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4 Capabilities of Current and Planned Ocean Color Sensor Missions T he previous chapter discussed the essential require- lifetime. While a foreign Type 2 sensor has recently been ments for a successful ocean color mission (including launched (Ocean Colour Monitor on-board Oceansat-2 requirements for sensor design, stability monitoring, [OCM-2]), questions of data access and data quality assur- vicarious calibration, and a calibration/validation program). ance need to be resolved, as discussed below. This chapter examines the capabilities of planned sen- Details of the character of each currently operating sors and missions to meet these requirements. We provide sensor are listed in Table 4.1. Characteristics of planned only a brief description of current and planned ocean color launches are listed in Table 4.2 (additional details in Appen- satellite sensors (for details see Appendix A). The chapter dix A). compares the sensors’ capabilities to the minimum require- As discussed in Chapter 2, the diverse set of data ments outlined by the committee, with commentary on specifications required to meet all ocean color user needs design approaches that enhance data accuracy and stability. requires different types of satellite sensors. Figure 4.1 shows In addition, it assesses the likelihood that these sensors will that many sensors will be available, but Tables 4.1, 4.2, and deliver products of sufficient quality. As discussed in Chapter 4.3 illustrate that they vary widely, each with its own capa- 2, a single sensor or mission cannot meet the needs of all bilities and limitations. Although nine ocean color satellite ocean color products. Because the charge to the committee missions have been launched to date, only four (CZCS, is limited to Type 1 and 2 sensors, this analysis focuses on SeaWiFS, MERIS, and MODIS-Aqua) have acquired high- their capabilities. quality global observations. This record raises concerns for the probability of success of the upcoming missions shown in Figure 4.1. CURRENT AND PLANNED OCEAN COLOR SENSORS For example, data from the MODIS-VIIRS line of sen- Figure 4.1 illustrates the timeline for past and planned sors can provide routine coverage of the global ocean at 1-km launches of U.S. and foreign ocean color sensors. Launches resolution. But U.S. users need access to other polar-orbiting of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) satellite ocean color data streams for (1) coastal and other a nd Moderate Resolution Imaging Spectroradiometer applications, (2) to improve coverage of the global ocean (MODIS) sensors were closely spaced, which resulted in using merged datasets from multiple sensors, and (3) as a a continuous U.S. data stream for remotely sensed ocean backup for global coverage in the event of a failure of a U.S. color. This close spacing resulted in sensor overlap, which sensor. The sensors with known or likely capabilities to serve made it possible to intercalibrate these sensors (for details these needs are MERIS on the European Space Agency’s see Chapter 3). However, SeaWiFS just recently ceased (ESA) Environmental Satellite (ENVISAT), Ocean Land operations and the only other U.S. sensor in orbit, MODIS, Colour Instrument (OLCI) to be flown on ESA’s Sentinel 3A is beyond its planned life span. The next U.S. sensor, Vis- and 3B satellites, Second-Generation Global Imager (S-GLI) ible Infrared Imager Radiometer Suite (VIIRS) on National to be flown by Japan Aerospace Exploration Agency (JAXA), Polar-orbiting Operational Environmental Satellite System and OCM-2, currently operating in space and maintained by (NPOESS) Preparatory Project (NPP), is not planned for the Indian Space Research Organization (ISRO). Data from launch until fall 2011 or later. Therefore, U.S. scientists other polar orbiting sensors may also be available, but their are at risk of losing access to an ocean color data stream. characteristics and mission operating procedures are less The Medium-Resolution Imaging Spectrometer (MERIS) well known to the U.S. community. is currently operating, although it is also beyond its design 46
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47 CURRENT AND PLANNED OCEAN COLOR SENSOR MISSIONS ??? ??? ??? ??? 10-year ??? Data Gap ??? ??? ??? Data Gap? FIGURE 4.1 The launch sequence of past, current, and planned ocean color sensors in polar orbit are displayed. The sensors still operational are shown with a one-sided arrow; the hatched area indicates when a sensor is beyond its design life. The gray shaded background indicates a S.1 (and 4-1).eps data gap in the past and a potential data gap arising if MODIS sensors and MERIS cease today. The question marks are used to indicate sensors that either do not yet meet the minimum requirements with vulnerablevector elementsallocation . Future sensors are shown having bitmap or are added to changes in funding either a five- or seven-year lifetime, according to their individual specifications. CZCS: Coastal Zone Color Scanner; OCTS: Ocean Color and Temperature Scanner; SeaWiFS: Sea-viewing Wide Field-of-view Sensor; OCM/OCM-2: Ocean Colour Monitor; MODIS-Terra/MODIS- Aqua: Moderate Resolution Imaging Spectroradiometer on Terra/Aqua, respectively; MERIS: Medium Resolution Imaging Spectrometer; GLI: Global Imager; VIIRS: Visible Infrared Imager Radiometer Suite; OLCI: Ocean Land Colour Instrument onboard Sentinel-3; PACE: Pre-Aerosol-Clouds-Ecosystem; GCOM-C: Global Change Observation Mission for Climate Research; JPSS: Joint Polar Satellite System. SOURCE: Based on data from http://www.ioccg.org/sensors_ioccg.html. TABLE 4.1 Current Sensors in Space Having Spectral Bands and Other Specifications That Provide Type a 1 or 2 Ocean Color Sensor Capabilities Spatial Resolutionb Swath Bands Spectral Coverage Sensor/Satellite/Type Agency Launch Date (km) (m) (visible/total) (nm) MODIS/Terra/1 NASA (USA) 1999 2,330 250/500/1,000 9/36 405-14,385 OCM-1/IRS-P4/2 ISRO (India) 1999 1,420 360/4,000 7/8 412-885 MERIS/2 ESA (Europe) 2002 1,150 300/1,200 12/15 412-1,050 MODIS/Aqua/1 NASA (USA) 2002 2,330 250/500/1,000 9/36 405-14,385 OCM-2/Oceansat2/2 ISRO (India) 2009 1,420 360/4,000 7/9 400-900 Listed in ascending order of launch date (for details see Appendix A). a Sensors are characterized into Type 1-4 based on their spatial and spectral coverage and orbit (see Table 2.1). b The sensor has some capability to sample at higher spatial resolution. Conclusion: U.S. research and operational users of satel- ANALYSIS OF CAPABILITIES AND GAPS lite ocean color data will have to rely on multiple sources, Our analysis of current and future capabilities is focused including sensors operated by non-U.S. space agencies, on the Type 1 and 2 sensors listed in Table 4.2 (those capable because the United States does not have approved mis- of providing global coverage approximately every two to sions that will sustain optimal ocean color data for all three days), because most of the past experience is limited to applications.
