Nikolaus Anderson

The Ground-Based Absolute Radiometric Calibration of Landsat 8 OLI

This paper presents the vicarious calibration results of Landsat 8 OLI that were obtained using the reflectance-based approach at test sites in Nevada, California, Arizona, and South Dakota, USA. Additional data were obtained using the Radiometric Calibration Test Site, which is a suite of instruments located at Railroad Valley, Nevada, USA. The results for the top-of-atmosphere spectral radiance show an average difference of −2.7, −0.8, 1.5, 2.0, 0.0, 3.6, 5.8, and 0.7% in OLI bands 1–8 as compared to an average of all of the ground-based measurements. The top-of-atmosphere spectral reflectance shows an average difference of 1.6, 1.3, 2.0, 1.9, 0.9, 2.1, 3.1, and 2.1% from the ground-based measurements. Except for OLI band 7, the spectral radiance results are generally within ±5% of the design specifications, and the reflectance results are generally within ±3% of the design specifications. The results from the data collected during the tandem Landsat 7 and 8 flight in March 2013 indicate that ETM+ and OLI agree to each other to within ±2% in similar bands in top-of-atmosphere spectral radiance, and to within ±4% in top-of-atmosphere spectral reflectance.

Design and calibration of field deployable ground-viewing radiometers

Three improved ground-viewing radiometers were built to support the Radiometric Calibration Test Site (RadCaTS) developed by the Remote Sensing Group (RSG) at the University of Arizona. Improved over previous light-emitting diode based versions, these filter-based radiometers employ seven silicon detectors and one InGaAs detector covering a wavelength range of 400-1550 nm. They are temperature controlled and designed for greater stability and lower noise. The radiometer systems show signal-to-noise ratios of greater than 1000 for all eight channels at typical field calibration signal levels. Predeployment laboratory radiance calibrations using a 1 m spherical integrating source compare well with in situ field calibrations using the solar radiation based calibration method; all bands are within ±2.7% for the case tested.

Early ground-based vicarious calibration results for Landsat 8 OLI

The Operational Land Imager (OLI) is one of two instruments onboard the Landsat 8 platform, which was launched on 11 February 2013 from Vandenberg Air Force Base in California. The multispectral bands of OLI retain the 30-m spatial resolution of Landsat 5 TM and Landsat 7 ETM+, but improvements to the system include 12-bit radiometric resolution, eight multispectral bands in the VNIR and SWIR spectral regions, and one panchromatic band. The earlier TM and ETM+ sensors use a whiskbroom configuration, while OLI uses a pushbroom configuration, which allows it to have a higher signal-to-noise ratio than previous Landsat instruments. This also creates challenges in radiometric calibration due to the large number of detectors on the 14 focal plane modules. Long-term data continuity is a crucial component of the 40-year Landsat series of satellites, and ground-based vicarious calibration has played an important role in ensuring that these sensors remain on the same radiometric scale. This work presents the early ground-based in-flight radiometric calibration of OLI, which was determined using the traditional and well-understood reflectance-based approach, as well as the Radiometric Calibration Test Site (RadCaTS), which is an automated suite of instruments located at Railroad Valley, Nevada.

On-orbit radiometric calibration of Earth-observing sensors using the Radiometric Calibration Test Site (RadCaTS)

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.

Accuracy assessment for the radiometric calibration of imaging sensors using preflight techniques relying on the sun as a source

The Remote Sensing Group (RSG) at the University of Arizona has performed high-accuracy radiometric calibration in the laboratory for more than 20 years in support of vicarious calibration of space-borne and airborne imaging sensors. Typical laboratory calibration relies on lamp-based sources which, while convenient to operate and control, do not simulate the solar spectrum that is the basic energy source for many of the imaging systems. Using the sun as a source for preflight radiometric calibration reduces uncertainties caused by the spectral mismatch between the preflight and inflight calibration, especially in the case in which a solar diffuser is the inflight calibration method. Difficulties in using the sun include varying atmospheric conditions, changing solar angle during the day and with season, and ensuring traceability to national standards. This paper presents several approaches using the sun as a radiometric calibration source coupled with the expected traceable accuracies for each method. The methods include direct viewing of the solar disk with the sensor of interest, illumination of the sensors inflight solar diffuser by the sun, and illumination of an external diffuser that is imaged by the sensor. The results of the error analysis show that it is feasible to achieve preflight calibration using the sun as a source at the same level of uncertainty as those of lamp-based approaches. The error analysis is evaluated and compared to solar-radiation-based calibrations of one of RSGs laboratory-grade radiometers.

