Surface emissivity in the thermal infrared (TIR) region is an important parameter for determining the land surface
temperature from remote sensing measurements. This work compares the emissivities measured by different field
methods (the Box method and the Temperature and Emissivity Separation, TES, algorithm) as well as emissivity data
from ASTER scenes and the spectra obtained from the ASTER Spectral Library. The study was performed with a field
radiometer having TIR bands with central wavelengths at 11.3 μm, 10.6 μm, 9.1 μm, 8.7 μm and 8.4 μm, similar to the
ASTER TIR bands. The measurements were made at two sites in southern New Mexico. The first was in the White
Sands National Monument, and the second was an open shrub land in the Jornada Experimental Range, in the northern
Chihuahuan Desert, New Mexico, USA. The measurements show that, in general, emissivities derived with the Box
method agree within 3% with those derived with the TES method for the spectral bands centered at 10.6 μm and 11.3
μm. However, the emissivities for the shorter wavelength bands are higher when derived with the Box method than those
with the TES algorithm (differences range from 2% to 7%). The field emissivities agree within 2% with the laboratory
spectrum for the 8-13 μm, 11.3 μm and 10.6 μm bands. However, the field and laboratory measurements in general
differ from 3% to 16% for the shorter wavelength bands, i.e., 9.1 μm, 8.6 μm and 8.4 μm. A good agreement between the
experimental measurements and the ASTER TIR emissivity data is observed for White Sands, especially over the 9 - 12
μm range (agreement within 4%). The study showed an emissivity increase up to 17% in the 8 to 9 μm range and an
increase of 8% in emissivity ratio of average channels (8.4 μm, 8.6 μm, 9.1 μm):(10.6 μm, 11.3 μm) for two gypsum
samples with different water content.
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) launched on NASA's Terra satellite in December 1999 provides anew tool for Earth observations. ASTER provides high-resolution, 15m(VNIR), 30m (SWIR) and 90m (TIR) coverage for limited areas with unique multispectal SWIR and TIR coverage and 15 m stereo coverage for DEM generation. These data have been used extensively for volcano and glacier monitoring. ASTER observations of over 1000 volcanoes around the world represent a significant increase in our ability to monitor volcanic activity and to map the products of eruptions. The SWIR channels have been used for mapping hot areas with temperatures up to 350 C and the multispectral TIR data have been used to map ash and SO<sub>2</sub> plumes. ASTER data are being used in the Global Land Ice Measurements from Space (GLIMS) project to map and catalog the approximately 80,000 glaciers. The objective is to acquire multiple observations to detect changes in ice margins and surface feature velocities. ASTER data acquired over the Jornada Experimental range in New Mexico have been used to extract spectral emissivities in the 8 to 12 micrometer range. These TIR data were also used in models to estimate the surface energy fluxes. Similar analysis of data acquired over the El Reno Oklahoma test site has shown that our satellite estimates of the surface fluxes agree reasonably well with ground measurements.
The Advanced Spaceborne Thermal Emission Reflectance Radiometer (ASTER) has acquired more than a dozen clear sky scenes over the Jornada Experimental Range in New Mexico since the launch of NASA's Terra satellite in December, 1999. To support the ASTER overpasses there were simultaneous field campaigns for the 5/09/00, 5/12/01, 9/17/01 and 5/15/02 scenes. Also, data from an airborne simulator, MASTER, were obtained for the 5/12/01 and 5/15/02 scenes to provide high resolution (3 m) data roughly coincident with ASTER. The Jornada Experimental Range is a long term ecological reserve (LTER) site located at the northern end of the Chihuahuan desert. The site is typical of a desert grassland where the main vegetation components are grass and shrubs. The White Sands National Monument is also within several of the scenes. ASTER has 5 channels in the 8 to 12 micrometer wave band with 90 meter resolution and thus is able to provide information on both the surface temperature and emissivity. The Temperature Emissivity Separation (TES) algorithm was used to extract emissivity values from the ASTER data for 5 sites on the Jornada and for the gypsum sand at White Sands. The results are in good agreement with values calculated from the lab spectra for gypsum and with each other. The results for sites in the Jornada show reasonable agreement with the lab results when the mixed pixel problem is taken into account. These results indicate ASTER and TES are working very well. The surface brightness temperatures from ASTER were in reasonable agreement with measurements made on the ground during the field campaigns.
