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C Community Letter to NASA and NOAA Regarding Concerns Over NPOESS Preparatory Project VIIRS Sensor and Response by S. Alan Stern COMMUNITY LETTER October 2, 2007 Dear Dr. Griffin and Admiral Lautenbacher, We are ocean biologists and biogeochemists who use NASA satellite data to study the ocean’s biosphere, its changes in time, and how it is affected by and responds to humankind’s activities. Our science requires satellite “ocean color” sensors that can quantify trends in global ocean biological and biogeochemical properties on intraseasonal to decadal time scales. Ocean color data are critical to address strategic needs of other federal agencies, including the National Oceanic and Atmospheric Administration (NOAA), the Environmental Protection Agency (EPA), and the U.S. Navy. High quality ocean color observations have applications such as detecting and monitoring changes in water quality, tracking harmful algal blooms, assessing underwater visibility for divers, and a variety of other applications related to ocean ecosystems, carbon and elemental cycling, coastal habitats, and coastal hazards. For the past decade, we have been privileged to use the internationally recognized highest quality NASA data sets from SeaWiFS and MODIS on Aqua. These NASA data sets have literally revolutionized our field and greatly enhanced our abilities to inform policymakers and the public of the changes to our oceans. We have become increasingly concerned that VIIRS (Visible Infrared Imager/Radiometer Suite), the ocean color sensor on the NPOESS Preparatory Project (NPP) mission, will be incapable of providing imagery for climate science applications. VIIRS on NPP was supposed to be the next ocean color mission to follow SeaWiFS and MODIS on Aqua and extend key biological and biogeochemical oceanographic observations. Importantly, this requirement of continuing the EOS-level climate science observations across all Earth Science disciplines was built into the recom- mendations of the NRC’s Decadal Survey committee. Recent summaries of VIIRS performance from the IPO and NASA, presented at the NPP Science Team Meeting in August 2007, clearly show that the present configuration of the VIIRS sensor will not come close to meeting the VIIRS design specifications and required spectral radiometric accuracies—which are minimum standards for ocean color sensor performance for climate science applications. The so-called “cherry-picked” configuration of the VIIRS’s filter array will just meet these standards but only if the rest of pre-flight testing is completely error free (which is unrealistic as VIIRS just entered its testing phase). Hence, we have little confidence that VIIRS on NPP will ever provide well-calibrated ocean color imagery. It may be able to provide qualitative imagery for descriptive purposes (detecting turbid water plumes, etc.), but not the high-quality ocean color imagery required for quantifying the vari- ability and impacts of natural and human-induced changes of our oceans. 167
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168 APPENDIX C We face the ominous likelihood that in the near future there will be a disruption in climate-quality ocean color data as both MODIS Aqua and SeaWiFS are beyond their designed mission lifetimes. Unfortunately, there is nothing in the first round of Earth Science Decadal Survey plans for EOS-level climate sensors beyond VIIRS on NPP (and then VIIRS on NPOESS). The Decadal Survey recommends a 2013-2016 launch date for the Aerosol/Cloud/Eco- systems (ACE) mission—a mission that if structured properly could significantly contribute to our community. Our understanding, however, is that even a 2016 launch for ACE is optimistic and would require funding for mission planning to begin no later than 2009. As it looks now, both MODIS-Aqua and SeaWiFS sensors are likely to be dead long before the ACE mission’s launch. In contrast to other international partnerships, such as with altimetry, we are not getting much help with global ocean color measurements from our international partners. Unfortunately, present ocean color missions are limited at best. For example, the European Space Agency’s (ESA) MERIS mission has a narrow swath (only a third of SeaWiFS’s) and has several image quality issues (radiometric inconsistencies, scan line dependence in derived products, no vicarious calibration procedure in place, etc.). We are not able to routinely receive global MERIS Level 1 data nor do we have detailed pre-flight characterization data, so there is no existing way for us to work with ESA to fix these problems. Nearly all other international ocean color missions in space today lack global coverage and are experimen- tal imagers with narrow swaths and poorly characterized imaging capabilities. Again, data access remains difficult and we have little ability to understand their capabilities. Future international missions are likewise problematic, as launch dates are far into the future, and we anticipate the same problems we have today with current international sensors (limited access to data, poor sensor characterization, etc.). We see three choices that the Space and Earth science agencies can make; each has serious and long-term conse- quences for our scientific discipline, the management of ocean resources, and society’s ability to understand climate change and mitigate human impacts on ocean habitats and ecosystems. These are: 1. Aggressively pursue and document improvements to the VIIRS sensor on NPP that enable it to meet the specifica- tions required for climate capable ocean color observatories, 2. Implement a stand-alone, global ocean color mission, or 3. Continue with plans of flying NPP, but in full recognition of its consequences. 