Vicarious techniques are used to provide supplemental radiometric calibration data for sensors with onboard calibration
systems, and are increasingly important for sensors without onboard calibration systems. The Radiometric Calibration
Test Site (RadCaTS) is located at Railroad Valley, Nevada. It is a facility that was developed with the goal of increasing
the amount of ground-based radiometric calibration data that are collected annually while maintaining the current level
of radiometric accuracy produced by traditional manned field campaigns. RadCaTS is based on the reflectance-based
approach, and currently consists of a Cimel sun photometer to measure the atmosphere, a weather station to monitor
meteorological conditions, and ground-viewing radiometers (GVRs) that are used the determine the surface reflectance
throughout the 1 × 1-km area. The data from these instruments are used in MODTRAN5 to determine the at-sensor
spectral radiance at the time of overpass.
This work describes the RadCaTS concept, the instruments used to obtain the data, and the processing method used to
determine the surface reflectance and top-of-atmosphere spectral radiance. A discussion on the design and calibration of
three new eight-channel GVRs is introduced, and the surface reflectance retrievals are compared to in situ
measurements. Radiometric calibration results determined using RadCaTS are compared to Landsat 7 ETM+, MODIS,
and MISR.
The Remote Sensing Group (RSG) at the University of Arizona is currently refining an automated system for the
absolute radiometric calibration of earth-observing sensors. The Radiometric Calibration Test Site (RadCaTS) relies on
semi-permanent instrumentation at the Railroad Valley (RRV) test site to collect data from which surface reflectance and
an atmospheric characterization is determined. Multispectral surface reflectance is determined from calibrated ground
viewing radiometers and assimilated to determine the hyperspectral reflectance used in radiative transfer calculations.
The reflectance retrieval algorithm relies on an accurate determination of the diffuse sky irradiance for the time of
interest. Currently, diffuse sky irradiance is modeled using the atmospheric characterization as input into MODTRAN5.
This work investigates the accuracy of the diffuse sky modeling by comparing modeled results to measurements made at
the test site. Diffuse sky irradiance from several alternative methods are also presented. Surface reflectance is computed
and compared to in-situ measurements taken with a portable spectoradiometer.
The Radiometric Calibration Test Site (RadCaTS) is an automated approach to ground-based vicarious calibration that
does not require on-site personnel during the overpass of an airborne or spaceborne sensor. The concept originates as an
attempt to increase the amount of ground-based data that are collected throughout the year. All-weather instruments are
used to measure atmospheric and surface conditions. The data are used in an automated processing scheme to produce
top-of-atmosphere spectral radiance, which are then compared to the sensor under test. RadCaTS has been located at
Railroad Valley, Nevada, since 2004, but the concept is applicable to any site that is suitable for vicarious calibration.
Railroad Valley was chosen to test the RadCaTS concept because it has been used by the Remote Sensing Group (RSG)
for over 15 years and is well understood.
This work describes the RadCaTS automated concept, and outlines the automated processing scheme that is used to
determine the surface reflectance. A description of the instrumentation used to measure the surface reflectance and
atmosphere is presented, followed by a discussion of their placement on the site, and also their calibration. Finally, the
RadCaTS ground-based results are compared to those from Aqua and Terra MODIS in 2008, and Landsat 7 ETM+ in
2009.
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 Hyperion's 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.
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.
A preferred method of ground-based vicarious calibration is the reflectance-based approach, which requires personnel to
be present at a test site during sensor overpass. The Remote Sensing Group at the University of Arizona developed an
instrumentation suite and methodology in 2004 to measure the surface and atmospheric characteristics in the absence of
personnel. Field campaigns typically occur at a rate of once per month during the academic year, and increase during the
summer months. The automated approach allows data to be collected during every overpass of large-footprint sensors
such as Terra and Aqua MODIS, and AVHRR, which are continuously collecting data.
The large-footprint-sensor site at Railroad Valley is 1 km2. In the absence of personnel, the surface bidirectional
reflectance factor is measured using five nadir-viewing radiometers that are currently located at the site. Their locations
are chosen based on the topography of the site in an effort to "completely" sample the 1-km2 area. This work quantifies
the uncertainty in predicting the surface reflectance of the 1-km2 area based on the point measurements of the automated
methodology. It also determines if the number of radiometers, and their positions, are suitable to characterize the site in a
spatial sense. These uncertainties are determined through the use of portable spectroradiometers, and high-spatial-resolution
QuickBird imagery.
The Remote Sensing Group (RSG) at the University of Arizona Optical Sciences Center has been performing high
accuracy laboratory calibration for over 20 years. This work has been done both in support of our work in vicarious
calibration of space-borne and airborne imaging sensors and as a standalone means of achieving NIST-traceable
radiometric calibration. The solar radiation-based calibration (SRBC) has in the past been a way for the RSG to verify
calibration results and to achieve continued calibration of field-grade instruments. This paper presents multiple SRBC
results for multiple laboratory-grade radiometers. These results are compared with laboratory calibrations and studied
for their merit as a sole means of high-accuracy calibration.
The Remote Sensing Group at the University of Arizona has been active in the vicarious calibration of numerous sensors
through the use of ground-based test sites. Recent efforts have included work to develop cross-calibration information
between these sensors using the results from the reflectance-based approach. The current work extends the cross-calibration
to the AVHRR series of sensors, specifically NOAA-17, and NOAA-18. The results include work done based
on data collected by ground-based personnel nearly coincident with the sensor overpasses. The available number of
calibrations for the AVHRR series is increased through a set of ground-based radiometers that are deployed without the
need for on-site personnel and have been operating for more than three years at Railroad Valley Playa. The spectral,
spatial, and temporal characteristics of the 1-km2 large-footprint site at Railroad Valley are well understood. It is
therefore well suited for the radiometric calibration of AVHRR, which has a nadir-viewing footprint of 1.1 x 1.1 km.
The at-sensor radiance is predicted via a radiative transfer code using atmospheric data from a fully-automated solar
radiometer. The results for AVHRR show that errors are currently larger for the automated data sets, but results indicate
that the AVHRR sensors studied in this work are consistent with the Aqua and Terra MODIS sensors to within the
uncertainties of each sensor.
The current generation of the Geostationary Operations Environmental Satellite (GOES) platform employs a total of 5
sensors to monitor and record atmospheric conditions used in predictions of upcoming weather events. Included in this
package is a 5-band imager that, from the 36,000-km geosynchronous orbit inhabited by GOES platform, enables
multiple fixed full-disc surface images of the earth during the course of a 24-hour day. There is currently no on-board
radiometric calibration for the visible bands of the imager and radiometric calibration relies on vicarious approaches. The
Remote Sensing Group (RSG) at the University of Arizona uses a vicarious approach that relies on ground-based
measurements to determine the radiometric calibration for multiple sun-synchronous and airborne visible and near-infrared
sensors. The current work extends the approach to the GOES I-M series of sensor. The paper presents the
methods and results of the reflectance-based method applied to the 1-km visible channel of GOES-11using large North
American high-desert test sites. Modifications to the RSG's methods to take into account the location of the test sites at
large zenith angles within the full-disk GOES image. The work provides an opportunity to evaluate uncertainties of the
spectral BRF of the test sites at large view angles and resulting importance to the accurate radiometric calibration of a
sensor. In addition, the impact of increased path length caused by the large view angle is evaluated with an emphasis on
the increased effect of the atmospheric characterization.
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