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48 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS TABLE 4.2 Planned Sensors Having Spectral Bands and Other Specifications That Provide Type 1 and 2 Ocean Color Capabilities Launch Swath Spatial Resolution Spectral Coverage Sensor/Satellite/Type Agency Date (km) (m) Bands (nm) VIIRS/NPP/1 NOAA/NASA (USA) 2011 3,000 370/740 22 412-11,800 OLCI/Sentinel-3A/2 ESA/EUMETSAT 2013 1,270 300/1,200 21 400-1,020 (Europe) S-GLI/GCOM-C JAXA (Japan) 2014 1,150-1,400 250/1,000 19 375-12,500 VIIRS/JPSS/1 NOAA/NASA (USA) 2016 3,000 370/740 412-11,800 OLCI/Sentinel-3B/2 ESA/EUMETSAT 2017 1,265 260 21 390-1,040 (Europe) PACE/2 NASA 2019 (USA) VIIRS/JPSS1/1 NOAA 2019 3,000 370/740 22 412-11,800 (USA) ACE/2 NASA 202X 1,000 Hyperspectral at 350-2,130 (USA) 5 nm; 3 discrete SWIR bands Listed in ascending order of scheduled launch date. TABLE 4.3 Planned Type 3 Sensor(s) Spectral Coverage Sensor/Satellite Agency Launch Date Swath Resolution Bands (nm) HyspIRI NASA Unknown 600 km 60 m Hyperspectral 380-2,500 (USA) at 10 nm sensors of that type and because our goal is to assess options sensing of coastal and shallow water habitats. For example, to ensure continuity in global ocean color data. The other a band at 640 nm is critical for semi-analytical inversion sensors are critical to advance research applications, but as models in coastal waters (Lee algorithm in IOCCG Report new sensors they are not essential to the continuity of the 5, 2006). Also, many sensors are missing the fluorescence long-term global ocean color time-series. bands. Based on freely available information, the committee The signal-to-noise ratio (SNR) of all wavebands on assessed whether current and planned sensors would meet MERIS and VIIRS (both on NPP and JPSS) are equivalent some of the elements essential to the success of an ocean to or better than the SNR for the wavebands on the SeaWiFS color mission, as listed in Chapter 3 (Table 4.4): pre-launch sensor (NOAA, 2010). In particular, reduction of the digi- instrument characterization and calibration, post-launch tization of the NIR channels in SeaWiFS was an important stability monitoring, sun-glint avoidance, vicarious calibra- source of noise in open ocean retrievals (Hu et al., 2004). The tion, data processing and reprocessing, freely available user- SNRs for 412-, 443-, 490-, and the 510-nm bands on OCM-2 friendly processing software, and availability of raw data and in the 761-nm band on OLCI and Second-Generation and information on instrument characterization. These last Global Imager (S-GLI) are worse than for respective bands two data-related requirements are of particular importance on SeaWiFS (NOAA, 2010). All sensors listed in Tables 4.2 to climate and other scientific applications. and 4.3 have been or are being designed to offer measures of Table 4.5 lists the spectral bands for the sensors identi- the satellite radiances in various visible, NIR and/or short- fied in Table 4.4, showing wide agreement among agencies wave infrared (SWIR) spectral bands, as well as for land and nations regarding the most important spectral capa - applications. These latter bands are and will continue to be bilities for ocean color that are required to sustain current important for atmospheric corrections. capabilities. A notable exception is the VIIRS sensor on NPP and Joint Polar Satellite System (JPSS), which is missing the ENSURING GLOBAL HIGH-QUALITY OCEAN 510- or 530-nm bands. Given the advances in ocean color COLOR DATA FOR THE NEXT TWO TO FIVE YEARS algorithms for turbid waters (Morel and Bélanger, 2006), the result of these missing wavebands will lead to sub-optimal The greatest risk identified by the committee is that retrievals of chlorophyll and possibly other derived products U.S. scientists and resource managers will lack access to in turbid coastal waters. Moreover, the large spectral gap high-quality ocean color data between now and the launch of between 555 and 665 nm has been problematic for remote Pre-Aerosol-Clouds-Ecosystem (PACE) (planned for 2019).
