Giulietta S. Fargion

Unique data repository facilitates ocean color satellite validation

The oceans play a critical role in the Earths climate, but unfortunately the extent of this role is only partially understood. One major obstacle is the difficulty associated with making high-quality globally distributed observations, a feat that is nearly impossible using only ships and other ocean-based platforms. The data collected by satellite-borne ocean color instruments, however, provide environmental scientists a synoptic look at the productivity and variability of the Earths oceans and atmosphere, respectively on high-resolution temporal and spatial scales. Three such instruments, the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) onboard ORBIMAGEs OrbView-2 satellite, and two Moderate Resolution Imaging Spectroradiometers (MODIS) onboard the National Aeronautic and Space Administrations (NASA) Terra and Aqua satellites, have been in continuous operation since September 1997, February 2000, and June 2002, respectively. To facilitate the assembly of a suitably accurate data set for climate research, members of the NASA Sensor Inter-comparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project and SeaWiFS Project Offices devote significant attention to the calibration and validation of these and other ocean color instruments. This article briefly presents results from the SIMBIOS and SeaWiFS Project Offices (SSPO) satellite ocean color validation activities and describes the SeaWiFS Bio-optical Archive and Storage System (SeaBASS),a state-of-the-art system for archiving, cataloging, and distributing the in situ data used in these activities.

Development of a consistent multi-sensor global ocean colour time series

The advent of a new generation of space-borne ocean colour sensors brings the prospect of global ocean measurements for decades into the future. These measurements will provide the basis for characterizing variability in the structure of the oceans phytoplanktonic communities and the response of those communities to climatic change. In addition, the measurements will allow development of the scientific basis necessary to manage the sustainable resources of marine ecosystems. These studies will require a merged, long-term, multi-satellite ocean colour time series extending beyond the operational lifetimes of individual instruments. The Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has been tasked to develop the tools required to create this time series. Among these tools are a comprehensive in situ (field collected) bio-optical dataset for validating ocean optics algorithms and associated models of oceanic properties; a programme to evaluate different atmospheric correction techniques; a programme to link the calibrations of individual satellite instruments; a programme to develop consistent calibration and validation datasets for satellite instruments; and a set of alternate methods to combine ocean colour measurements from different sources into a single time series. We report on the progress of this development work by the SIMBIOS Project.

Sun-Pointing-Error Correction for Sea Deployment of the MICROTOPS II Handheld Sun Photometer

Handheld sun photometers, such as the MICROTOPS II (manufactured by Solar Light, Inc.), provide a simple and inexpensive way to measure in situ aerosol optical thickness (AOT), ozone content, and water vapor. Handheld sun photometers require that the user manually point the instrument at the sun. Unstable platforms, such as a ship at sea, can make this difficult. A poorly pointed instrument mistakenly records less than the full direct solar radiance, so the computed AOT is much higher than reality. The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has been collecting maritime AOT data since 1997. As the dataset grew, a bias of the MICROTOPS II data with respect to other instrument data was noticed. This bias was attributed to the MICROTOPS II measurement protocol, which is intended for land-based measurements and does not remove pointing errors when used at sea. Based upon suggestions in previous literature, two steps were taken to reduce pointing errors. First, the measurement protocol was changed to keep the maximum (rather than average) voltage of a sequence of measurements. Once on shore, a second screening algorithm was utilized to iteratively reject outliers that represent sun-pointing errors. Several versions of this method were tested on a recent California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise, and were compared to concurrent measurements using the manufacturer-supplied protocol. Finally, a separate postprocessing algorithm was created for data previously gathered with the manufacturer-supplied protocol, based on statistics calculated by the instrument at the time of capture.

SIMBIOS Program in Support of Ocean Color Missions: 1997-2003

The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Program had a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS data collection takes place via the SIMBIOS Science Team. In addition, SIMBIOS had a calibration and product validation component (Project Office). The primary purpose of these calibration and product validation activities were to (1) reduce measurement error by identifying and characterizing true error sources, such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical and atmospheric correction algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternate methods when possible, and provides the results to the appropriate project office.

Three years of ocean color instrument intercomparisons and cross-calibrations by the SIMBOIS project (1997-2000)

The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS dat collection takes place via the SIMBIOS Science Team and the National Aeronautics and Space Administration (NASA) Aerosol Robotic Network. In addition, SIMBIOS has a calibration and product validation component. The primary purpose of these calibration and product validation activities are to (1) reduce measurement error by identifying and characterizing true error sources such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternate methods when possible, and provides the results to the appropriate project office.

Ocean color instrument intercomparisons and cross-calibrations by the SIMBIOS project (1999-2000)

The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project has a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS data collection takes place via the SIMBIOS Science Team and the NASA Aerosol Robotic Network (AERONET). In addition, SIMBIOS has a calibration and product validation component. The primary purpose of these calibration and product validation activities are to (1) reduce measurement error by identifying and characterizing true error sources such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternative methods when possible, and provides the results to the appropriate project office.