The Goddard Laser for Absolute Measurement of Radiance (GLAMR) is a mobile spectral and radiometric sensor characterization facility based at NASA/Goddard Space Flight Center. Based on NIST’s traveling Spectral Irradiance and Radiance Calibration using Uniform Sources (SIRCUS), GLAMR consists of a system of tunable lasers to generate quasi-monochromatic energy between 310 and 2500nm, a large integrating sphere to provide a full aperture uniform source, a control system to automate operations and a data system to record and serve telemetry. GLAMR was used to characterize the Ocean Color Instrument (OCI) to be launched aboard the Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission. The test of the OCI flight instrument took place in October 2022. GLAMR will be used to characterize the CLARREO Pathfinder (CPF) instrument in September 2023. Both programs had stringent calibration requirements on GLAMR, necessitating additional characterization of GLAMR radiometric uncertainty and improvements in the NIST traceability. This paper will discuss the improvement in the GLAMR uncertainty budget and the performance of GLAMR for the OCI instrument as well as the upcoming test for CPF.
The NASA GSFC Code 618 Calibration Laboratory maintains instruments and National Institute of Standards and Technology (NIST) traceable calibrated sources and detectors to calibrate, characterize, and monitor remote sensing instrumentation throughout NASA and the larger scientific community. Under the Calibration Laboratory umbrella, we operate the Radiometric Calibration Lab (RCL) focused on calibrating instrument radiometers, the Diffuser Calibration Lab (DCL) specializing in NIST traceable calibration of reflective and transmissive space diffusers. The RCL uses broadband sources as well as an array of options for monochromatic spectral calibration to provide regular NIST traceable calibration services to ground, flight, and remote sensing missions at NASA GSFC. The DCL uses scatterometers to measure the Bidirectional Reflectance and Transmittance Distribution Functions (BRDF & BTDF) of flight diffusers and witness samples. As we look to the future, the Calibration Laboratory will be automating routine processes throughout the facility and updating our online data collection and distribution capabilities. We are adding monitoring radiometers to our Grande calibration sphere to improve NIST traceability. Hardware updates to our scatterometers will keep us aligned with the diffuser calibration capabilities being developed at NIST.
Based on the recent success of our strained-layer superlattice (SLS)-based infrared (IR) camera that performed Earth imaging from the International Space Station (ISS) in 2019 we have built, what we consider, to be the next generation multi-band SLS imaging system. The Compact Thermal Imager (CTI) was installed on the Robotic Refueling Mission 3 (RRM3) and attached to the exterior of the ISS. From this location we were able to capture 15 million images of a multitude of fires around the globe in 2019. This unexpected trove of data initiated quite a bit of scientific interest to further utilize this imaging capability but would include features to more precisely monitor terrestrial fires and other surface phenomena. To this end, we developed a technique to install specific bandpass filters directly onto the SLS detector hybrid assembly. Utilizing this technique we have built a CTI-2 camera system with two filters, 4 and 11μm, and have made a second detector assembly with six filter bands from 4- 12μm. This second system will also be used to supplement Landsat remote imaging monitoring approximate land surface temperatures, monitor evapotranspiration, sea ice and glacier dynamics. The CTI-2 camera is based on a 1,024x1,024 (1kx1k) format SLS detector hybridized to a FLIR ISC0404 readout integrated circuit (ROIC). The six band SLS focal plane array is based on the 640x512 FLIR ISC 9803 ROIC. This camera system is based on the Landsat 8 and 9 Thermal IR Sensors (TIRS) instrument and one of its purposes is to perform ground truthing for the Landsat 8/9 data at higher spectral resolution. Both Landsat TIRS instruments are dual band thermal IR sensors centered on 11 and 12μm (each with about a 1μm bandpass). Both of our SLS systems utilize a Ricor K548 cryocooler. To streamline costs and development time we used commercial optics and both commercial and custom NASA electronic components. A primary feature of these camera systems is the incorporation of specific filters to collect fire data at ~3.9μm and thermal data at ~11μm. The CTI- 2 instrument is designed for 37 m /pixel spatial resolution from 410km orbit (ISS orbit). In this paper, we will present the design and performance of the focal plane, optics, electronics and mechanical structure of the dual-band CTI-2 and the focal plane performance of the six-band focal plane.