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TABLE 4.4 Comparing SeaWiFS with Current and Planned Sensors Against Some Important Sensor and Mission Requirements MODIS- MODIS- VIIRS VIIRS SeaWiFS Aqua Terra NPP JPSS-I PACE ACE MERIS OLCI OCM OCM-2 S-GLI Pre-launch Yes Yes Yes Yes TBD Yes Yes TBD TBD TBD Instrument Mission Mission Characterization requirement requirement Stability Yes; monthly Yes; Yes; No; TBD; Yes; dual Yes; dual No Planned Yes; solar Monitoring lunar look single solar single solar single solar single solar Mission Mission solar solar two times diffuser and (lunar calibration diffuser diffuser diffuser diffuser requirement requirement diffuser diffuser per year monthly lunar or solar diffuser) with with with with look stability stability stability stability monitor; monitor; monitor, monitor, monthly monthly occasional occasional views of views of views of views of the the moon the moon the moon moon Sun-Glint Yes; tilts Mid-AM/ Mid-AM/ No No No Yes, Yes; tilts Yes; but Yes Avoidance mid-PM mid-PM Mission Mission permanent optimized orbits orbits requirement requirement across for the compensate compensate track tilt Indian Ocean Vicarious MOBY MOBY and MOBY and TBD; TBD; Planned Planned No Asked for Assumed use Calibration SeaWiFS SeaWiFS assumed assumed use Mission Mission for 2011 to use MOBY of MOBY use of of MOBY requirement requirement to use MOBY, MOBY MOBY, Boussole Boussole Data Yes Yes Yes No TBD Yes Yes Unknown TBD Planned Reprocessing Mission Mission requirement requirement Participates in a Yes Yes (via Yes (via No TBD Yes Yes No Member of Yes Continuity Plan SeaWiFS) SeaWiFS) Mission Mission OCR-VC for Heritage requirement requirement Missions Data Exchange Yes Yes Yes No TBD Likely Likely TBD TBD TBD TBD Yes agreement for L-0 and L-1 data Sensors from U.S. agencies are listed in white fields, from the European Space Agency in purple, from India in teal-colored fields, and from the Japanese space agency in blue. SOURCE: Adapted from National Oceanic and Atmospheric Administration’s Report, Ocean Color Satellite Continuity Mitigation Plan Revision 2, Final Report . 49
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50 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS TABLE 4.5 SeaWiFS Spectral Bands and Those of Current and Planned Sensors Ocean Color Data Sources OLCl MODIS- MODIS- MERIS OCM-2 Sentinel 3A S-GLI VIIRS VIIRS SeaWiFS Aqua Terra Envisat Oceansat & 3B GCOM NPP NPOESS/JPSS Band Center Band Band Band Band Band Band Band Band* Band* 412 412 412 412 412.5 412 413 412 412 412 443 443 443 443 442.5 443 443 443 445 445 490 490 488 488 490 490 490 490 488 488 510 510 531 531 510 510 510 530 555 555 551 551 560 555 560 565 555 555 670 670 667 667 665 620 665 674 672 672 678 678 678 681 765 765 748 748 778.8 740 778 763 746 746 865 865 869 869 865 865 865 869 865 865 Note band differences among the sensors. SOURCE: Adapted from National Oceanic and Atmospheric Administration’s Report, Ocean Color Satellite Continuity Mitigation Plan Revision 2, Final Report. This high risk results from a combination of factors: the loss (Bouée pour l’acquisition de Séries Optiques à Long Terme; of SeaWiFS, the quality and age of the MODIS sensors, Antoine et al., 2006, 2008) were used for this purpose and concerns with aspects of the VIIRS/NPP mission, and lack the calibration has been completed. BOUSSOLE is a joint of adequate data access to foreign sensors. Concerns with European ocean color calibration and validation activity to the VIIRS/NPP mission result from uncertainty regarding the which NASA also contributes. quality of and access to data. To minimize the risk of a data The MERIS sensor is not tilted to avoid sun glint. Pixels gap, this section will assess the three sensors most likely in with “moderate sun glint” are identified and a correction is orbit and capable of delivering ocean color data in the near applied to increase the coverage area (Bézy et al., 2000). term, with regard to their ability to meet key requirements. Pixels with higher levels of sun glint are simply flagged and users are left to judge whether they can use the data or not. Level 0 data generally are not available to users. This is MERIS Assessment only of concern to those users who may want to process and As discussed in the previous chapter and indicated in reprocess the data with the original raw data (see previous Table 4.5, the MERIS sensor was well characterized and discussion in this report). Level 1 data are available in near- calibrated pre-launch (Rast et al., 1999). In fact, the commit- real time (about three hours after acquisition) and again after tee concludes that this careful characterization contributed processing for accuracy, within about three weeks (Bézy et significantly to the mission’s success. al., 2000). Level 2 data also are freely available and mapped. Although the MERIS mission is not designed to use MERIS data have been used in particular in producing lunar looks for stability monitoring, its approach of using the multi-sensor global GlobColour merged data products dual solar diffusers appears to be adequate to monitor the (Maritorena et al., 2010). A major reprocessing of the entire sensor’s stability. MERIS uses one solar diffuser every two mission has been achieved, which includes a Level 2 vicari- weeks; the second is used every three months to check the ous calibration similar to the one applied to the NASA Sea- stability of the first. The second diffuser is assumed to offer WiFS and MODIS instruments (e.g., Gordon, 1998; Franz et constant reflectance, which provides a check on the reflec- al., 2007) that improves compatibility across these missions. tance change of the first. The last calibration analysis shows The ocean color group at the National Aeronautics and a 1.5 percent degradation of the diffuser between 2002 and Space Administration’s (NASA’s) Goddard Space Flight 2010 in the blue (443 nm) band and no detectable degrada- Center (GSFC) typically prefers to work with Level 0 data tion in other bands. but can use as an alternative Level 1B imagery. Such imagery Because of the careful pre-launch characterization and is available so long as ESA provides updates and the GSFC calibration, a vicarious calibration was initially assumed group has access/insight to the sensor issues, as is currently to be unnecessary. However, the benefit of the vicarious the case through GSFC participation in the MERIS “Data calibration has been subsequently recognized (Antoine et Quality Working Group” (DQWG). Although Level 3 data al., 2008). Marine Optical Buoy (MOBY) and BOUSSOLE have become available recently, some U.S. users were dis-
