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/Ecosystems (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 experimental 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 consequences 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:
Aggressively pursue and document improvements to the VIIRS sensor on NPP that enable it to meet the specifications required for climate capable ocean color observatories,
Implement a stand-alone, global ocean color mission, or
Continue with plans of flying NPP, but in full recognition of its consequences.
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.
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) mission 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.
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 understanding 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