At the moment, two sensors (MODIS and MERIS) are providing ocean color data at the same time. This redundancy has served the climate research community well because it has enabled scientists to intercalibrate the sensors and improve the reprocessing to ensure data continuity. However, the sensors are beyond their anticipated life spans. SeaWiFS recently stopped delivering data and has been terminated. It is uncertain how much longer the other two sensors can deliver high-quality observations. Therefore, it is plausible that, should MODIS and MERIS sensors fail, OCM-2 will be the only new sensor in space before VIIRS on NPP is launched. In addition, it is likely that OCM-2 and VIIRS/NPP will be the only Type 1 and 2 ocean color missions in orbit before Sentinel-3A is launched in 2013.
Conclusion: Because OCM-2 and VIIRS could be the only sensors in orbit until launch of Sentinel-3A in 2013, access to data from OCM-2 is a high priority for U.S. scientists. The committee notes that neither NOAA nor NASA is aggressively pursuing routine access to OCM-2 ocean color data for U.S. users, although preliminary discussion is ongoing and an MOU is in place.
The timeline of current and future ocean color sensors shown in Figure 4.1 does not necessarily represent the current and future availability of ocean color data, because several of the sensors have unusable and/or inaccessible data. For example, as a result of uncertainties and instabilities in the pre-launch and on-orbit characterization of MODIS-Terra, these data have been largely unusable (Franz et al., 2008). The data from India’s OCM sensor has generally not been available to the international community (Wilson, 2011), and there also are serious issues with its calibration (Lyon, 2009). It is anticipated that data from OCM-2 will be more accessible to the international community, but this remains to be seen. However, the OCM-2 is primarily a regional-scale mission intended for the Indian fishing community, not as a global mission.
Although Level 1-3 MERIS data are available to U.S. scientists, access to Level 0 data remains an issue (Wilson, 2011). While most data users only desire access to Level 1-3 data, some space-agency projects working with multiple international satellite datasets and with access to multiple sources of calibration data want access to Level 0 data, or to an appropriate substitute (Level 1B in the case of MERIS). The Level 0 data (or its substitute) are needed so users can go through identical data processing steps for different sensors. Not having access to Level 0 data has been a source of contention in the past. It might also become an issue when attempting to develop an international merged ocean color dataset—as is proposed for the virtual constellation—that requires access to all data/metadata for reprocessing and merging (see discussion in Chapter 5). International Traffic in Arms Regulations (ITAR) restrictions may force limits to the distribution of VIIRS Level 0 data that may contribute to friction between national satellite projects and present a barrier to full international cooperation for ocean color data processing.
Conclusion: Data access is a major issue that needs to be resolved before many of the sensors listed in Table 4.4 meet requirements.
NASA and NOAA have good relations with ESA and JAXA and a longstanding tradition of exchanging satellite data. Relations with ISRO for data exchange are evolving in a positive way.
Nevertheless, issues arise with all partners on the details of data access. For example, there are generally few if any restrictions related to the exchange of Level 3 data products, once the mission teams have established confidence in the quality of the products. However, Level 0 and Level 1 data present problems. Issues related to data volume, proprietary software, ITAR restrictions (for VIIRS), etc. make it more difficult for U.S. and foreign agencies to exchange complete Level 0 or Level 1 datasets. These issues also can impede the full exchange of information on calibration, characterization, and processing details. When merging data from multiple sensors, it is impossible to generate climate-quality data products without full access to Level 0 and Level 1 datasets and without complete information on calibration, characterization, and processing details.
In addition, these data exchange issues can make it difficult for U.S. ground stations to downlink raw data from non-U.S. sensors for U.S. coastal waters. Without direct downlink capability to U.S. ground stations, it is extremely difficult, if not impossible, to generate true real-time products for applications for the United States.
In recognition of these challenges, the international Committee on Earth Observation Satellites3 (CEOS) has formed several “virtual constellations,” including the Ocean Colour Radiometry Virtual Constellation (OCR-VC). Chapter 5 discusses in greater detail how this virtual constellation presents unique opportunities to overcome some of these challenges.