Supplementary material to “Achieving Satellite Instrument Calibration for Climate Change”

George Ohring, NOAA National Environmental Satellite, Data, and Information Service (NESDIS), Camp Springs, Md., E-mail: George.Ohring@noaa.gov; Joe Tansock, Utah State University, Logan; William Emery, University of Colorado, Boulder; James Butler, NASA Goddard Space Flight Center, Greenbelt, Md.; Lawrence Flynn and Fuzhong Weng, NOAA NESDIS, Camp Springs, Md.; Karen St. Germain, National Polar-Orbiting Environmental Satellite System Integrated Program Office, Silver Spring, Md.; Bruce Wielicki, NASA Langley Research Center, Hampton, Va.; Changyong Cao and Mitchell Goldberg, NOAA NESDIS, Camp Springs, Md.; Jack Xiong, NASA Goddard Space Flight Center, Greenbelt, Md.; Gerald Fraser, National Institute of Standards and Technology (NIST), Gaithersburg, Md.; David Kunkee, Aerospace Corporation, Silver Spring, Md.; David Winker, NASA Langley Research Center, Hampton, Va.; Laury Miller, NOAA NESDIS, Camp Springs, Md.; Stephen Ungar, NASA Goddard Space Flight Center, Greenbelt, Md.; David Tobin, University of Wisconsin, Madison; James G. Anderson, Harvard University, Cambridge, Mass.; David Pollock, University of Alabama, Huntsville; Scott Shipley, Raytheon, Lanham, Md.; Alan Thurgood, Utah State University, Logan; Greg Kopp, University of Colorado, Boulder; Philip Ardanuy, Raytheon, Lanham, Md.; and Tom Stone, U.S. Geological Survey, Flagstaff, Ariz.

Citation:
Ohring, G., J. Tansock, W. Emery, J. Butler, L, Flynn, F. Weng, K. St. Germain, B. Wielicki, C. Cao, M. Goldberg, J. Xiong, G. Fraser, D. Kunkee, D. Winker, L. Miller, S. Ungar, D. Tobin, J. G. Anderson, D. Pollock, S. Shipley, A. Thurgood, G. Kopp, P. Ardanuy, and T. Stone (2007), Achieving Satellite Instrument Calibration for Climate Change, Eos Trans. AGU, 88(11), 136. [Full Article (pdf)]

The Workshop on Achieving Satellite Instrument Calibration for Climate Change (ASIC3) was held at the National Conference Center, Lansdowne, Va., 16–18 May 2006. The major objective of the workshop was to formulate a national road map for developing calibration systems that will enable us to monitor long-term global climate change. The workshop was sponsored by the National Oceanic and Atmospheric Administration (NOAA), NASA, the National Institute of Standards and Technology (NIST), the Integrated Program Office for the National Polar-orbiting Operational Environmental Satellite System (NPOESS), and the Space Dynamics Laboratory of Utah State University.

The workshop established the following goal: Develop a satellite-based climate monitoring system that will ensure global, long-term climate records of high accuracy, tested for systematic errors on-orbit, and tied to irrefutable international standards such as those maintained in the U.S. by the National Institute of Standards and Technology.

Two overarching recommendations emerged from the workshop:

  1. Conduct satellite benchmark missions to create indisputable climate records and calibrate other satellite sensors and
  2. Establish a National Center for Calibration (NCC).

Motivation

Climate change is probably today’s most compelling issue since it is the single issue that will significantly affect all of humanity. For the most part, satellite observations of climate are not presently sufficiently accurate to establish a climate record that is indisputable and hence capable of determining whether and at what rate the climate is changing, as well as to establish a baseline for testing long-term trend predictions of climate models. Satellite observations do provide a clear picture of the relatively large signals associated with interannual climate variations such as El Niño–Southern Oscillation (ENSO), and they have also been used to diagnose gross inadequacies of climate models, such as their cloud generation schemes. However, satellite contributions to measuring long-term change have been limited and, at times, controversial, as in the case of differing atmospheric temperature trends derived from NOAA’s microwave radiometers.

Measuring long-term global climate change from space must be addressed from the fundamental physics of metrology, as practiced at the National Institute of Standards and Technology (NIST). The climate signals we are trying to detect are small: e.g., temperature trends of only a few tenths of a degree Celsius per decade, ozone changes as little as 1% per decade, and variations in the Sun’s output as tiny as 0.1% per decade or less. Current satellite systems are not up to the task. Sensors and onboard calibration sources degrade in orbit, long-term data sets must be stitched together from a series of overlapping satellite observations, orbital drift introduces artifacts into long-term time series, and insufficient attention is paid to meeting the high-accuracy, high-stability instrument requirements for monitoring global climate change.