The advanced baseline imagers (ABIs) on board the geostationary operational environmental satellite (GOES)-R series, GOES-16 and GOES-17 satellites, have accumulated long data series that have been updated throughout their missions to enhance their utility for imaging the Earth’s weather, oceans, and environment. In preparing for a reprocessing effort aimed at improving the consistency and image quality and reducing radiometric uncertainties necessary for some Earth science applications, we developed a vicarious validation technique for five of the six reflective solar bands. This technique uses Sonoran desert surface reflectance data derived from the moderate resolution imaging spectrometer bidirectional reflectance distribution function parameters product along with other atmospheric products to generate a predicted at-sensor radiance for both GOES-16 and GOES-17 for comparison with ABI observations and to enable ABI intercomparisons. This technique identified known instrument biases and demonstrated absolute validation over the course of a year, showing its capability to detect changes, monitor stability, and assess radiometric uncertainties for operational data applications and future reprocessed data sets.
The GOES-17 Advanced Baseline Imager (ABI) has an anomaly with its on-board cooling system that prevents it from maintaining its Focal Plane Modules (FPMs) at cold, optimal temperatures. Because of this, during certain times of the year the FPMs and their detectors warm and cool throughout the day. Changing the detectors’ temperature changes their response to incoming radiance, which leads to calibration errors over time and degrades the imagery. Numerous mitigation strategies have been implemented to reduce the solar insolation on the instrument and to mitigate image degradation, including semi-annual yaw flips and changing the integration time of the detectors twice daily. These and other mitigations all work with the baseline calibration algorithms currently in place on the GOES-R Ground System. In an attempt to reduce the image degradation even further, the ABI vendor designed a new calibration scheme that predicts key parameters forward in time to account for the drifting FPM temperatures. These parameters, the linear gain term and dark current scene, are nominally updated on orbit every 5 minutes and 30 seconds, respectively. However, even at these relatively short cadences the detectors can change temperature, thereby rendering the parameters invalid for accurate calibration. By projecting these parameters forward in time the radiometric bias is reduced and image quality improves. This Predictive Calibration modification was deployed to operations on July 25, 2019, following several months of extensive testing and optimization by the GOES-R science teams. During this time several parameters and thresholds were tuned to ensure Predictive Calibration was turning on and off at the optimal times. Since going into operations users have seen noticeable improvement to the imagery and its calibration. This paper will discuss the fundamental assumptions behind the baseline equations and highlight the changes introduced by Predictive Calibration. Results will show the improvements to the calibration of the operational L1b products and reduction in image degradation.
Validation results from a reflectance-based field campaign at the Salar de Uyuni in Bolivia (September 2018) are presented for GOES-17 and GOES-16 Advanced Baseline Imagers (ABI) reflective channels. The in situ measurements were used to characterize the surface reflectance and the atmosphere in order to constrain a radiative transfer model and predict the reflectance at the top of the atmosphere (TOA), which was then compared to the ABI measurements. The field campaign provides TOA reflectance estimates over several days, allowing assessment of the calibration accuracy and stability of channels 1, 2, 3, 5 and 6 for GOES-17 and GOES-16 ABI. Channel 1 of GOES-17 ABI shows -5.5% bias in comparison to the ground-based predicted TOA. Over 6% bias in GOES-17 B2 was confirmed. A comparison to NOAA-20 VIIRS was also carried on during a near nadir overpass.
KEYWORDS: Landsat, Earth observing sensors, Space operations, Stray light, Sensors, Signal to noise ratio, Observatories, Short wave infrared radiation, Calibration
Landsat 9 is currently undergoing testing at the integrated observatory level in preparation for launch from Vandenberg Air Force Base in 2021. Landsat 9 will replace Landsat 7 in orbit, 8 days out of phase with Landsat 8. Landsat 9 is largely a copy of Landsat 8 in terms of instrumentation, with an Operational Land Imager (OLI), model #2 and a Thermal Infrared Sensor (TIRS), model #2. The TIRS-2 is more significantly changed from TIRS with increased redundancy, as well as changes to the telescope baffling to improve stray light control and a revised scene select mirror encoder mechanism. Data quality of the Landsat 9 instruments is comparable to, or better than the Landsat 8 ones, with an increase to 14 bits of data transmitted and more detailed pre-launch characterization for OLI-2, and with more detailed characterization of the TIRS-2 pre-launch, in addition to the improved stray light control. The performance of the two instruments is summarized and compared to that of the Landsat 8 instruments.
The large focal plane arrays used in the new generation of the Geostationary Operational Environmental Satellites (GOES) advanced baseline imager (ABI) introduce new calibration challenges compared with the heritage GOES imagers. The increased number of detectors allows for increased spatial, spectral, and temporal performance, but as a trade-off, it has an increased risk of image striping. We detail the development of a new postlaunch relative calibration capability for ABI reflective solar bands that utilizes ABI special scans to generate a set of relative gains that can be applied to improve image quality and reduce image striping. Results demonstrate that the method reduces image striping in the ABI solar reflective bands over varying scene content and time, both diurnally and over an extended period. This methodology ensures a calibration strategy that is consistent with heritage approaches yet adapts to the new postlaunch validation challenges presented by the new class of operational imagers in the GOES-R series. The developed approach is ready for operational use, as needed, and can be easily implemented into operations to support the operational production of geostationary imagery of the Earth.
