Earth Observation Magazine

SPECTROMETRY
CERES Experiment Provides NASA Valuable Data
Detailed information measures incoming solar radiation
By Robert Wheeler, Don R. Cahoon, and Gerald C. Purgold

To a casual observer, it may appear that NASA focuses solely on space exploration. While galactic treks, such as the recently launched Cassini mission to Saturn, invite copious media coverage, earthbound programs are of great importance. Often overshadowed, Earth observation can provide a basis for the assessment of global climate change. One research project - the Clouds and the Earth's Radiant Energy System (CERES) - promises to aid farmers, foresters, environmental scientists and others who may benefit from solar and thermal radiation mapping.
    CERES - a NASA Langley Research Center Initiative - is one component of the space agency's Earth Observing System (EOS) project. The largest element of NASA's Mission to Planet Earth Program, EOS is a long-term research effort to study Earth as a global system. EOS is part of an international program for studying the Earth from space using a multiple-instrument, multiple-satellite approach. The EOS project is critical for providing a scientific understanding of ongoing natural and human-induced global changes. EOS analysis will be used in the development of coherent global environmental policies and the improvement of quantitative measurements from space.
    The CERES experiment is one of the highest priority scientific satellite instruments being developed for EOS. CERES will measure solar-reflected and Earth-emitted radiation from the top of the atmosphere to the Earth's surface. It will also determine cloud properties including the amount, height, thickness, particle size, and phase of clouds using simultaneous measurements by other EOS instruments.
    Analyses of these data, building on the foundation laid by previous missions such as the Earth Radiation Budget Experiment (ERBE), will lead to a better understanding of the roles of clouds and the energy cycle in global climate change. The first of three planned CERES instruments was launched on the Tropical Rainfall Measuring Mission (TRMM) in November 1997. The second and third CERES instruments will be launched on the NASA EOS satellites starting in 1998 and extending over at least 15 years.
    The CERES instrument scans, or maps, the Earth, providing measurements of incoming solar radiation versus visible reflected and thermal emissions from the Earth, clouds, and atmosphere. In doing so, CERES generates data that provide a detailed characterization of the role clouds play as reflectors or absorbers of radiation. CERES succeeds ERBE, a project that began in the 1980s.
    Though clearly the most suitable means of providing global coverage - particularly since a limited number of ground measurement sites are available - CERES satellite scanners derive measurements at numerous viewing angles, solar zenith angles, and wavelengths for all seasons. Surface brightness is strongly influenced by all of these variables, especially viewing and solar zenith angles. For example, at higher solar zenith angles, the forward and backward scattering radiation peaks become more pronounced. Consequently, all satellite measurements require angular correction to ensure the accuracy of radiation measurements.
    Understanding the anisotropic characteristics of a surface is an important component of satellite observation validation. Anisotropy refers to the angular asymmetry of the radiance from a given surface. It is a function of the scene type, azimuthal and zenith viewing geometry, zenith angle of incident solar radiation, and the aerosol composition, concentration, and distribution of the intervening atmosphere.
    The CERES Airborne Radiometer Scanner (ARS) provides high angular and spectral resolution directional radiance measurements, fixed narrow band channels that approximate the response of the first 4 bands of the Landsat TM (Thematic Mapper), and surface radiometric temperatures from a versatile Bell UH-1 helicopter platform.
    The ARS utilizes internal GPS navigation and computer-aided surface target tracking systems. GPS navigation affords orientation at precise angles with respect to the sun and surface targets. Graphical GPS information is shown on a pilot's overhead display panel in the helicopter cockpit. Actual flight path information is continuously updated and plotted over the computed patterns. This allows the pilot to perform on-the-fly flight path adjustments.
    A major component of the ARS employed to help validate the angular corrections of CERES satellite data are two compact, portable spectroradiometers produced by Boulder, Colorado-based Analytical Spectral Devices, Inc. (ASD). The two FieldSpec FR spectroradiometers - one part of NASA's helicopter-mounted ARS, the other part of a ground-based Surface Radiation and Atmospheric Characterization System (SRACS) - provide high spectral resolution radiation measurements for a variety of conditions.