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51 CURRENT AND PLANNED OCEAN COLOR SENSOR MISSIONS Conclusion: Many issues are unresolved with regard to the couraged that they did not have access to Level 3 data from high quality of and access to OCM-2 data. the mission’s start. Conclusion: The MERIS mission and sensor design meet Conclusion: Because of the age of MERIS and MODIS, most requirements and show great promise for data to data availability from these sensors may be lost soon. reach climate quality. Incorporating NASA scientists into Therefore, OCM-2 and VIIRS on NPP may be the only ESA’s MERIS Data Quality Working Group is a very posi- Type 1 and 2 ocean color missions in orbit until the launch tive development that ideally would continue. of JAXA’s S-GLI and ESA’s OLCI sensors. Therefore, data access to OCM-2 may be the only option to mitigate a data gap, if VIIRS/NPP fails to meet the requirements and both OCM-2 Assessment MERIS and MODIS stop operating. All sensors currently in polar orbit are beyond their Recommendation: NASA and NOAA’s current efforts to design lives, with the exception of OCM-2. If these sensors resolve international data access issues should continue fail, OCM-2 will be the only ocean color sensor in space and be given high priority. NASA, NOAA, and ESA should until the launch of VIIRS on NPP. Therefore, this sensor continue to include foreign scientists as part of their mis- might become an important component of a mitigation plan sion science teams to foster information and data exchange. to minimize a data gap. The committee was not able to review the status of the pre-launch characterization and calibration due to lack of VIIRS/NPP Assessment information. Importantly, OCM-2 assesses sensor stability using solar and lunar calibrations. ISRO has established a In 2007, the ocean color community articulated prob- cal/val optical buoy in the Lakshadweep Sea to perform a lems with the NPOESS’ NPP VIIRS (Siegel and Yoder, vicarious calibration. 2007). Users were concerned about the ability of the VIIRS OCM-2 tilts the sensor twice per year to avoid the sun on NPP to deliver multi-spectral data of sufficient quality to glint over the Indian Ocean. Thus, the current sun-glint sustain the time-series of oceanographic products derived avoidance mechanism is optimized for the Indian Ocean. from SeaWiFS and MODIS-Aqua. The community reached Because of this constraint, the mission is not designed to these conclusions based on an NPOESS Integrated Program acquire global data. Office (IPO) report regarding VIIRS on NPP laboratory per- formance tests. Optical cross-talk was of greatest concern.1 Although India is a member of the virtual ocean color constellation group and has plans to make products avail- The community suggested two options to mitigate risk able online, no data agreements have yet been established to of a disruption of the ocean color data record: access data from OCM-2 for U.S. investigators. However, the 1) Aggressively pursue and document improvements to the U.S. and Indian space agencies have held productive discus- VIIRS sensor on NPP that enable it to meet the specifications sions to negotiate data exchange agreements. required for climate capable ocean color observatories; or Because OCM-2 is the only ocean color sensor in orbit 2) Implement a stand-alone, global ocean color mission. that has not exceeded its design life, negotiating data access (Siegel and Yoder, 2007) from the Indian Space Agency is the only option to avoid los- ing near-term access to ocean color data, if the older sensors The second option was not pursued. This study aims fail. Even if U.S. users acquire access to the data, at present to assess whether improvements to the VIIRS sensor and it is difficult to assess whether this sensor could produce mission that have been and continue to be made will allow climate-quality ocean color products. For example, the tilt VIIRS data to meet the requirements for climate-quality of OCM-2 is currently optimized only for the Indian Ocean; ocean color data. however, OCM-2 views the moon and the sun to assess sen- As this report was being written, VIIRS/NPP was suc- sor stability. Before it can be determined whether OCM-2 cessfully integrated to the spacecraft; it now awaits launch. products will be of equivalent high-quality to SeaWiFS After integration with the spacecraft, NIST conducted full products, access to data, including characterization and cali- system tests of VIIRS radiometric performance. These tests bration data, needs to be worked out. In addition, it is unclear included evaluation of relative spectral response (RSR), at this point if the mission’s operations can include global polarization sensitivity, and stray light characterization. coverage. For OCM-2 to meet the requirements of a vicari- These tests quantified VIIRS’ non-compliance of integrated ous calibration, routine reprocessing and stability monitoring out-of-band (OOB) response and cross-talk among relevant need to be implemented. Therefore, it is not yet possible ocean color spectral bands. Dynamic and static electrical to determine whether OCM-2 can provide climate-quality global data. Finally, there is no plan to routinely access 1 Optical cross-talk is scattering of light from one band to another, caused OCM-2 data, although NOAA has successfully negotiated by defects in the manufacturing of the VIIRS Integrated Filter Assembly access to vector wind data from Oceansat-2. (IFA).
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52 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS and optical cross-talk was observed. Quantitative analysis bias in the calibrated [top of atmosphere] TOA radiances, then use direct OOB corrections of the water-leaving radi- of optical cross-talk is in progress. In parallel, Northrop ances to remove residual OOB biases. Grumman Aerospace Systems (NGAS) proposed a data- (Presentation to NRC by J. Gleason, November 2010) processing correction method that is in government peer review. It is our expectation that the optical cross-talk that is Although the committee is pleased that JPSS plans a a result of defects in the VIIRS Integrated Filter Assembly vicarious calibration program to address ocean color issues (IFA) will be corrected for the second VIIRS sensor. The on NPP VIIRS, it is critical that plans for vicarious calibra- hardware issue will remain in the VIIRS/NPP. The VIIRS/ tion, as well as plans for routine reprocessing and stability NPP optical cross-talk and OOB shortfalls compared with monitoring, are in fact implemented. the VIIRS specification affect the performance of ocean color As concluded in the previous chapter, a vicarious cali- and aerosol observations. bration is critical to overcoming some of the sensor’s short- Otherwise, the pre-launch VIIRS/NPP tests conducted comings and to ensure accuracy requirements for Lw are met. by NIST indicate that the sensor meets SNR, dynamic range, Currently, MOBY (or a MOBY-like approach) is the only linearity, uncertainty, stability, and polarization specifica- proven and operational approach to undertake such a vicari- tions (Turpie, 2010). Tests detected minor variances for ous calibration. However, at the time that this report was gain transition, but gain transition points are well charac- completed, funding allocations for a MOBY-like vicarious terized. Tests also identified potential “striping.” Striping, calibration program were insufficient. The committee was evident in MODIS Level 1a and 2 imagery, is typically not aware of any definitive plans to conduct a MOBY-like seen in dark ocean