Joel McCorkel

Temporal, spectral, and spatial study of the automated vicarious calibration test site at Railroad Valley, Nevada

The Remote Sensing Group at the University of Arizona has developed an automated methodology and instrument suite to measure the surface reflectance of the vicarious calibration test site at Railroad Valley, Nevada. Surface reflectance is a critical variable used as one of the inputs into a radiative transfer code to predict the top-of-atmosphere radiance, and inexpensive and robust ground-viewing radiometers have been present at the site since 2004. The goal of the automated approach is to retain RSGs current 2-3% level of uncertainty while increasing the number of data sets collected throughout the year without the need for on-site personnel. A previous study was completed to determine if the number and positions of the four radiometers were adequate to spatially sample the 1-km2 large-footprint site at Railroad Valley. The preliminary study utilized one set of panchromatic data from Digital Globes QuickBird satellite. Results from this one day showed that the positions of the four ground-viewing radiometers adequately sample the site. The work presented here expands in a spectral and temporal sense by using high-spatial-resolution data from Ikonos, QuickBird, and Landsat-7 ETM+ to determine if the locations of the ground-viewing radiometers correctly sample the site. The multispectral capability of these sensors is used to establish if there are any spectral effects, which will also help RSG to determine what spectral bands should be chosen for the new ground-viewing radiometers that are currently in development for the automated test site at Railroad Valley.

Radiometric calibration of Advanced Land Imager using reflectance-based results between 2001 and 2005

The Landsat series of sensors have supplied the remote sensing community with a continuous data set dating to the early 1970s. An important aspect of retaining the continuity of these data is that a Landsat follow-on as well as current Landsat instruments must be understood radiometrically throughout their mission. The Advanced Land Imager (ALI), for example, was developed as a prototype for the next generation of Landsat Instruments, and as such there was a significant effort to understand its radiometric characteristics as well as how it compares with previous Landsat sensors. The Remote Sensing Group at the University of Arizona has been part of this effort since the late 2000 launch of ALI through the use of the reflectance-based method of vicarious calibration. The reflectance-based approach consists of ground-based measurements of atmospheric conditions and surface reflectance at the time of satellite overpass to predict the at-sensor radiance seen by the sensor under study. The work compares results from the reflectance-based approach obtained from well-characterized test sites such as Railroad Valley Playa in Nevada and Ivanpah Playa in California as applied to ALI, Landsat-5 TM, and Landsat-7 EMT+. The results from the comparison use a total of 14 ALI dates spanning in time from 2001 to late 2005 and show that ALI agrees with the current radiometric results from TM and ETM+ to within 5%.

Vicarious calibration of the ASTER SWIR sensor including crosstalk correction

The Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) sensor on the Terra spacecraft has been providing remote sensing data for the past five years. ASTER has three separate sensor sections including a sensor with six bands in the shortwave infrared section of the spectrum. The radiometric calibration of the SWIR sensor has been updated from preflight values based on the on-board calibration sources. The SWIR sensor shows evidence of crosstalk between SWIR bands which is probably optical in origin. The crosstalk was present during preflight calibration and is present in all data collected in-flight including calibration data. The effects of crosstalk can be partially removed by applying a crosstalk correction program. This correction changes the calibration of the system. In this paper we apply a vicarious calibration to crosstalk corrected ASTER imagery over high reflectance desert test sites using a reflectance based method. The updated calibration provides for better retrieval of spectral reflectance or radiance of ground targets in ASTER SWIR imagery.