The JPSS-2 VIIRS sensor has completed its pre-launch test program and is now awaiting launch in the 2022 timeframe. The VIIRS spectral characterization, in the form of band averaged and supporting detector level relative spectral response (RSR) for each VIIRS band, was completed in 2019 and is based upon independent SpMA dual monochromator (all bands) and GSFC GLAMR laser system (reflectance bands only) spectral measurements, including first time measurements of the VIIRS SWIR bands by a laser system. The measurements and subsequent analysis effort by subject matter experts of the VIIRS DAWG has led to the July 2019 VIIRS Version 2 RSR release, the official at-launch RSR characterization for the JPSS-2 VIIRS mission. Version 2 replaces and improves upon the August 2018 Version 1 release by incorporating the GLAMR measurements into the analysis to produce an updated “fused” RSR for reflective solar bands (M1- M10, I1-I3, DNBLGS, DNBMGS) and by applying a CO2 absorption correction to the SpMA measurements for thermal band M13. For all other bands (M11, M12, M14-M16, I4, I5), the Version 1 characterization, based entirely upon the SpMA measurements, is carried forward into the Version 2 release. An assessment on compliance with spectral performance metrics finds that VIIRS is compliant on nearly all metrics, with a few minor exceptions. The version 2 RSR release includes band average (over all detectors and subsamples) RSR plus supporting RSR for each detector and subsample, and is available under EAR99 restrictions to the science community at a restricted access NASA eRoom site.
The Landsat-9 satellite will carry the Operational Land Imager-2 (OLI-2) as one of its payloads. This instrument is a clone of the Landsat-8 OLI and its mission is to continue the operational land imaging of the Landsat program. The OLI-2 will continue to populate an archive of Landsat earth images that dates back to 1972. The OLI-2 instrument is not significantly different from OLI though the instrument-level pre-launch spectral characterization process was much improved. While OLI was characterized by a double monochromator system, the OLI2 spectral characterization made use of the Goddard Laser for Absolute Measurement of Radiance (GLAMR), a system of tunable lasers that cover 350-2500nm which are fiber-coupled to a 30-in integrating sphere and is monitored by NISTtraceable radiometers. GLAMR allowed the spectral characterization of every detector of the OLI-2 focal plane in nominal imaging conditions. The in-band relative spectral responses were sampled at 1 or 2nm wavelength increments and the out-of-band responses at 10 or 20nm wavelength increments (increment is dependent on spectral band/region). The final relative spectral responses (RSRs) represent the best characterization any Landsat instrument spectral response. This paper will cover the results of the instrument-level spectral characterization, including in-band response, out-of-band response, spectral cross-talk and spectral uniformity.
The Goddard Laser for Absolute Measurement of Radiance (GLAMR) is a transportable calibration facility that provides characterization of spectral and radiometric response of airborne and satellite-based instruments operating in the solarreflective spectral region. In this work, the time resolved output of GLAMR’s integrating sphere coupled with a modelocked source was measured. The 76-cm diameter sphere with 30-cm output aperture was illuminated using a modelocked pulse train at 76 MHz, with single pulse durations of 12 ps. The time constant of the sphere was found to be 36 ns, and the resultant temporal averaging of pulses produced a maximum time varying irradiance at the output of 20% of the mean. A comparison of instrument calibration data generated with this integrating sphere using both a mode-locked source and a continuous-wave source is also given.