scenes where small errors in detector effort for VIIRS/NPP in the near term. gain and offset correction cause brightness variations from The committee heard arguments that, due to time pres- one detector to the next over a cross-track scan. Plans for sure in delivering ocean color to the operational community post-launch striping correction similar to those applied to quickly and the limited number of match-ups during the first Landsat and MODIS data are in place if needed. Overall, year, the project office may rely more heavily on SeaWiFS2 VIIRS/NPP meets requirements for noise-equivalent radi- and MODIS data during the first year to set the gain factor. ance, dynamic range, gain transition, linearity, uniformity, It is true that during the first year obtaining enough absolute radiometric difference, and stability. Therefore, match-ups to set the gain may be difficult (vicarious cali- the VIIRS environmental data records (EDRs) are expected bration for SeaWiFS over 13 years yielded 160 match-ups). to meet Integrated Operational Requirements Document Nonetheless, it remains imperative that MOBY be main- thresholds, with the possible exception of ocean color and tained until an alternative proven approach has been tested aerosol optical depth. and deployed. A MOBY or a MOBY-like approach needs to be maintained continuously to develop the required dataset Conclusion: Because of the VIIRS/NPP issues described of match-ups. Indeed, because it takes many years and many above, the committee expects that deriving high-quality vicarious calibration points to get down to 0.3 percent abso- ocean color products from VIIRS/NPP is possible but will lute accuracy, the vicarious calibration effort will yield high- be challenging. The importance of vicarious calibration, quality products sooner if it begins immediately after launch. stability monitoring, and a vigorous calibration/validation effort cannot be overstated and are discussed in more detail Conclusion: Without a MOBY-like approach to vicari- in Chapter 3. ous calibration, the accuracy requirements of the climate research community cannot be met. In November 2010, the NPP project scientist told the committee: There is an overwhelming probability that a disruption in funding will result in the disbanding of the group with the technical and institutional memory to operate a system such We are optimistic that the [NPP] VIIRS instrument may as MOBY. To date, an appropriate alternative approach to still be a viable ocean color instrument, provided that conduct a vicarious calibration has not been demonstrated. the calibration and validation infrastructure of heritage Because NOAA is responsible for the calibration of VIIRS NASA EOS missions is in place. This infrastructure includes a plan and support for vicarious calibration site(s), a data/ and for maintaining climate data records, NOAA is also validation program, on-orbit calibration maneuvers, regular responsible for maintaining the capability to conduct a vicar- mission-level data reprocessing, and the use of NASA select- ious calibration. Funding to maintain the proven vicarious ed operational algorithms. The VIIRS on-orbit performance, calibration approach seems to be easily justified, consider- due to the OOB calibration biases alone, should be no worse ing the importance of the vicarious calibration to the overall than SeaWiFS. If the VIIRS OOB calibration biases are not success of the VIIRS/NPP mission, and the small cost of the adversely complicated by the cross-talk, the heritage OOB program compared to the overall mission cost. mitigation approaches that were developed for SeaWiFS and MODIS-Aqua should work for VIIRS. These approaches use vicarious calibration as the primary correction for the OOB 2 These comments were made prior to the failure of SeaWiFS.
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53 CURRENT AND PLANNED OCEAN COLOR SENSOR MISSIONS Conclusion: Based on experience with SeaWiFS and the requirement. To monitor the sensor degradation, a MODIS, a MOBY-like approach for a vicarious calibration 15-degree roll maneuver of the spacecraft eight to nine is the proven method to meet the accuracy requirements times a year would be sufficient to meet the requirement of for climate-quality data. Because of a funding shortage, obtaining monthly lunar looks. VIIRS/NPP may use MODIS for the vicarious calibration during the first year, primarily for operational applications. Recommendation: JPSS should conduct spacecraft maneu- However, this approach does not meet the accuracy require- vers to collect monthly lunar looks for VIIRS/NPP. ments listed in the previous chapter. The Sensor Intercomparison and Merger for Biological Conclusion: Based on NIST’s most recent and thorough and Interdisciplinary Oceanic Studies (SIMBIOS) program instrument characterization, the VIIRS sensor on NPP has contributed to the success of the SeaWiFS and MODIS continues to have problems with the filter cross-talk and missions. The SIMBIOS program led the effort to validate out-of-band response. The approach with the best chance the ocean color products. Besides calibrating the sensor with for obtaining ocean color CDRs from VIIRS/NPP is to solar and lunar views and MOBY match-ups, it collected and i mplement a vicarious calibration program based on archived a global dataset of in situ data to ground-truth the MOBY match-ups and to monitor sensor stability with a satellite products. In addition, the program was very success- monthly lunar look. ful in developing working relationships with the international community and foreign space agencies (McClain, 2010). Conclusion: If it is NOAA’s goal to produce climate-quality Although plans presented to the committee for calibra- ocean color data from VIIRS/NPP, NOAA funding for the tion and validation included all necessary elements, the fund- vicarious calibration needs to be sufficient to support the ing to support these efforts for VIIRS/NPP was not available. current level of MOBY operations and the development and In addition, the plans were relatively limited in scope and deployment of a replacement unit. lacked details to ensure a successful implementation. They appeared limited to U.S. coastal waters. Considering the fast- VIIRS is patterned after MODIS, with an improved solar approaching launch date of the VIIRS sensor, the committee diffuser design based on lessons learned from MODIS-Terra. concludes that a high level of uncertainty remains regarding In addition to providing deep space and lunar views available the availability of a high-quality calibration and validation roughly quarterly without spacecraft maneuvers, VIIRS also program such as the SIMBIOS program. contains the MODIS-derived solar diffuser stability monitor. The committee is most concerned about the current However, the experience with MERIS demonstrates that the uncertainty regarding the timing and level of NOAA’s stability of the solar diffuser needs to be monitored, and this financial support for a MOBY-type vicarious calibration ability depends on how well the stability monitor on VIIRS program (DiGiacomo and Guenther, personal communica- will