Radiometric characterization of hyperspectral imagers using multispectral sensors

The Remote Sensing Group (RSG) at the University of Arizona has a long history of using ground-based test sites for the calibration of airborne and satellite based sensors. Often, ground-truth measurements at these tests sites are not always successful due to weather and funding availability. Therefore, RSG has also employed automated ground instrument approaches and cross-calibration methods to verify the radiometric calibration of a sensor. The goal in the cross-calibration method is to transfer the calibration of a well-known sensor to that of a different sensor. This work studies the feasibility of determining the radiometric calibration of a hyperspectral imager using multispectral imagery. The work relies on the Moderate Resolution Imaging Spectroradiometer (MODIS) as a reference for the hyperspectral sensor Hyperion. Test sites used for comparisons are Railroad Valley in Nevada and a portion of the Libyan Desert in North Africa. Hyperion bands are compared to MODIS by band averaging Hyperions high spectral resolution data with the relative spectral response of MODIS. The results compare cross-calibration scenarios that differ in image acquisition coincidence, test site used for the calibration, and reference sensor. Cross-calibration results are presented that show agreement between the use of coincident and non-coincident image pairs within 2% in most bands as well as similar agreement between results that employ the different MODIS sensors as a reference.

Transmittance measurement of a heliostat facility used in the preflight radiometric calibration of Earth-observing sensors

Ball Aerospace and Technologies Corporation in Boulder, Colorado, has developed a heliostat facility that will be used to determine the preflight radiometric calibration of Earth-observing sensors that operate in the solar-reflective regime. While automatically tracking the Sun, the heliostat directs the solar beam inside a thermal vacuum chamber, where the sensor under test resides. The main advantage to using the Sun as the illumination source for preflight radiometric calibration is because it will also be the source of illumination when the sensor is in flight. This minimizes errors in the pre- and post-launch calibration due to spectral mismatches. It also allows the instrument under test to operate at irradiance values similar to those on orbit. The Remote Sensing Group at the University of Arizona measured the transmittance of the heliostat facility using three methods, the first of which is a relative measurement made using a hyperspectral portable spectroradiometer and well-calibrated reference panel. The second method is also a relative measurement, and uses a 12-channel automated solar radiometer. The final method is an absolute measurement using a hyperspectral spectroradiometer and reference panel combination, where the spectroradiometer is calibrated on site using a solar-radiation-based calibration.

Intercomparison of Imaging Sensors using Automated Ground Measurements

The reflectance-based method is a vicarious approach providing absolute radiometric calibration. A desire to increase the number of possible reflectance-based calibrations led the University of Arizona Remote Sensing Group (RSG) to deploy multispectral, downlooking radiometers at the RSGs Railroad Valley test site in Nevada. The radiometers are coupled with data from a sun photometer to provide the information needed for reflectance-based calibration without the need for on-site personnel. Results from these radiometers show similar uncertainties as on-site methods, and early results have led to their use with geostationary sensors, for derivation of surface bi-directional reflectance effects, and for comparisons of biases between sensors. The results show that radiometers with a single view angle are sufficient to characterize BRDF effects for Railroad Valley Playa. The results also give confidence in the automated approach as a means for cross-calibration relative to the vicarious results providing similar intercomparison results as with on-site personnel.

RETRIEVAL OF SURFACE BRDF FOR REFLECTANCE-BASED CALIBRATION

The University of Arizona has recently deployed a set of automated, downlooking radiometers to retrieve surface reflectance of the Railroad Valley test site in Nevada. Results from these radiometers have been combined with atmospheric data from the same site to provide a reflectance-based, vicarious calibration of multiple sensors. The accuracy of the calibrations is similar to those obtained from on-site personnel. Past work has emphasized near-nadir views by the satellite sensors under study to match more closely the view geometry of the automated radiometers to minimize the effect of bi-directional effects in the surface reflectance. Extension to off-nadir views requires an accurate understanding of the surface BRDF. Surface bi-directional reflectance effects have always played a key role in the accuracy of the vicarious calibration of imaging sensors. Such effects are especially important for the large, off-nadir views of sensors such as AVHRR and MODIS. The current work presents a method for retrieving the BRDF using the nadir-viewing data from the automated radiometers throughout the day. The concept of reciprocity is used to derive the reflectance as a function of view angle based on the measurements as a function of solar zenith angle. Comparisons of the results from this approach are compared to MODIS-derived BRDF data as well as ground-based measurements.