Landsat-8 has been operating on-orbit for 5+ years. Its two sensors, the Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS), are continuing to produce high quality data. The OLI has been radiometrically stable at the better than 0.3% level on a band average basis for all but the shortest wavelength (443 nm) band, which has degraded about 1.3% since launch. All on-board calibration devices continue to perform well and consistently. No gaps in across track coverage exist as 100% operability of the detectors is maintained. The variability over time of detector responsivity within a band relative to the average is better than 0.05% (1 sigma), though there are occasional detectors that jump up to 1.5% in response in the Short-Wave InfraRed (SWIR) bands. Signal-to-Noise performance continues at 2-3x better than requirements, with a small degradation in the 443 nm band commensurate with the loss in sensitivity. Pre-launch error analysis, combined with the stability of the OLI indicates that the absolute reflectance calibration uncertainty is better than 3%; comparisons to ground measurements and comparisons to other sensors are consistent with this. The Landsat-8 TIRS is similarly radiometrically stable, showing changes of at most 0.3% over the mission. The uncertainty in the absolute calibration as well as the detector to detector variability are largely driven by the stray light response of TIRS. The current processing corrects most of the stray light effects, resulting in absolute uncertainties of ~1% and reduced striping. Efforts continue to further reduce the striping. Noise equivalent delta temperature is about 50 mK at typical temperatures and 100% detector operability is maintained. Landsat-9 is currently under development with a launch no earlier than December 2020. The nearly identical OLI-2 and upgraded TIRS-2 sensors have completed integration and are in the process of instrument level performance characterization including spectral, spatial, radiometric and geometric testing. Component and assembly level measurements of the OLI-2, which include spectral response, radiometric response and stray light indicate comparable performance to OLI. The first functional tests occurred in July 2018 and spatial performance testing in vacuum is scheduled for August 2018. Similarly, for TIRS-2, partially integrated instrument level testing indicated spectral and spatial responses comparable to TIRS, with stray light reduced by approximately an order of magnitude from TIRS.
The Thermal Infrared Sensor-2 (TIRS-2) aboard Landsat 9 will continue Landsat’s four decade-long legacy of providing moderate resolution thermal imagery from low earth orbit (at 705 km) for environmental applications. Like the Thermal Infrared Sensor aboard Landsat 8, it is a pushbroom sensor with a cross-track field of view of 15° and provides two spectral channels at 10.8 and 12 μm. To ensure radiometric, spatial, and spectral performance, a comprehensive pre-launch testing program is being conducted at NASA Goddard Space Flight Center at the component, subsystem, and instrument level. This paper will focus on the results from the subsystem level testing where the instrument is almost completely assembled. This phase of testing is specifically designed to assess imaging performance including focus and stray light rejection, but is also used to provide a preliminary assessments of spatial and spectral performance. The calibration ground support equipment provides a flexible blackbody illumination source and optics to conduct these tests. The spectral response test setup has its own illumination source outside the chamber that propagates through the calibration ground support equipment in an optical configuration designed for this purpose. This test configuration with the calibration ground support equipment and TIRS-2 subsystem in the thermal vacuum chamber enables a large range of illumination angles for stray light measurements. The results show that TIRS-2 performance is expected to meet all of its performance requirements with few waivers and deviations.
The Landsat-9 Operational Land Imager 2 (OLI-2) instrument, currently under development for launch in late 2020, is a clone of the Landsat-8 OLI instrument, which was launched in 2013. Ball Aerospace built and rigorously characterized the Landsat-8 OLI and is repeating the process for the Landsat-9 OLI-2. A major difference between the testing for OLI and OLI-2 will be spectral test equipment. The instrument-level spectral test for OLI made use of a double monochromator; the OLI-2 test will use of Goddard Laser for the Absolute Measurement of Radiance (GLAMR). The GLAMR system is a set of lasers, which collectively cover the entire spectral range of the OLI-2 spectral bands. The laser outputs are fed to a 30” integrating sphere via fiber optic cables, which OLI-2 can view from its position inside the thermal vacuum chamber. The laser-based spectral characterization offers several major advantages over the monochromator-based methods: (1) higher signal levels as compared to the lamp in the double monochromator providing better signal to noise and capabilities to measure out of band response, (2) full aperture illumination and flood illumination of multiple focal plane modules so that all detectors are tested and crosstalk effects can be observed, as opposed to the approximately 60 detectors illuminated by the slit image of the monochromator (3) an absolute spectral response characterization as opposed to relative spectral response. OLI-2 spectral testing with GLAMR should begin in late 2018. This work describes the spectral-radiometric test plan, test requirements, and GLAMR performance demonstrated prior to OLI-2 characterization.
Landsat-9, the next in the series of Landsat satellites, will have the same complement of two sensors as Landsat-8: The Operational Land Imager (OLI) that covers the reflective solar part of the spectrum in 9 spectral bands and the Thermal Infrared Sensor (TIRS) with two bands in the thermal infrared region. The main changes to the sensors for Landsat-9 will be to increase redundancy in the TIRS instrument, called TIRS-2, to bring it up to a five year design lifetime and fixes for anomalies observed on-orbit on Landsat-8 TIRS: Stray light and scene select mechanism encoder degradation. This work reports on the multi-pronged approach that will be used to ensure that stray light is reduced to required levels and properly characterized. Baffles to reduce stray light were designed and tested at several stages of sensor development. In parallel, optical modeling by NASA and independent teams was used to predict performance of the design changes to hold against test results as well as Landsat 8 TIRS on-orbit performance for model validation. A new subsystem-level test allows a large angular range to be tested to characterize out-of-field stray light that was not available during the first TIRS build. Combined, characterization results from modeling and ambient-, component-, subsystem-, and instrument-level testing will fully characterize TIRS-2 performance.