perform. The SeaWiFS and MODIS experiences indicate tion), as well as NOAA’s apparent lack of commitment and that monthly lunar looks will be required despite the solar capability to reprocess the VIIRS data (NOAA, 2010). Both diffuser and stability monitor, because instrument degrada- are absolutely essential if VIIRS is to produce climate- and tion is not always predictable. Furthermore, vicarious cali- science-quality data. These concerns were echoed by all bration cannot be used to determine the rate of degradation of workshop and meeting participants throughout the study the atmospheric correction bands, because the water-leaving period and had not been resolved at the time this report was radiance signal is so low at those wavelengths. Thus, the only concluded (sixth months prior to launch date). option for VIIRS to monitor degradation in those bands is NOAA plans to process, archive, and distribute VIIRS with a lunar look. A presentation to the committee by Fred data. However, based on its report to the committee (NOAA, Patt stated VIIRS will image the moon three to four times a 2010), NOAA does not have (nor has it demonstrated) the year with no maneuvering of the spacecraft. This does not technical and infrastructure capabilities to do end-to-end meet the requirement of monthly sampling frequency. Patt processing and reprocessing of ocean color data. also stated that small roll maneuvers (fewer than 15 degrees) Moreover, reprocessing of the data is not included in are required to acquire eight to nine additional lunar calibra - the data management plans contemplated for VIIRS/NPP. tion views per year (Patt VIIRS ATBD document). If imple- Reprocessing of data is required for the development of mented these roll maneuvers would increase the number of climate-quality data records. In addition, NOAA has not lunar views to the required monthly frequency. At the time yet developed the mechanisms to engage experts in the this report was prepared, no final decision had been made academic and international community to provide feedback whether to implement roll maneuvers. However, the original and revise algorithms and methods for product development. instructions were to not implement roll maneuvers. NOAA has recognized this deficiency and is attempting to develop the in-house capacity for end-to-end data processing Conclusion: VIIRS/NPP is currently scheduled to collect (NOAA, 2010). However, the current management structure four lunar looks per year, which is insufficient to meet of JPSS (separate from NOAA/National Environmental
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54 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS Recommendation: If VIIRS/NPP is to continue to provide Satellite, Data, and Information Service [NESDIS] presents SeaWiFS/MODIS-quality ocean color data, NOAA should an additional challenge to NOAA to implement the agency’s immediately implement the following: own recommendations (NOAA, 2010). The NASA Ocean Color Group has the expertise and • spacecraft maneuvers throughout the life of the resources to do end-to-end processing of VIIRS data. NASA mission to provide monthly lunar looks to quantify sensor is capable of developing a processing system by launch, and stability; VIIRS ocean color products would be processed and avail- • fund MOBY and a new MOBY-like program to able through mechanisms that are familiar to the research replace the aging MOBY; and and operational community. Further, as of today, no module • a capability equivalent to the NASA Ocean Color exists to provide access to Level 3 data from VIIRS/NPP. Group to process, reprocess, and distribute VIIRS data in The NASA Ocean Color Group has built such modules for a manner consistent with the heritage missions (CZCS, SeaWiFS, MODIS, and MERIS. It has a code in place to bin SeaWiFS, and MODIS). VIIRS data to Level 3 and the expertise to make the module available for VIIRS/NPP data. To build its own in-house Conclusion: NOAA’s ocean color mission would benefit capacity for end-to-end processing, NOAA is well advised from engaging the NASA Ocean Color Group at Goddard to engage NASA to enable the knowledge transfer (NOAA, Space Flight Center to process, archive, distribute, and 2010). reprocess NPP/VIIRS data in the near term. Initially, this could be accomplished through subcontracting its services, Conclusion: NOAA is responsible for data management although this is not a long-term solution. of VIIRS ocean color products for the nation but has not yet demonstrated that it has the required expertise An option to ensure the availability of a capacity equiva- or infrastructure to successfully achieve this task. The lent to NASA’s Ocean Color Group is to contract with that NASA Ocean Color Group does not have the funding to group. Such a contract should include but not be restricted process, reprocess, and distribute VIIRS data, but has to these tasks: the unique expertise and infrastructure. Contracting the NASA Ocean Color Group to manage VIIRS/NPP data is • process, reprocess, distribute the data, and generate the only option for the foreseeable future to ensure high- new and improved products; quality management of VIIRS data. NOAA and NASA • work with the VIIRS calibration team to assess trends should work together to shift this capability to NOAA as in sensor performance and to evaluate anomalies; soon as possible, or they should develop a partnership for • implement a process to engage experts in the field of ocean color processing that serves the missions of both ocean color research to revisit standard algorithms and prod- agencies. ucts, including those for atmospheric correction, to ensure consistency with heritage instruments and for implementing Conclusion: As of now, the following requirements are not improvements; and met for VIIRS/NPP: • form a data product team to work closely with the • Stability monitoring; calibration and validation teams to implement vicarious • Data processing, reprocessing and distribution; and lunar calibrations, expand global validation efforts and • Vicarious Calibration program; provide oversight of reprocessing. • Global validation program throughout the life span of the mission; and Recommendation: NOAA should extend the validation • Algorithm development and research. program to cover the full range of global ocean conditions. Conclusion: The VIIRS sensor on NPP continues to ENSURING GLOBAL HIGH-QUALITY OCEAN have problems with the filter cross-talk and out-of-band COLOR DATA FOR THE NEXT FIVE TO TEN YEARS response. It remains to be seen whether optical cross-talk issues can be overcome on orbit via software corrections. While the most immediate need and highest priority is VIIRS on NPP has the potential to meet requirements only to ensure that the VIIRS/NPP mission is of highest quality if a vicarious calibration is undertaken and the sensor sta- possible, some near-term missions also hold great promise. bility is monitored with a monthly lunar viewing. The most promising are two foreign missions. The third mission we consider here is the second VIIRS sensor, to be launched as part of the JPSS1 mission.
