be used to support marine spectrometric applications, track the satellite sensor stability, and enable continuity and consistency of ocean color products between several satellites.
The combination of increased spectral resolution for in situ ocean optical instrumentation as well as future ocean remote sensing missions (e.g., PACE) provides an opportunity to examine new methods of analysis and ocean monitoring that were not feasible during the multispectral satellite era. For example, hyperspectral data enables a much more precise determination of the apparent true color for natural waters, one based on the full spectral shape of water-leaving radiance distributions. Herein we provide examples of how specific integrated biogeo-optical and physical processes in the northern Gulf of Mexico have characteristic hyperspectral signatures, and thusly, characteristic true color identifiers. Our emergent hypothesis is that once the characteristic hyperspectral color signature of a specific biophysical process is known, it can be detected and monitored even with multispectral or broad-band response digital imaging systems. To test this hypothesis, we examine archived imagery from MODIS and HICO to identify putative bottom boundary layer ventilation events along divergent shelf-frontal boundaries across the northern Gulf continental margin. Whereas on-demand in situ physical data that provide spatiotemporal correspondence with archived images are not available, we employ the data-assimilative Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) as a physical data surrogate. Preliminary results of this method appear to support the hypothesis, with the caveat that model results must be interpreted with due caution.
Standard oceanographic processing of the visible infrared imaging radiometer suite (VIIRS) and the moderate resolution imaging spectroradiometer (MODIS) data uses established atmospheric correction approaches to generate normalized water-leaving radiances (nLw) and bio-optical products. In many cases, there are minimal differences between temporally and spatially coincident MODIS and VIIRS bio-optical products. However, due to factors such as atmospheric effects, sensor, and solar geometry differences, there are cases where the sensors’ derived products do not compare favorably. When these cases occur, selected nLw values from one sensor can be used to vicariously calibrate the other sensor. Coincident VIIRS and MODIS scenes were used to test this cross-sensor calibration method. The VIIRS sensor was selected as the “base” sensor providing “synthetic” in situnLw data for vicarious calibration, which computed new sensor gain factors used to reprocess the coincident MODIS scene. This reduced the differences between the VIIRS and MODIS bio-optical measurement. Chlorophyll products from standard and cross-sensor calibrated MODIS scenes were fused with the VIIRS chlorophyll product to demonstrate the ability for this cross-sensor calibration and product fusion method to remove atmospheric and cloud features. This cross-sensor calibration method can be extended to other current and future sensors.
Navy operational ocean color products of inherent optical properties and radiances are evaluated for the Suomi–NPP VIIRS and MODIS-Aqua sensors. Statistical comparisons with shipboard measurements were determined in a wide variety of coastal, shelf and offshore locations in the Northern Gulf of Mexico during two cruises in 2013. Product consistency between MODIS-Aqua, nearing its end-of-life expectancy, and Suomi-NPP VIIRS is being evaluated for the Navy to retrieve accurate ocean color properties operationally from VIIRS in a variety of water types. Currently, the existence, accuracy and consistency of multiple ocean color sensors (VIIRS, MODIS-Aqua) provides multiple looks per day for monitoring the temporal and spatial variability of coastal waters. Consistent processing methods and algorithms are used in the Navy’s Automated Processing System (APS) for both sensors for this evaluation. The inherent optical properties from both sensors are derived using a coupled ocean-atmosphere NIR correction extending well into the bays and estuaries where high sediment and CDOM absorption dominate the optical signature. Coastal optical properties are more complex and vary from chlorophyll-dominated waters offshore. The in-water optical properties were derived using vicariously calibrated remote sensing reflectances and the Quasi Analytical Algorithm (QAA) to derive the Inherent Optical Properties (IOP’s). The Naval Research Laboratory (NRL) and the JPSS program have been actively engaged in calibration/validation activities for Visible Infrared Imager Radiometer Suite (VIIRS) ocean color products.
As part of the Joint Polar Satellite System (JPSS) Ocean Cal/Val Team, Naval Research Lab - Stennis Space Center (NRL-SSC) has been working to facilitate calibration and validation of the Visible Infrared Imaging Radiometer Suite (VIIRS) ocean color products. By relaxing the constraints of the NASA Ocean Biology Processing Group (OBPG) methodology for vicarious calibration of ocean color satellites and utilizing the Aerosol Robotic Network Ocean Color (AERONET-OC) system to provide in situ data, we investigated differences between remotely sensed water leaving radiance and the expected in situ response in coastal areas and compare the results to traditional Marine Optical Buoy (MOBY) calibration/validation activities.
An evaluation of the Suomi National Polar-Orbiting Partnership (SNPP)-VIIRS ocean color products was performed in coastal waters using the time series data obtained from the Northern Gulf of Mexico AERONET-OC site, WaveCIS. The coastal site provides different water types with varying complexity of CDOM, sedimentary, and chlorophyll components. Time series data sets were used to develop a vicarious gain adjustment (VGA) at this site, which provides a regional top of the atmospheric (TOA) spectral offset to compare the standard MOBY spectral calibration gain in open ocean waters.
The Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory Project (NPP) (SNPP) was launched in October 2011 to continue monitoring the globe in a similar fashion as the heritage sensors, such as the MODerate resolution Imaging Spectroradiometer (MODIS). This paper applies a scene-based technique to examine in-orbit radiometric stability of VIIRS relative to Aqua MODIS. The cross-comparison is made over global deep ocean waters. This cross-comparison allows for a comprehensive examination of the sensors’ radiometric responsivity at relatively low signal levels (over oceanic waters). The study is further extended to L2/L3 products, including remote sensing reflectance and the inherent optical properties (IOPs) of waters under investigation, derived from the top-of-atmosphere (TOA) radiance (L1B). The temporal analyses give insights into the trends in the relative radiometric stability and the resulting discrepancies in the corresponding products.
Same day ocean color products from the S-NPP and MODIS provide for a new capability to monitor changes in the bio-optical processes occurring in coastal waters. The combined use of multiple looks per day from several sensors can be used to follow the water mass changes of bio-optical properties. Observing the dynamic changes in coastal waters in response to tides, re-suspension and river plume dispersion, requires sequential ocean products per day to resolve bio-optical processes. We examine how these changes in bio-optical properties can be monitored using the NPP and MODIS ocean color products. Additionally, when linked to ocean circulation, we examine the changes resulting from current advection compared to bio-optical processes. The inter-comparison of NPP and MODIS ocean products are in agreement so that diurnal changes surface bio-optical processes can be characterized.
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.
We examine the impact of incorrect atmospheric correction, specifically incorrect aerosol model selection, on
retrieval of bio-optical properties from satellite ocean color imagery. Uncertainties in retrievals of bio-optical properties
(such as chlorophyll, absorption and backscattering coefficients) from satellite ocean color imagery are related to a
variety of factors, including errors associated with sensor calibration, atmospheric correction, and the bio-optical
inversion algorithms. In many cases, selection of an inappropriate or erroneous aerosol model during atmospheric
correction can dominate the errors in the satellite estimation of the normalized water-leaving radiances (nLw), especially over turbid, coastal waters. These errors affect the downstream bio-optical properties. Here, we focus on only the
impact of incorrect aerosol model selection on the nLw radiance estimates, through comparisons between Moderate-
Resolution Imaging Spectroradiometer (MODIS) satellite data and in situ measurements from AERONET-OC (Aerosol
Robotic NETwork - Ocean Color) sampling platforms.
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