The ability of sensors to detect changes in the Earth’s environment is dependent on retrieving radiometrically consistent and calibrated measurements from its surface. Intercalibration provides consistency among satellite instruments and ensures fidelity of scientific information. Intercalibration is especially important for spaceborne satellites without any on-board calibration, as accuracy of instruments is significantly affected by changes that occur postlaunch. To better understand the key parameters that impact the intercalibration process, this paper describes a simulation environment that was developed to support the primary mission of the Algodones Dunes campaign. Specifically, measurements obtained from the campaign were utilized to create a synthetic landscape to assess the feasibility of using the Algodones Dunes system as an intercalibration site for spaceborne instruments. The impact of two key parameters (differing view-angles and temporal offsets between instruments) on the intercalibration process was assessed. Results of these studies indicate that although the accuracy of intercalibration is sensitive to these parameters, proper knowledge of their impact leads to situations that minimize their effect. This paper concludes with a case study that addresses the feasibility of performing intercalibration on the International Space Station’s platform to support NASA’s CLARREO, the climate absolute radiance and refractivity observatory, mission.
Several sites from around the world are being used operationally and are suitable for vicarious calibration of space-borne imaging platforms. However, due to the proximity of these sites (e.g., Libya 4), a rigorous characterization of the landscape is not feasible, limiting their utility for sensor intercalibration efforts. Due to its accessibility and similarities to Libya 4, the Algodones Sand Dunes System in California, USA, was identified as a potentially attractive intercalibration site for space-borne, reflective instruments such as Landsat. In March 2015, a 4-day field campaign was conducted to develop an initial characterization of Algodones with a primary goal of assessing its intercalibration potential. Five organizations from the US and Canada collaborated to collect both active and passive airborne image data, spatial and temporal measurements of spectral bidirectional reflectance distribution function, and in-situ sand samples from several locations across the Algodones system. The collection activities conducted to support the campaign goal is summarized, including a summary of all instrumentation used, the data collected, and the experiments performed in an effort to characterize the Algodones site.
This paper describes the current ground-based calibration results of Landsat 7 Enhanced Thematic Mapper Plus (ETM+), Landsat 8 Operational Land Imager (OLI), Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), Suomi National Polar orbiting Partnership Visible Infrared Imaging Radiometer Suite (VIIRS), and Sentinel-2A Multispectral Instrument (MSI), using an automated suite of instruments located at Railroad Valley, Nevada, USA. The period of this study is 2012 to 2016 for MODIS, VIIRS, and ETM+, 2013 to 2016 for OLI, and 2015 to 2016 for MSI. The current results show that all sensors agree with the Radiometric Calibration Test Site (RadCaTS) to within ±5% in the solar-reflective regime, except for one band on VIIRS that is within ±6%. In the case of ETM+ and OLI, the agreement is within ±3%, and, in the case of MODIS, the agreement is within ±3.5%. MSI agrees with RadCaTS to within ±4.5% in all applicable bands.
Many inter-consistency efforts force empirical agreement between satellite and airborne sensors viewing a source nearly coincident in time and geometry that ensures consistency between sensors rather than relying on a physical understanding of the source. Several research groups organized a campaign at Algodones Dunes in March 2015 in an effort to measure and characterize parameters that can be used for a source model that will enable this physical understanding. This work will provide an overview of the parameters retrieved from airborne and ground-based measurements made during the campaign. Examples of model-based predictions of at-sensor radiance will be shown for Landsat and MODIS. This approach will provide insight into uncertainties of sensor inter-consistency studies and allow for documented SI-traceability and associate error budget. The Algodones model and subsequent test site models can be used for the assessing inter-calibration accuracies of the upcoming Climate Absolute Reflectance and Refractivity Observatory (CLARREO) Pathfinder mission.