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55 CURRENT AND PLANNED OCEAN COLOR SENSOR MISSIONS Sentinel Mission NPP should be applied to VIIRS/JPSS1 to ensure improve- ments are made. The VIIRS sensor on the JPSS1 mission ESA is developing an operational mission as part of its needs to include an improved filter array to avoid the cross- Global Monitoring for Environment and Security (GMES) talk problems associated with VIIRS on NPP. As discussed program. Because the Sentinel mission is operational, the in the previous section, many issues remain regarding the requirements for revisit, coverage, and mission life cycle overall mission planning and design, including stability are stringent. Two OLCIs are being built as part of this pro- monitoring, vicarious calibration based on a MOBY-like gram; the first OLCI (3A) is to be launched in April 2013. standard, pre-launch characterization, and data validation/ The design follows MERIS with a dual Spectralon solar dif- calibration and processing/reprocessing. fuser stability monitoring system and deep space looks. The Based on what we have learned to date, all recommenda- committee hopes this will include a vicarious calibration of tions for VIIRS on NPP need to be implemented for VIIRS/ sensor gains, as is being done now for MERIS. The OLCI JPSS1, in particular: comprises five independent narrow field-of-view cameras arranged in a fan configuration to offer a total 68.5-degree • Stability monitoring; field-of-view, tilted 12.5 degrees off-nadir to avoid sun glint. • Vicarious calibration based on a MOBY-like It will have 21 spectral channels compared to the 15 on approach; MERIS. Based on the quality of the MERIS mission and the • Pre-launch characterization of VIIRS/NPP applied to design features of these sensors, data likely will be of high all follow-on missions; and quality, especially because climate research is one of its main • Processing, reprocessing, and distribution (for details applications. This assumes that a vicarious calibration will be see Chapter 5). pursued as has been recently done for MERIS. However as with MERIS, questions regarding data access will need to be If the problems with the first VIIRS can be avoided resolved before Sentinel will fully satisfy the requirements during the JPSS phase, these follow-on missions can deliver of U.S. resource managers and scientists. climate-quality ocean color data. Conclusion: The Sentinel sensor and mission description are promising, but data access needs to be resolved. PACE and ACE ACE is one of NASA’s Decadal Survey missions and S-GLI/GCOM-C1 and GCOM-C2 would include an advanced ocean color capability to serve the research community. PACE was announced in 2010 as The Japanese Global Change Observation Mission part of NASA’s Climate Initiative and is an advanced ocean (GCOM) also plans to launch two ocean color sensors for color imager with requirements similar to those planned for climate monitoring purposes. As discussed, this goal is ACE. Very little information about PACE or ACE was avail- associated with stringent mission and sensor requirements. able to the committee. Based on a draft document provided To monitor the sensor’s stability, the SGLI Visible and by C.R. McClain (NASA’s GSFC), the ocean color radiom- Near-Infrared Radiometer (VNR) offers a Spectralon solar etry requirements for ACE (and presumably for PACE) are diffuser, internal light-emitting diodes (LED) sources, and given below. The requirements are stringent; if they are met, deep space views. With planned satellite maneuvers, lunar PACE and ACE would satisfy many of the ocean color needs looks also are anticipated. The sensor will have 19 wave- of the research community discussed in Chapter 5. bands (375 to 12,500 nm) and a spatial resolution of 250 m, The ocean radiometer requirements are outlined below. with the goal to improve coastal and aerosol observations. The first list provides general sensor performance and mis- Following an initial evaluation period, the data products are sion support requirements. Table 4.6 provides specific data likely to be openly available, as was the case for OCTS and on multispectral bands, bandwidths, typical clear sky TOA GLI, although it is not yet clear if near-real time access will radiances over the ocean, saturation radiances, and mini- be an option for U.S. coastal waters. mum SNRs (based on the analyses above). In Table 4.6, the SNR value at 350 nm is lower than in the other UV bands Conclusion: Based on the sensor and mission description because its application for detecting absorbing aerosols does and operation for S-GLI, and given past history with data not require a value of 1,000 nm. Also, the SNR at 678 nm access to OCTS, S-GLI could meet all requirements and be is set at 1,400 nm based on analysis of MODIS retrievals an excellent Type 1 and 2 sensor. (the bio-optical sensitivity analyses above did not include fluorescence line height). In the wavelength domain of VIIRS/JPSS1 and JPSS2 345-755 nm, multispectral bands are aggregations of 5-nm hyperspectral bands. Because VIIRS/NPP was designed as the preparatory Below are general requirements for ocean radiometer mission for the operational program, lessons from VIIRS/ and mission support:
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56 SUSTAINED OCEAN COLOR RESEARCH AND OPERATIONS Radiometer Spectral Attributes • 0.1 percent radiometric stability (1-month pre-launch verification) • 26 multispectral bands (Table 4.6) including: o 10-nm fluorescence bands (667-, 678-, 710-, and Spatial Coverage 748-nm band centers) o 10- to 40-nm bandwidth aerosol correction bands • Two-day global coverage (58.3-degree cross-track at 748, 765, 865, 1,245, 1,640, and 2,135 nm scanning) o 820-nm band for estimation of column water • 1-km resolution at center of swath vapor concentration o 350-nm band for absorbing aerosol detection Other • 5 nm resolution 345 to 755 nm (functional group derivative analyses) • Sensor tilt (±20 degree) for sun-glint avoidance • Polarization: <1.0 percent sensor radiometric sensi- • Five-year minimum design lifetime tivity, 0.2 percent pre-launch characterization accuracy • Monthly lunar imaging at 7-degree phase angle • No saturation in multispectral bands through Earth-view sensor port Accuracy and Stability CONCLUSIONS • <2 percent pre-launch radiance calibration accuracy To date, MODIS-Aqua is the only sensor in orbit • On-orbit vicarious calibration accuracy to 0.2 percent that meets all requirements for sustaining climate-quality • 0.1 percent radiometric stability knowledge (mission water-leaving radiances and ocean