Preflight and Vicarious Calibration of Artemis

Pre-flight and on-orbit calibration of the spectral imagery acquired with the Advanced Responsive Tactically Effective Military Imaging Spectrometer (ARTEMIS) will make use of solar radiation-based methods. Preflight spectral calibration relies on views of a specially-coated to provide a set of known absorption features. Radiometric calibration is determined from views of a spectrally-flat panel illuminated by the sun. At-aperture radiance from the panel is determined through combined measurements by multi-spectral, well-calibrated transfer radiometers and a hyperspectral, field-portable spectrometer. ARTEMIS will rely on vicarious methods for the long-term evaluation of the sensors calibration. The on-orbit radiometric calibration uses the reflectance-based method to provide at-sensor, hyperspectral radiance for the entire spectral range of ARTEMIS. Vicarious calibration of the spectral response will make use atmospheric absorption features and solar Fraunhofer lines across the solar reflective spectrum. The absolute uncertainty of the solar-based radiometric calibrations is less than 3% in spectral regions not affected by strong absorption.

The absolute radiometric calibration of the Landsat 8 Operational Land Imager using the reflectance-based approach and the Radiometric Calibration Test Site (RadCaTS)

Landsat 8 was launched on 11 February 2013 as the newest platform in the Landsat program. It contains two Earthobserving instruments, one of which is the Operational Land Imager (OLI). OLI includes an onboard radiometric calibration system that is used to monitor changes in its responsivity throughout the mission lifetime, and it consists of Spectralon solar diffuser panels as well as tungsten lamp assemblies. External techniques are used to monitor both OLI and its calibration system, and they include lunar views, side slither maneuvers of the satellite, and ground-based vicarious calibration. This work presents the absolute radiometric calibration results for Landsat 8 OLI that were obtained using two ground-based measurement techniques. The first is the reflectance-based approach, where measurements of atmospheric and surface properties are made during a Landsat 8 overpass, and it requires personnel to be on site during the time of measurement. The second uses the Radiometric Calibration Test Site (RadCaTS), which was developed by the Remote Sensing Group in the College of Optical Sciences at the University of Arizona so that radiometric calibration data can be collected without the requirement of on-site personnel. It allows more data to be collected annually, which increases the temporal sampling of trending results.

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.

Design of an ultra-portable field transfer radiometer supporting automated vicarious calibration

The University of Arizona Remote Sensing Group (RSG) began outfitting the radiometric calibration test site (RadCaTS) at Railroad Valley Nevada in 2004 for automated vicarious calibration of Earth-observing sensors. RadCaTS was upgraded to use RSG custom 8-band ground viewing radiometers (GVRs) beginning in 2011 and currently four GVRs are deployed providing an average reflectance for the test site. This measurement of ground reflectance is the most critical component of vicarious calibration using the reflectance-based method. In order to ensure the quality of these measurements, RSG has been exploring more efficient and accurate methods of on-site calibration evaluation. This work describes the design of, and initial results from, a small portable transfer radiometer for the purpose of GVR calibration validation on site. Prior to deployment, RSG uses high accuracy laboratory calibration methods in order to provide radiance calibrations with low uncertainties for each GVR. After deployment, a solar radiation based calibration has typically been used. The method is highly dependent on a clear, stable atmosphere, requires at least two people to perform, is time consuming in post processing, and is dependent on several large pieces of equipment. In order to provide more regular and more accurate calibration monitoring, the small portable transfer radiometer is designed for quick, one-person operation and on-site field calibration comparison results. The radiometer is also suited for laboratory calibration use and thus could be used as a transfer radiometer calibration standard for ground viewing radiometers of a RadCalNet site.

Ground viewing radiometer characterization, implementation and calibration applications - a summary after two years of field deployment

In 2011, three improved ground-viewing radiometers (GVRs) were built and deployed to support the Radiometric Calibration Test Site (RadCaTS) developed by the Remote Sensing Group (RSG) at the University of Arizona. The GVRs are filter-based radiometers with eight spectral channels covering a wavelength range of 400-1550 nm. They are automated, field-deployable instruments capable of long-term, standalone operation. The radiometers are temperature controlled and designed for greater stability and lower noise than their light emitting diode (LED) based predecessors. This work describes the deployment period of these radiometers with particular attention paid to the in-field performance, reliability, and results from these instruments. Using other RadCaTS inputs including meteorological station data and Aerosol Robotic Network (AERONET) Cimel sun photometer data, select vicarious calibration results are presented. With these results, an assessment of the calibration applications of the RadCaTS during new GVR deployment is discussed. In addition, GVR calibration and characterization results, including solar radiation based calibration (SRBC), are presented as another means of assessing the performance of the radiometers over deployment periods.