While global climate models predict gradual climate change over the next century, the possibility of more abrupt climate change—large changes on decadal timescales—cannot be ruled out [National Research Council, 2002]. Early warning of such changes is also dependent on highly accurate observations. Until such capabilities are achieved we must rely on satellite instruments whose stability on-orbit is established as best we can: by checking the instrument against stable external sources—Moon, stars, stable Earth-based reference sites—or special aircraft under-flights and observational campaigns, and by ensuring sufficient satellite overlap periods to allow intercalibration of the records of a series of satellites. It is essential to realize, though, that the backbone of any long-term global climate record that can detect small trends associated with climate forcing and response must be built from high-accuracy observations that are traced on-orbit to international measurement standards.

Background

ASIC3 was a follow-up to a 2002 workshop [Ohring et al., 2004, 2005] that had developed the measurement requirements for a number of global climate variables. The 2002 workshop defined the accuracies (absolute) and long-term stabilities of global climate data sets that are needed to detect expected trends.

Organization

ASIC3 brought together some 100 participants, including experts in satellite instrument calibration, metrology scientists from the U.S. and U.K. national standards institutes, and remote sensing specialists. The workshop format consisted of plenary sessions with invited papers and breakout groups that reported to plenary sessions. Invited papers covered the following topics: Agency Roles, Review of Requirements for Measuring Global Climate Change, Calibration Status for Current Instruments and Plans for Future Instruments, and Concepts and Methodologies for Achieving Calibration of Global Climate Change Measurements.

Breakout groups were organized primarily according to the spectral regions used in space-based measurements, and they discussed current satellite instrument calibration capabilities, impediments to progress, and recommendations to accelerate progress. Breakout groups were formed on infrared instruments; ultraviolet, visible, and near-infrared instruments; microwave instruments; active instruments (such as radars and lidars); and broadband instruments (such as Earth radiation budget and total solar irradiance). Two additional breakout groups were created: A group on intercalibration of instruments focused on techniques for intercalibrating sensors on different satellites; and the national road map group prepared an outline of a road map to implement the recommendations of the workshop.

Recommendations

The workshop developed two overarching recommendations as well as a large number of technical recommendations on advancing the state of climate monitoring from satellite instruments.

Overarching recommendation 1:

Conduct a set of satellite benchmark missions to create irrefutable records and calibrate other satellite sensors. This is a new paradigm for achieving satellite instrument calibration for measuring long-term global climate change. The basic concept is to place in space a series of highly accurate benchmark instruments (see Goody [2001] for characteristics of climate benchmark measurements) to measure with high spectral resolution the energy reflected and emitted by the Earth. These instruments would not only provide reliable long-term records in their own right, but would also serve as a reference standard in space to calibrate other environmental satellite sensors. These spectral instruments would be joined in space by several other critical benchmark measurements.

Specifically, the workshop recommended the following with respect to benchmark measurements:

  • Initiate absolute spectrally resolved measurements of Earth’s emission spectrum
  • Initiate spectrally resolved measurements of Earth’s solar reflectance spectrum simultaneously calibrated/validated against multiple/redundant calibration targets (multiple diffusers, lamps, Moon, selected Earth targets)
  • Ensure continuity of global sea level measurements with overlap of altimeter missions
  • Ensure continuity of overlapped broadband Earth Radiation Budget measurements
  • Ensure continuity of overlapped total solar irradiance observations

The final three recommendations are especially critical in view of the fact that these instruments have been dropped from the NPOESS program. The NPOESS satellites themselves will be designed in such a way that if money is found elsewhere to pay for the sensors they could be placed on the satellites. NASA and NOAA are currently investigating mitigation strategies.

Overarching recommendation 2:

Establish a National Center for Calibration (NCC). It was recommended that the center be organized by NOAA, NASA, and NIST and be a distributed center; that is, the center’s program would be conducted at the partner agencies. Administrative headquarters would be established at one of the participating agencies, possibly NOAA. As demonstrated by the NASA-NOAA-DOD Joint Center for Satellite Data Assimilation, this kind of distributed joint center has been a very successful model for integrating federal activities that cross several agencies. A NOAA-NIST program to improve satellite instrument calibration, scheduled to begin in fiscal year 2009, will be an initial step toward formation of a national center. The program will bring NIST’s instrument calibration and standards capabilities and NOAA’s postlaunch instrument monitoring expertise to bear on improving satellite instrument calibration.

The mission of the center would be to advance the state of the art of satellite instrument calibration. Its activities would include carrying out the technical recommendations of the ASIC3 workshop, implementing the U.S. component of the World Meteorological Organization’s Global Satellite Inter-Calibration System (GSICS)—an evolving international program to intercalibrate instruments on different Earth-observing satellites—and championing satellite benchmark measurements for climate monitoring.

Benefits

The benefits of implementing the recommendations of the ASIC3 workshop would be:

Copies of the invited workshop presentations may be viewed at http://www.asic3.sdl.usu.edu/ with the username “guest” and password “asic32006”. A complete workshop report is being prepared and will be published by NOAA.

Acknowledgment

We thank Stephen Leroy for his critical review of this manuscript.

References