color products for U.S. duration) scientists. MERIS data access is much improved, and as of March 2011, discussions are under way between NASA and ESA for a bulk data exchange to include MERIS Level 1B data. Experts at NASA/GSFC believe that Level 1B data is a realistic substitute for access to Level 0 data. Data access TABLE 4.6 OES Multispectral Band Centers, (as discussed below) is a potential issue with almost every Bandwidths, Typical TOA Clear Sky Ocean Radiances foreign sensor, especially with proprietary sensor design (Ltyp), Saturation Radiances (Lmax), and Minimum SNRs at information. Ltyp Until PACE is launched (currently planned for 2019), λ Δλ the VIIRS series on NPP and JPSS1 will be the only U.S. Ltyp Lmax SNR-spec ocean color sensors in orbit that are not beyond their design 350 15 7.46 35.6 300 life spans (e.g., MODIS-Aqua). If the appropriate steps are 360 15 7.22 37.6 1,000 385 15 6.11 38.1 1,000 not taken now to ensure that all requirements are met for a 412 15 7.86 60.2 1,000 successful mission, U.S. scientists will not have access to a 425 15 6.95 58.5 1,000 research/climate-quality dataset for ocean color from U.S. 443 15 7.02 66.4 1,000 sensors. In addition, U.S. resource managers, for example 460 15 6.83 72.4 1,000 those at NOAA’s NMFS and National Ocean Service (NOS) 475 15 6.19 72.2 1,000 490 15 5.31 68.6 1,000 and at state and local agencies, will not have access to opera- 510 15 4.58 66.3 1,000 tional products in near-real time. 532 15 3.92 65.1 1,000 555 15 3.39 64.3 1,000 Recommendation: To mitigate the risk of a data gap, NOAA 583 15 2.81 62.4 1,000 should ensure that VIIRS meets all requirements for a suc- 617 15 2.19 58.2 1,000 cessful mission, including: 640 10 1.90 56.4 1,000 655 15 1.67 53.5 1,000 • Stability monitoring; 665 10 1.60 53.6 1,000 • Vicarious calibration based on a MOBY-like 678 10 1.45 51.9 1,400 710 15 1.19 48.9 1,000 approach; 748 10 0.93 44.7 600 • Pre-launch characterization; 765 40 0.83 43.0 600 • Global validation program throughout the life span 820 15 0.59 39.3 600 of the mission; and 865 40 0.45 33.3 600 • Processing, reprocessing, and distribution of the 1,245 20 0.088 15.8 250 1,640 40 0.029 8.2 250 data. 2,135 50 0.008 2.2 100 Radiance units are mW/cm2 µm str.
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57 CURRENT AND PLANNED OCEAN COLOR SENSOR MISSIONS At the moment, two sensors (MODIS and MERIS) are the distribution of VIIRS Level 0 data that may contribute providing ocean color data at the same time. This redundancy to friction between national satellite projects and present a has served the climate research community well because barrier to full international cooperation for ocean color data it has enabled scientists to intercalibrate the sensors and processing. improve the reprocessing to ensure data continuity. However, Conclusion: Data access is a major issue that needs to be the sensors are beyond their anticipated life spans. SeaWiFS resolved before many of the sensors listed in Table 4.4 meet recently stopped delivering data and has been terminated. requirements. 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 NASA and NOAA have good relations with ESA and be the only new sensor in space before VIIRS on NPP is JAXA and a longstanding tradition of exchanging satellite launched. In addition, it is likely that OCM-2 and VIIRS/ data. Relations with ISRO for data exchange are evolving in NPP will be the only Type 1 and 2 ocean color missions in a positive way. orbit before Sentinel-3A is launched in 2013. Nevertheless, issues arise with all partners on the details of data access. For example, there are generally few if any Conclusion: Because OCM-2 and VIIRS could be the only restrictions related to the exchange of Level 3 data products, sensors in orbit until launch of Sentinel-3A in 2013, access once the mission teams have established confidence in the to data from OCM-2 is a high priority for U.S. scientists. quality of the products. However, Level 0 and Level 1 data The committee notes that neither NOAA nor NASA is present problems. Issues related to data volume, proprietary aggressively pursuing routine access to OCM-2 ocean software, ITAR restrictions (for VIIRS), etc. make it more color data for U.S. users, although preliminary discussion difficult for U.S. and foreign agencies to exchange complete is ongoing and an MOU is in place. Level 0 or Level 1 datasets. These issues also can impede the full exchange of information on calibration, characterization, The timeline of current and future ocean color sensors and processing details. When merging data from multiple shown in Figure 4.1 does not necessarily represent the current sensors, it is impossible to generate climate-quality data and future availability of ocean color data, because several products without full access to Level 0 and Level 1 datasets of the sensors have unusable and/or inaccessible data. For and without complete information on calibration, character- example, as a result of uncertainties and instabilities in the ization, and processing details. pre-launch and on-orbit characterization of MODIS-Terra, In addition, these data exchange issues can make it dif- these data have been largely unusable (Franz et al., 2008). ficult for U.S. ground stations to downlink raw data from The data from India’s OCM sensor has generally not been non-U.S. sensors for U.S. coastal waters. Without direct available to the international community (Wilson, 2011), downlink capability to U.S. ground stations, it is extremely and there also are serious issues with its calibration (Lyon, difficult, if not impossible, to generate true real-time prod- 2009). It is anticipated that data from OCM-2 will be more ucts for applications for the United States. accessible to the international community, but this remains to In recognition of these challenges, the international Committee on Earth Observation Satellites3 (CEOS) has be seen. However, the OCM-2 is primarily a regional-scale mission intended for the Indian fishing community, not as a formed several “virtual constellations,” including the Ocean global mission. Colour Radiometry Virtual Constellation (OCR-VC). Chap- Although Level 1-3 MERIS data are available to U.S. ter 5 discusses in greater detail how this virtual constellation scientists, access to Level 0 data remains an issue (Wilson, presents unique opportunities to overcome some of these 2011). While most data users only desire access to Level challenges. 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 sen- sors. 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 3 http://www.ceos.org/.