The goal of the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission is to provide high-accuracy data for evaluation of long-term climate change trends. Essential to the CLARREO project is demonstration of SI-traceable, reflected measurements that are a factor of 10 more accurate than current state-of-the-art sensors. The CLARREO approach relies on accurate, monochromatic absolute radiance calibration in the laboratory transferred to orbit via solar irradiance knowledge. The current work describes the results of field measurements with the Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) that is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO. Recent measurements of absolute spectral solar irradiance using SOLARIS are presented. The ground-based SOLARIS data are corrected to top-of-atmosphere values using AERONET data collected within 5 km of the SOLARIS operation. The SOLARIS data are converted to absolute irradiance using laboratory calibrations based on the Goddard Laser for Absolute Measurement of Radiance (GLAMR). Results are compared to accepted solar irradiance models to demonstrate accuracy values giving confidence in the error budget for the CLARREO reflectance retrieval.
The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission is formulated to determine long-term climate trends using SI-traceable measurements. The CLARREO mission will include instruments operating in the reflected solar (RS) wavelength region from 320 nm to 2300 nm. The Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO and facilitates testing and evaluation of calibration approaches. The basis of CLARREO and SOLARIS calibration is the Goddard Laser for Absolute Measurement of Response (GLAMR) that provides a radiance-based calibration at reflective solar wavelengths using continuously tunable lasers. SI-traceability is achieved via detector-based standards that, in GLAMR’s case, are a set of NIST-calibrated transfer radiometers. A portable version of the SOLARIS, Suitcase SOLARIS is used to evaluate GLAMR’s calibration accuracies. The calibration of Suitcase SOLARIS using GLAMR agrees with that obtained from source-based results of the Remote Sensing Group (RSG) at the University of Arizona to better than 5% (k=2) in the 720-860 nm spectral range. The differences are within the uncertainties of the NIST-calibrated FEL lamp-based approach of RSG and give confidence that GLAMR is operating at <5% (k=2) absolute uncertainties. Limitations of the Suitcase SOLARIS instrument also discussed and the next edition of the SOLARIS instrument (Suitcase SOLARIS- 2) is expected to provide an improved mechanism to further assess GLAMR and CLARREO calibration approaches.
The reflected solar instrument that is part of the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission is being formulated with a goal of providing SI-traceable measurement of radiance that is an order of magnitude more accurate than the current imaging sensors. The Goddard Laser for Absolute Measurement of Radiance (GLAMR) is a key element to reaching such accuracy along with transferring the laboratory calibration to on-orbit measurements. Results from field reflectance retrievals using three separate instruments all of which have been calibrated using GLAMR are shown. The instruments include a commercial field spectrometer and a portable version of CLARREO’s calibration demonstration system. The third instrument is NASA Goddard’s Lidar, Hyperspectral and Thermal Imager (G-LiHT) which is an airborne system. All three were operated during a March 2013 measurement campaign at Red Lake Playa, Arizona as part of the on-orbit commissioning phase of Landsat 8. Reflectance is derived from near-coincident measurements by the three sensors for a small area of the playa. The retrieved results are SI-traceable and demonstrate the ability to transfer the GLAMR calibration to the field. Use of the G-LiHT data in the calibration of Landsat-7 and -8 sensors permits them both to be placed on the GLAMR-scale as well.
Climate change studies require long-term, continuous records that extend beyond the lifetime, and the temporal resolution, of a single remote sensing satellite sensor. The inter-calibration of spaceborne sensors is therefore desired to provide spatially, spectrally, and temporally homogeneous datasets. The Digital Imaging and Remote Sensing Image Generation (DIRSIG) tool is a first principle-based synthetic image generation model that has the potential to characterize the parameters that impact the accuracy of the inter-calibration of spaceborne sensors. To demonstrate the potential utility of the model, we compare the radiance observed in real image data to the radiance observed in simulated image from DIRSIG. In the present work, a synthetic landscape of the Algodones Sand Dunes System is created. The terrain is facetized using a 2-meter digital elevation model generated from NASA Goddard's LiDAR, Hyperspectral, and Thermal (G-LiHT) imager. The material spectra are assigned using hyperspectral measurements of sand collected from the Algodones Sand Dunes System. Lastly, the bidirectional reflectance distribution function (BRDF) properties are assigned to the modeled terrain using the Moderate Resolution Imaging Spectroradiometer (MODIS) BRDF product in conjunction with DIRSIG's Ross-Li capability. The results of this work indicate that DIRSIG is in good agreement with real image data. The potential sources of residual error are identified and the possibilities for future work are discussed.