    The FieldSpec FR spectroradiometers obtain essential high spectral and angular-resolution Bi-directional Reflectance Distribution Function (BRDF) measurements of particular surfaces under cloud-free conditions. All of the angular dependent radiances are then combined to produce hemispherical flux measurements represented by anisotropic 3-D plots. These measurements provide highly accurate determinations of the anisotropy for a given scene and provide a source of input data for critical angular distribution models used to correct satellite radiance measurements.
    Accurate corrections require precise characterizations of the state of the atmosphere at the time of the BRDF measurements. Particulate aerosols, water vapor, ozone, and carbon dioxide absorb and scatter incident radiation. The ARS and SRACS instrumentation provide measurements within the boundary layer. These measurements are compared with satellite observations by accounting for the influences of the intervening atmosphere attributable to scattering and absorption.
    Coincident SRACS micropulse lidar measurements and radiosonde soundings provide atmospheric water vapor and aerosol observations and solar photometer data measurements, of constituent aerosol optical depths. Through accurate characterization of atmosphere state at the time of the ARS measurements intervening influences may be accounted for and removed so that BRDFs can be taken to the top of the atmosphere (TOA). These clear-sky TOA directional radiance measurements then become baseline assessments used to adjust computer models. A review of these assessments enables NASA to correct the satellite observations, and allows for a more precise detection and characterization of cloud radiation effects.
    The CERES ARS and SRACS permit larger scale measurements less sensitive to small-scale surface inhomogeneities than some available alternatives. The helicopter-based ARS takes BRDF measurements at an altitude of 1,000 feet AGL. NASA combines 5¡ field-of-view, 1-nanometer resolution spectroradiometer data from 350-2500 nm, narrow-band Landsat TM wavelengths of 456-522 nm, 524-595 nm, 629-687 nm and 762-897 nm, and pyrometer surface radiometric skin temperatures. Traveling at a speed of 50 knots, the ARS records data at 4 Hz intervals resulting in better than 1¡ viewing zenith angle resolution for 10 separate viewing azimuths.
    By exchanging the FieldSpec FR 5¡ foreoptic for the hemispheric Remote Cosine Receptor (RCR), the spectroradiometer and additional broadband hemispheric radiometers provide wide-area albedo measurements for a variety of scene types as defined by the International Geosphere Biosphere Programme (IGBP). Albedo measurements are expressed as a ratio of the reflected (or upwelling) hemispheric solar radiation reflected by the surface versus the amount of incident (or downwelling) hemispheric radiation. The higher the ratio, the brighter the surface. The IGBP characterizes scenes based on general homogeneous surface characteristics. Examples include agricultural crops, woody savannas, oceans, or coniferous forests. The accompanying IGBP chart indicates some of the scene types and their approximate distribution worldwide.
    The ARS BRDF flight profiles are composed of five, 2-kilometer legs at azimuths of 0¡, 25¡, 45¡, 90¡ and -35¡ with respect to the solar plane. Ten separate azimuths with respect to a given target can then be measured. As the ARS flies along each flight leg, the narrow beam instruments are locked onto the fixed target of interest using a GPS aided aiming system. Viewing zeniths for each leg are measured from +75¡ to -75¡. The entire five-leg pattern takes approximately 10 minutes to complete.
    The narrow-band FieldSpec FR units give the ARS the capability to resolve spectral radiances in 1-nm increments, resulting in more accurate results. Additionally, the high-resolution measurements from the FieldSpec FR data can be integrated with relative instrument response functions to provide simulated responses for a wide variety of space-borne or Earth-bound sensors. This minimizes the need to carry large volumes of equipment into the field to support field missions.
    The effect clouds have on the global radiation balance is still largely debatable and understanding this role is critical in our understanding of global climate change. Accurate measurements and analysis of cloud/radiation effects on the Earth's climate system are essential to improvements in weather forecasting, precision farming techniques and the formulation of coherent global environmental policies. Toward this end, comprehensive field studies utilizing high-resolution remote sensing devices provide important information leading to refinements in space-based monitoring of the Earth's surface and atmosphere.

About the Authors:
Robert J. Wheeler is a senior scientist for Analytical Services and Materials, Inc. Don R. Cahoon is a senior research scientist for the Atmospheric Science Division, NASA Langley Research Center. Gerald C. Purgold is the CERES ARS project engineer, Atmospheric Science Division, NASA Langley Research Center.

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