Accurate, spatially distributed surface temperatures are required for modeling evapotranspiration (ET) over agricultural fields under wide ranging conditions, including stressed and unstressed vegetation. Modeling approaches that use surface temperature observations, however, have the burden of estimating surface emissivities. Emissivity estimation, the subject of much recent research, is facilitated by observations in multiple thermal infrared bands. But it is nevertheless a difficult task. Using observations from multiband thermal sensors, ASTER and MASTER, estimated surface emissivities and temperatures are retrieved in two different ways: the temperature emissivity separation approach (TES), and the normalized emissivity approach (NEM). Both rely upon empirical relationships, but the assumed relationships are different. TES relies upon a relationship between the minimum spectral emissivity and the range of observed emissivities. NEM relies upon an assumption that at least one thermal band has a predetermined emissivity (close to 1.0). Experiments comparing TES and NEM were performed using simulated observations from spectral library data, and with actual data from two different landscapes-- one in central Oklahoma, USA, and another in southern New Mexico, USA. The simulation results suggest that TES's empirical relationship is more realistic than NEM's assumed maximum emissivity, and therefore TES temperature estimates are more accurate than NEM estimates. But when using remote sensing data, TES estimates of maximum emissivities are lower than expected, thus causing overestimated temperatures. Work in progress will determine the significance of this overestimation by comparing ground level measurements against the remote sensing observations.
The recent availability of multi-band thermal infrared imagery from the Advanced Spaceborne Thermal Emission & Reflection radiometer (ASTER) on NASA's Terra satellite has made feasible the estimation of evapotranspiration at 90 meter resolution. One critical variable in evapotranspiration models is surface temperature. With ASTER the temperature can be reliably determined over a wide range of vegetative conditions. The requirements for accurate temperature measurement include minimization of atmospheric effects, correction for surface emissivity variations and sufficient resolution for the type of vegetative cover. When ASTER imagery are combined with meteorological observations, these requirements are usually met and result in surface temperatures accurate within 1-2 C. ASTER-based evapotranspiration estimates were made during September 2000 over a sub-humid regions at the USDA/ARS Grazinglands research laboratory near El Reno in central Oklahoma. Daily evapotranspiration was estimated by applying instantaneous ASTER surface temperatures, as well as ASTER-based vegetation indices from visible-near infrared bands, to a two-source energy flux model and combining the result with separately acquired hourly solar radiation data. The estimates of surface fluxes show reasonable agreement (within 50-100 W/m<SUP>2</SUP>) with ground-based Bowen Ratio Energy Balance measurements and illustrate how ASTER measurements can be applied to heterogeneous terrain. There are some significant discrepancies, however, and these may in part be due to difficulty quantifying fractional cover of senescent vegetation.
On several days in 2000 & 2001 the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) on the Terra satellite obtained data over the Jornada Experimental Range test site along the Rio Grande river and the White Sand National Monument in New Mexico. ASTER has 14 channels from the visible (VNIR) through the thermal infrared (TIR) with 15 m resolution in the VNIR and 90 m in the TIR. The overpass time is approximately 11 AM (MST). With 5 channels between 8 and 12 micrometers these multispectral TIR data from ASTER provide the opportunity to separate the temperature and emissivity effects observed in the thermal emission from the land surface. Ground measurements during these overflights included surface temperature, vegetation type and condition and limited surface emissivity measurements. Preliminary results indicate good agreement between ASTER emissivities and ground measures. Analysis of earlier aircraft data has shown that the multispectral TIR data are very effective for estimating both the surface temperature and emissivity. These results will be compared with those obtained from the ASTER data for this site. With multispectral thermal infrared observations provided by ASTER it is possible for the first time to estimate the spectral emissivity variation for these surfaces on a global basis at high spatial resolution.
Recovery of land surface temperature (LST) from remotely sensed data requires correction for atmospheric effects and decoupling surface temperature and emissivity. In this study, we have applied the Temperature Emissivity Separation (TES) method to several flight lines of the Thermal Infrared Multispectral Scanner (TIMS) acquired as part of the HAPEX- Sahel experiment. Atmospheric correction of at-sensor radiances is done by means of nearly coincident radiosondes and the MODTRAN radiative transfer code. The sensitivity of the method to the atmospheric corrections has been checked by using different radiosonde data. Even for low altitude flights, ignorance of atmospheric correction can lead to large errors in the retrieved emissivities and temperatures. Errors depend on the surface type, but in all cases channel 1 and 6 of TIMS are the most affected. The TES method is based on an empirical relationship relating the maximum-minimum emissivity difference (or contrast) with the minimum value for the 6 TIMS channels. Residual atmospheric effects dictate the max-min difference, especially for flat targets (e.g. vegetation). Since channels 1 and 6 have shown a greater sensitivity to atmospheric effects, a modified version using only the 4 central channels has been proposed and applied to the TIMS scenes. Preliminary results suggest that this modified version yields better values for vegetation targets, with emissivities around 0.98 and very little spectral variation.
