capabilities, many important research and operational uses are compromised, including the capability to detect impacts of climate change on primary productivity. Therefore, it is imperative to maintain and improve the capability of satellite ocean color missions at the accuracy level required to understand changes to ocean ecosystems that potentially affect living marine resources and the ocean carbon cycle, and to meet other operational and research needs. Given the importance of maintaining the data stream, the National Oceanic and Atmospheric Administration (NOAA), NASA, the National Science Foundation (NSF), and the Office of Naval Research (ONR) asked the National Research Council to convene an ad hoc study committee to review the minimum requirements to sustain global ocean color radiance measurements for research and operational applications and to identify options to minimize the risk of a data gap (see Box S.1 for the full statement of task). Because the ability to sustain current capabilities is at risk, the report focuses on minimum requirements to sustain ocean color observations of a quality equivalent to the data collected from SeaWiFS. Meeting these requirements will mitigate the risk of a gap in the ocean color climate data record but will be insufficient to explore the full potential of ocean color research and will fall short of meeting all the needs of the ocean color research and operational community. To meet all these needs, a constellation of multiple sensor types2 in polar and geostationary orbits will be required. Note that satellite requirements for research leading to the generation of novel products would vary depending on the question addressed and are difficult to generalize.
THE REQUIREMENTS TO OBTAIN HIGH-QUALITY GLOBAL OCEAN COLOR DATA
Satellite ocean color sensors measure radiance at different wavelengths that originate from sunlight and are backscattered from the ocean and from the atmosphere. Deriving the small ocean component from the total radiance measured by satellite sensors is a complex, multi-step process. Each step is critical and needs to be optimized to arrive at accurate and stable measurements. Using a set of algorithms (starting with removal of the contribution from the atmosphere, which is most of the signal), radiance at the top of the atmosphere is converted to water-leaving radiance (Lw) and then to desired properties such as phytoplankton abundance and primary productivity. To detect long-term climactic trends from these properties, measurements need to meet stringent accuracy requirements. Achieving this high accuracy is a challenge, and based on a review of lessons learned from the SeaWiFS/MODIS era, requires the following steps to sustain current capabilities:
1. The sensor needs to be well characterized and calibrated prior to launch.
2. Sensor characteristics, such as band-set and signal-to-noise, need to be equivalent to the combined best attributes from SeaWiFS and MODIS.
3. Post-launch vicarious calibration3 using a Marine Optical Buoy (MOBY)-like approach with in situ measurements that meet stringent standards is required to set the gain factors of the sensor.
4. The sensor stability and the rate of degradation need to be monitored using monthly lunar looks.4
5. At least six months of sensor overlap are needed to transfer calibrations between space sensors and to produce continuous climate data records.
6. The mission needs to support on-going development and validation of atmospheric correction, bio-optical algorithms, and ocean color products.
7. Periodic data reprocessing is required during the mission.
8. A system needs to be in place that can archive, make freely available, and distribute rapidly and efficiently all raw,5 meta- and processed data products to the broad national and international user community.
9. Active research programs need to accompany the mission to improve algorithms and products.
10. Documentation of all mission-related aspects needs to be accessible to the user community.
Meeting these requirements would contribute to sustaining the climate-quality global ocean color record for the open ocean. However, further enhancements to sensors and missions, such as higher spectral and spatial resolution, will be required to meet the research and operational needs for imaging coastal waters and for obtaining information about the vertical distribution of biomass or particle load. High frequency sampling (e.g., imagery every 30 minutes for a fixed ocean area), such as can be obtained from geostationary orbit, are desirable enhancements for applications such as ecosystem and fisheries management, as well as naval applications.
2 Type 1: Polar orbiting sensors with relatively low spatial resolution (1 km) with 8 (or many more) wave bands.
Type 2: Polar orbiting sensors with medium spatial resolution (250-300 m) and more bands to provide a global synoptic view at the same time as allowing for better performance in coastal waters.
Type 3: Hyper-spectral sensors with high spatial resolution (~30m) in polar orbit.
Type 4: Hyper- or multi-spectral sensors with high spatial resolution in geostationary orbit.
3 Vicarious calibration refers to techniques that use natural or artificial sites on the surface of Earth and models for atmospheric radiative transfer to provide post-launch absolute calibration of sensors.
4 Monthly lunar looks refers to the spacecraft maneuver that looks at the surface of the moon once a month as a reference standard to determine how stable the sensor’s detectors are. The information from the lunar looks is then used for determining temporal changes in sensor calibration.
5 Raw data is defined as data in engineering units to which new calibration factors can be applied to generate radiance values at the top of the atmosphere.