Calibration and validation play an essential role during the acquisition and processing of satellite data for Earth Observing System satellites in addition to being an integral part of maintaining scientific values of archived satellite data. The Advanced Spaceborne Thermal Emission and Reflection and Radiometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) are two of five sensors onboard the Terra platform. ASTER has a swath width of 60 km with 8 spectral bands in the visible and near infrared (VNIR) and thermal infrared (TIR) spectral range with a spatial resolution of 15-m (bands 1-3) and 90-m (bands 10-14), respectively while MODIS has a swath width of 2300 km with 36 spectral bands from visible to infrared spectral range with a spatial resolution of 250 m (bands 1-2), 500 m (bands 3-7), and 1 km (bands 8-36). ASTER is the ‘zoom’ lens and MODIS is the ‘keystone’ instrument for Terra; they provide quantitative measurements of various earth system variables to the scientific and to the broader community. The simultaneous view of the sensors simplifies the intercomparison between them and the current work relies on the use of the Railroad Valley Playa test site to reduce uncertainties caused by spatial heterogeneity and spectral differences in the sensors. The fact that Railroad Valley is a calibration test site for ASTER ensures that ASTER was tasked at a higher rate over this area providing more scenes for an intercomparison. The study compares ASTER L1B data for the three VNIR bands reprocessed with recent calibration updates and MODIS 02 Collection 6 data products for the similar bands. No correction for geometry angle is needed and coincident 3-km by 3-km regions are used to reduce the impact of spatial heterogeneity. A correction for spectral differences between the sensors is applied based on seasonal averages of EO-1 Hyperion spectral range. Results indicate that the calibrated radiance products from the two sensors agree to within the combined absolute uncertainties. There is no statistically significant temporal trend between the two and this should allow the correction between the two to within 0.5% over the 10 years studied here.
The polarization sensitivity of the Visible/NearIR (VISNIR) bands in the Joint Polar Satellite Sensor 1 (J1) Visible Infrared Imaging Radiometer Suite (VIIRS) instrument was measured using a broadband source. While polarization sensitivity for bands M5-M7, I1, and I2 was less than 2.5 %, the maximum polarization sensitivity for bands M1, M2, M3, and M4 was measured to be 6.4 %, 4.4 %, 3.1 %, and 4.3 %, respectively with a polarization characterization uncertainty of less than 0.38%. A detailed polarization model indicated that the large polarization sensitivity observed in the M1 to M4 bands is mainly due to the large polarization sensitivity introduced at the leading and trailing edges of the newly manufactured VISNIR bandpass focal plane filters installed in front of the VISNIR detectors. This was confirmed by polarization measurements of bands M1 and M4 bands using monochromatic light. Discussed are the activities leading up to and including the two polarization tests, some discussion of the polarization model and the model results, the role of the focal plane filters, the polarization testing of the Aft-Optics-Assembly, the testing of the polarizers at the National Aeronautics and Space Administration’s (NASA) Goddard center and at the National Institute of Science and Technology (NIST) facility and the use of NIST’s Traveling Spectral Irradiance and Radiance responsivity Calibrations using Uniform Sources (T-SIRCUS) for polarization testing and associated analyses and results.
The Radiometric Calibration Test Site (RadCaTS) at Railroad Valley Playa, Nevada is being developed by the University of Arizona to enable improved accuracy and consistency for airborne and satellite sensor calibration. Primary instrumentation at the site consists of ground-viewing radiometers, a sun photometer, and a meteorological station. Measurements made by these instruments are used to calculate surface reflectance, atmospheric properties and a prediction for top-of-atmosphere reflectance and radiance. This work will leverage research for RadCaTS, and describe the requirements for an online database, associated data formats and quality control, and processing levels.
NASA Goddard’s Lidar, Hyperspectral and Thermal Imager (G-LiHT) facilitates simultaneous measurements beneficial to variety of applications. Of the suite of “off-the shelf” instruments of G-LiHT, the Visible Near-Infrared (VNIR) Imaging Spectrometer acquires high resolution spectral measurements (1.5 nm resolution) from 0.4 to 1 μm. Goddard Space Flight Center’s Laser for Absolute Measurement of Response (GLAMR) was used to measure the absolute spectral response (ASR) of the G-LiHT’s imaging spectrometer. Continuously tunable lasers coupled to an integrating sphere allow a radiance-based calibration for the detectors at reflective solar wavelengths. GLAMR measurements, covering a wavelength range from 0.58 to 0.99 μm were acquired between July 30 to August 2, 2013. In order to account for the large field-of-view (50°), G-LiHT was rotated in 2 degree increments so that the same area of the sphere is viewed by all detectors. Using this data along with the coincident Silicon trap radiometer measurements, the ASR was computed. The derived calibration parameters for G-LiHT’s Imaging Spectrometer are to be transferred to near-simultaneous measurements of Landsat sensors. Calibration uncertainty of G-LiHT is 1-3% depending spectral region and transferring this traceability to coincident satellite sensors has 3-5% depending on spectral region.