We present a simple, three-dimensional vegetation canopy thermal infrared exitance model for agricultural scenes. Computer graphics and ray-tracing techniques are used to estimate three-dimensional canopy view factors and scene shadows. The view factors are used to weight the individual contributions of soil and vegetation emission computed by steady-state energy budget formulations. We compare the three- dimensional model results to a one-dimensional formulation for an agricultural test site from the Hydrologic Atmospheric Pilot Experiment and Modelisation du Bilan Hydrique. The root mean square error is daylight brightness temperature for the one dimensional model was 2.5 degrees Celsius and 2.0 degrees Celsius for the three dimensional model.
This paper presents a review of how data from the advanced spaceborne thermal emission radiometer (ASTER) can be used to estimate the energy fluxes from the land surface. The basic concepts of the energy balance at the land surface are presented along with an example of how remotely sensed surface brightness temperatures can be used to estimate the sensible heat. The example is from the Monsoon 90 experiment conducted over an arid watershed in the state of Arizona in the United States.
Water stored in the soil serves as the reservoir for the evapotranspiration process, thus the interest in trying to map its spatial and temporal variations in experiments studying the soil- plant-atmosphere interactions at the GCM grid scale. During the 8 week intensive observation period (IOP) of HAPEX-Sahel (Hydrologic Atmospheric Pilot Experiment in the Sahel), this was done with two airborne microwave radiometer systems. The five frequency (5 to 90 GHz) PORTOS radiometer on the French ARAT aircraft and the single frequency (1.42 GHz) multibeam pushbroom microwave radiometer (PBMR) on the NASA C-130 were used. These aircraft measurements were supported by ground based observations at the central sites and, because of several rains during the IOP, covered a good range of soil wetness conditions that existed. The PBMR and the 5.05 GHz PORTOS channel in H polarization show a large dynamic range of TB on each day and between different days in response to variations in rainfall and drying conditions ranging from low TBs of 210 to 220 K for the wettest conditions to values of 280 to 290 K for the driest.
A major application for a 21 cm radiometer is the remote sensing of soil moisture which is possible because of the large contrast between the dielectric constant of dry soil (approximately equals 3.5) and that of liquid water ( approximately equals 80). One of the major problems with the utilization of long wavelength radiometers from satellite platforms has been the large antenna size required with its substantial mass. For example, at satellite altitudes an antenna size of at least 10 m is required to obtain resolutions in the 10-20 km range. The size requirement is fundamental but the mass can be reduced by using unfilled arrays or as will be described here a thinned array antenna. Such a system operating at L-Band ((lambda) equals 21 cm or 1.42 GHz) has been developed and tested from an aircraft platform. It is called ESTAR (Electronically Scanned Thinned Array Radiometer) and it uses linear (stick) antennas in the along-track direction and aperture synthesis between pairs of sticks separated by odd multiples on half wavelengths in the cross track direction. The approximate dimensions of the antenna are 1 meter by 1 meter. Results from an evaluation series of flights over a study watershed in Oklahoma indicate that such a system can provide useful soil moisture information.
The HAPEX-DBILHY program is studying the hydrological budget arid evapotranspiration
(ET) flux at the scale of the GG4 grid square i.e. 1O<sup>4</sup>km<sup>2</sup>. For the
program several surface and subsurface networks operated from mid 1985 to early
1987 to monitor soil moisture, surface energy budget, and surface meteorological
parameters. During the Special Observing Period (SOP) from 7 May to 15 July 1986,
there were additional measurements including detailed observations of atmospheric
fluxes at the surface and with two well instrumented aircraft: the NCPR King-Air
for eddy correlation flux measurements and the NTSA C-l30 for remote sensing
observations. Brief descriptions of measurement systems and some results from the
SOP will be presented here concentrating on the remote sensing of surface