Validation of Thermal Infrared Data and Products from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) over Land Simon J. Hook1, Fred J. Prata2 and Geoffrey Schladow3 MS 183-501, Jet Propulsion Laboratory, Pasadena, CA 91109 USA CSIRO Atmospheric Research, PMB 1 Aspendale, Vic. 3195 Australia University of California, Davis, Department of Civil and Environmental Engineering, 116 Everson Hall, Davis, CA 95616 USA Introduction In 1997 a proposal was submitted to the Satellite Remote Sensing Measurement Accuracy, Variability, and Validation Studies NASA Research Announcement entitled: "Validation of Thermal Infrared Data and Products from MODIS and ASTER over Land" The objective of the proposal was to validate the thermal infrared data and products acquired over land from the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) and the Moderate Resolution Imaging Spectroradiometer (MODIS) using a set of automated validation sites. The main advantage of this approach is the validation data are acquired automatically allowing validation whenever satellite data are acquired and monitoring of accuracy and precision of the satellite data and products over time. The proposal was accepted and four automated validation sites were identified throughout the world. Lake Tahoe was identified as one of the sites for several reasons: Large size. High elevation. Does not freeze in winter. Excellent logistical support. The large size permits validation of sensors with large footprints; for example ASTER has a footprint size of 90 m while MODIS has a footprint size of 1 km. The high elevation minimizes any errors associated with propagation of the ground measurements through the atmosphere for comparison with the satellite instrument measurements. The lack of freezing allows validation throughout the year and coupled with the excellent logistical support from the Tahoe Research Group has allowed us to install a comprehensive set of measuring devices allowing year-round validation of thermal infrared data from satellite as well as aircraft instruments. Measurements To characterize the radiative and bulk temperatures of Lake Tahoe, four rafts have been deployed arranged in a box centered on the Lake. Each raft has a radiometer and several temperature loggers (Figure 1). The data from the temperature probes and radiometer is stored on a solar powered data logger. The logger has cellular access and the data are downloaded to NASA/JPL automatically on a daily basis. Measurements of the radiative and bulk temperatures are made every 2.5 minutes.   Figure 1. Mk IV raft at L. Tahoe with double solar panels, radiometer and temperature loggers on thermistor chain.   In order to perform a comparison with the satellite data it is necessary to measure the bulk and radiative temperatures and also characterize the atmosphere between the surface and the satellite. This has necessitated the installation of additional atmospheric monitoring equipment that supplements the measurements already being made at the US Coast Guard station (Figure 2). The additional measurements include a Total Sky Imager (TSI) and Multi Filter Rotating Shadowband Radiometer (MFRSR). The TSI takes an image of the sky every couple of minutes allowing us to determine the amount of cloud present and the MFRSR provides information on the amount and types of particles (aerosols) in the air as well as the total column water vapor. Figure 2. A YankeeTotal Sky Imager (TSI) and Yankee Multi Filter Rotating Shadowband Radiometer (MFRSR) on new mount above gauging station.   Early Science Results The primary objective of this proposal is to validate the thermal infrared data and products from the MODIS and ASTER sensors on the Terra platform. The Terra platform was launched in December 1999 but, due to technical difficulties, did not reach orbit until several months later. As a result, data suitable for validation are only just becoming available. In the interim, the data from the site is being used to validate other satellite and airborne sensors. Below is an example of some results from validation work involving a spaceborne instrument: the Landsat Enhanced Thematic Mapper (ETM+). Landsat ETM+ was launched approximately 7 months prior to the launch of Terra and thus validation data are available over the annual skin temperature cycle at Lake Tahoe (~5 — 25 º C). The temperature data from Lake Tahoe were provided to F. Palluconi (Landsat Science Team member) and used by Palluconi to validate the thermal infrared channel of Landsat ETM+. The results are summarized in Figure 3 and indicate a substantial error of approximately 2.5-3 degrees in the calibration of the Landsat thermal channel 6. While this error is clearly serious, it appears to be constant over time and temperature (Figure 3). Therefore it should be possible to update the calibration constants at the data processing center so that subsequent datasets are better calibrated or provide a correction algorithm for users. It should be emphasized that while the error appears nearly constant over time and temperature at present, it could change due to some change in the instrument and therefore requires further monitoring.   Figure 3 In Situ Derived Kinetic Temperature and Kinetic Temperature Difference (Image derived minus In situ) versus Image Derived Kinetic Temperature. (Plot courtesy of F. Palluconi)   Future Plans Obviously future work is focused primarily on the validation of the ASTER and MODIS data. Work is also underway to add additional measurement capability at both the rafts and the US Coast Guard Station. A full meteorological station providing wind speed, wind direction, relative humidity and net radiation, that can be deployed on a floating platform, has been purchased and will be installed on one of the rafts in the fall of 2000. These data will be valuable not only in validating the satellite data but also in understanding the lake climatology. An upward looking radiometer has also been added to the US Coast Guard Station that provides information on changes in the temperature of the sky with angle. Recent Publications and Media Hook, S. J. 1998. In Flight Validation of Thermal Infrared Data over Land. European Symposium on Remote Sensing: SENSORS, SYSTEMS AND NEXT GENERATION SATELLITES IV, Barcelona, Spain, September 21-25. (Abstract). Hook, S. J., Myers, J. J., Thome, K. J., Fitzgerald, M. and A. B. Kahle, 1999. The MODIS/ASTER Airborne Simulator (MASTER) — A New Instrument for Earth Science Studies. Accepted Remote Sensing of Environment. Hook, S. J., Schladow, G., Abtahi, A. F. Prata and B. Richards. In-Flight Validation of Remotely Sensed Thermal Infrared Data for Hydrological Applications. American Geophyscial Union, San Francisco. (Abstract) Hook, S. J., G. Schladow, A. Abtahi, F. Prata, B. Richards and S. Palmarsson. Validation of ASTER and MODIS Thermal Infrared Products. Sixth International Conference Remote Sensing for Marine and Coastal Environments. Charleston, South Carolina, May 1-3 2000. (Abstract) Hook, S. J. 2000. MASTER — A New Instrument for Hyperspectral Analysis from the Visible to Thermal Infrared. 2nd EARSeL Workshop on Hyperspectral Imaging, Enschede, The Netherlands, July 11-13. (Abstract). Tahoe World January 13 2000. 1.3 billion satellite links Tahoe to NASA By Shannon Darling, Tahoe World Staff. Tahoe World March 10, 2000. NASA rafts await satellite data By Shannon Darling, Tahoe World Staff. HomePage