Terra: Instrument Characterization 2

On-orbit maneuvers of the Terra satellite for additional calibration and characterization
The Moon will be used by EOS and other remote sensing instruments as a common, stable, on-orbit radiance reference target. The United States Geological Survey (USGS) and Northern Arizona University (NAU) are making radiometric measurements of the Moon with the goal of producing a lunar radiance model that will draw upon lunar radiance measurements over a period of 4.5 years. Consequently, on-orbit measurements of the Moon by Terra and other remote sensing instruments can be directly compared to the output of the lunar radiance model. In order for Terra instruments to view the Moon at optimum lunar phase and geometry, a calibration attitude maneuver (CAM) of the Terra satellite is planned.

The accuracy and precision of Earth remote sensing data sets produced by different instruments are only achieved by the consistent use of common on-orbit calibration sources and measurement methodologies. Terra will perform an on-orbit, pitch-based CAM to enable the instruments to view deep space and the Moon. The deep space view provides CERES and ASTER an accurate determination of the DC offsets in their thermal bands; and it provides MODIS the opportunity to characterize the dependence of thermal infrared reflectance on the angle of incidence (AOI) of the scan mirror. Determination of the dependence of thermal infrared reflectance versus AOI is necessary to ensure that all of the MODIS thermal bands are within or near specification. These on-orbit characterizations result in significant improvements in the accuracy of the radiometric data products from these instruments.

The same pitch-based CAM also provides the Terra instruments the opportunity to view the Moon at a lunar phase of 22°. The Moon affords the ability to monitor radiometric responsivity and stability in the visible, near-infrared, and shortwave infrared wavelength regions. The appearance of the Moon as a bright target in a dark surround provides EOS instruments an on-orbit target for the determination of scattered light sensitivity, size of source effect, and Modulation Transfer Function (MTF).

[ER-2 over San Francisco]

NASA's Lockheed ER-2 formation flight of three high-altitude research aircraft over San Francisco, CA.

[Bell Helicopter]

Wallops Flight Facility's Bell Huey helicopter, carrying two side platforms with a POLDER instrument, a modular multi-band radiometer (MMR), an SE-90 spectrometer, infrared thermal (IRT) sensors, and video recording equipment. Pictured here, the helicopter is participating in the BOREAS campaign, near Prince Albert, Saskatchewan, Canada (NASA Ames Research Center).

Validating the calibration
The on-orbit calibration will be validated occasionally by cross-comparing aircraft observations with those of the EOS satellites. Additionally, targets on the Earth's surface and in the atmosphere that have known, stable, or measured physical properties will be remotely sensed by Terra sensors. Discrepancies between actual and expected measurements may suggest that the calibration has drifted. Known as vicarious calibration, these on-orbit techniques typically involve deployment of calibrated ground-based or airborne radiometers on or above a spectrally and spatially homogeneous target to make simultaneous measurements during periods of satellite instrument overpasses. The ground-based and airborne radiometric measurements are corrected for atmospheric effects to produce a top-of-the-atmosphere radiance that can be registered and compared to the measured satellite radiance. Participating vicarious calibration instruments are absolutely and relatively calibrated before, during, and after field campaigns using integrating spheres, diffuse reflectors with irradiance standards, and diffuse reflectors with solar illumination. Alternative techniques for vicarious calibration include the use of known and stable reflectance characteristics of molecular scattering in the atmosphere, and stable spectral properties of ocean glint and clouds in the visibleto-near-infrared parts of the spectrum.

Intercomparison with future EOS sensors
Cross-calibration of the Terra instruments with each other and with instruments on future EOS satellites is necessary for the production of a continuous 18-year Earth remote sensing data set. One method of cross-calibrating EOS instruments is to have the instruments view identical Earth targets at the same time to compare the resulting measured radiances to some agreed-upon reference radiance. This type of cross-calibration activity is planned for MODIS, MISR, and ASTER. Follow-on EOS missions will also use lunar observations, enabling Terra to be cross-calibrated with them as well.

The Marine Optical Buoy (MOBY) being deployed off the south-west coast of Lanai, HI. The primary purpose of the buoy is to measure visible and infrared solar radiation entering and emanating from the ocean. By monitoring variations in the reflected radiation, other quantities can be derived, such as the abundance of microscopic marine plants (phytoplankton). Under the direction of Dennis Clark of NOAA, MOBY data will be used to validate both MODIS and SeaWiFS ocean color products (Ed King, NOAA).