1. An aggressive rehabilitation of VIIRS on NPP is theoretically possible, and scientists and engineers at NASA GSFC have long been considering it. As you likely know, there are many issues (institutional, contractual, fiscal, schedule, interagency agendas, etc.) that will make the successful rehabilitation of VIIRS extremely difficult. This strategy will require a much improved and transparent pre-launch testing and on-orbit calibration/validation program (including on-orbit maneuvers for lunar calibrations) than is in place today for the NPP program. Even if this plan were to be pursued, there is no certainty that it will work in the end. For example, the MODIS sensor on the Terra platform has yet to provide stable ocean color data products despite many person years of effort. MODIS on Terra has shown substantial radiometric signal degradation on orbit (up to 40%). This degradation varies with scan angle and mirror-side and may have resulted from damage done to the mirror coatings in pre-launch testing after its pre- launch characterization. Limitations with the on-orbit calibration system (e.g., solar diffuser door anomaly, inability to track polarization changes) have resulted in temporal variations in calibrated radiances that obfuscate ocean color trends. Seven years after its launch, researchers at the Goddard Space Flight Center (GSFC) are still working to produce high-quality, ocean color data products from MODIS on Terra. 2. A gap-filling mission in the spirit of SeaWiFS can be implemented and flown. SeaWiFS is a success story of the “better, faster, cheaper” version of NASA. A dedicated, single-instrument ocean mission can be flown easily and cost-effectively. NASA has launched “quick recovery” missions before. The Quick Scatterometer (QuikSCAT) mis- sion was launched to fill the critical data gap created after the NASA Scatterometer (NSCAT) failed. The ocean color science community has learned much over the last decade about what is required for a successful mission. NASA HQ has requested a detailed concept study for this type of mission and this work is being conducted at the Goddard Space Flight Center. This gap-filling sensor could also be placed on a mission of opportunity. This is clearly the lowest-risk option for insuring that high-quality ocean color data are available for the next decade of science and applications. 3. The last choice is the situation we find ourselves in today, and arguably the worst-case scenario. Staying the course with NPP will clearly not result in climate-quality ocean color data. This decision must be made with the un- derstanding that these data will not be available at the time when efforts to address climate change, coastal hypoxia, harmful algal blooms, fisheries health, ocean acidification, and many other issues are accelerating. The bottom line
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APPENDIX C 169 is that essential research-quality ocean color data will not be available in the very near future unless immediate and substantive actions are taken. The lack of global ocean color observations at the accuracy and quality of which NASA is currently capable will impact critical climate research conducted in the U.S. Ocean Carbon and Biogeochemistry (OCB) program as well as ocean observatory efforts currently coming on-line from the National Science Foundation (OOI) and NOAA (IOOS). In particular, there will be no way to integrate the invaluable in situ regional observations of the ocean observatory nodes with global observations of ocean plants, animals, and overall health and chemistry for better understanding and prediction of Earth System responses to climate variability and change. Without global and high-resolution coastal ocean color data, there will unquestionably be an impact on implementation and delivery of results to support goals of the Climate Change Science Program (e.g., carbon cycle and ecosystem science), the U.S. Ocean Action Plan and recently released Ocean Research Priorities Plan themes, and the Global Earth Observa- tion System of Systems objectives. We thank you for your attention and consideration of this matter. Please let us know what we can do to help. Sincerely, David A. Siegel, Professor of Marine Science and Director of the Institute for Computational Earth System Science, University of California, Santa Barbara, and member of the NASA NPP Science Team James Yoder, Senior Scientist, Woods Hole Oceanographic Institution; former Chair of the Carbon Cycle Scien- tific Steering Group, current Chair of the International Ocean Color Coordinating Group, and former member of ESSAAC [Also included in the letter were 56 supporting signatures; they are not shown here.]
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170 APPENDIX C RESPONSE App C