The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission addresses the need to observe highaccuracy, long-term climate change trends and to use decadal change observations as a method to determine the accuracy of climate change. A CLARREO objective is to improve the accuracy of SI-traceable, absolute calibration at infrared and reflected solar wavelengths to reach on-orbit accuracies required to allow climate change observations to survive data gaps and observe climate change at the limit of natural variability. Such an effort will also demonstrate National Institute of Standards and Technology (NIST) approaches for use in future spaceborne instruments. The current work describes the results of laboratory and field measurements with the Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) which is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO. SOLARIS allows testing and evaluation of calibration approaches, alternate design and/or implementation approaches and components for the CLARREO mission. SOLARIS also provides a testbed for detector technologies, non-linearity determination and uncertainties, and application of future technology developments and suggested spacecraft instrument design modifications. Results of laboratory calibration measurements are provided to demonstrate key assumptions about instrument behavior that are needed to achieve CLARREO’s climate measurement requirements. Absolute radiometric response is determined using laser-based calibration sources and applied to direct solar views for comparison with accepted solar irradiance models to demonstrate accuracy values giving confidence in the error budget for the CLARREO reflectance retrieval.
Scientists in the Biospheric Sciences Laboratory at NASA’s Goddard Space Flight Center have undertaken a unique instrument fusion effort for an airborne package that integrates commercial off the shelf LiDAR, Hyperspectral, and Thermal components. G-LiHT is a compact, lightweight and portable system that can be used on a wide range of airborne platforms to support a number of NASA Earth Science research projects and space-based missions. G-LiHT permits simultaneous and complementary measurements of surface reflectance, vegetation structure, and temperature, which provide an analytical framework for the development of new algorithms for mapping plant species composition, plant functional types, biodiversity, biomass, carbon stocks, and plant growth. G-LiHT and its supporting database are designed to give scientists open access to the data that are needed to understand the relationship between ecosystem form and function and to stimulate the advancement of synergistic algorithms. This system will enhance our ability to design new missions and produce data products related to biodiversity and climate change. G-LiHT has been operational since 2011 and has been used to collect data for a number of NASA and USFS sponsored studies, including NASA’s Carbon Monitoring System (CMS) and the American ICESat/GLAS Assessment of Carbon (AMIGA-Carb). These acquisitions target a broad diversity of forest communities and ecoregions across the United States and Mexico. Here, we will discuss the components of G-LiHT, their calibration and performance characteristics, operational implementation, and data processing workflows. We will also provide examples of higher level data products that are currently available.
NASA Goddard’s SOLARIS (Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer) sensor is the calibration demonstration system for CLARREO (Climate Absolute Radiance and Refractivity Observatory), a mission that addresses the need to make highly accurate observations of long-term climate change trends. The SOLARIS instrument will be designed to support a primary objective of CLARREO, which is to advance the accuracy of absolute calibration for space-borne instruments in the reflected solar wavelengths. This work focuses on the development of a simulated environment to facilitate sensor trade studies to support instrument design and build for the SOLARIS sensor. Openly available data are used to generate geometrically and radiometrically realistic synthetic landscapes to serve as input to an image generation model, specifically the Digital Imaging and Remote Sensing Image Generation (DIRSIG) model. Recent enhancements to DIRSIG’s sensor model capabilities have made it an attractive option for performing sensor trade studies. This research takes advantage of these enhancements to model key sensor characteristics (e.g., sensor noise, relative spectral response, spectral coverage, etc.) and evaluate their impact on SOLARIS’s stringent 0.3% error budget for absolute calibration. A SOLARIS sensor model is developed directly from measurements provided by NASA Goddard and various synthetic landscapes generated to identify potential calibration sites once the instrument achieves orbit. The results of these experiments are presented and potential sources of error for sensor inter-calibration are identified.
The Advanced Spaceborne Thermal Emissive and Reflection Radiometer (ASTER) and Moderate Resolution Imaging Spectrometer (MODIS) are two of the five sensors onboard the Earth Observing System’s Terra satellite. These sensors share many similar spectral channels while having much different spatial and operational parameters. ASTER is a tasked sensor and sometimes referred to a zoom camera of the MODIS that collects a full-earth image every one to two days. It is important that these sensors have a consistent characterization and calibration for continued development and use of their data products. This work uses a variety of test sites to retrieve and validate intercalibration results. The refined calibration of Collection 6 of the Terra MODIS data set is leveraged to provide the up-to-date reference for trending and validation of ASTER. Special attention is given to spatially matching radiance measurements using prelaunch spatial response characterization of MODIS. Despite differences in spectral band properties and spatial scales, ASTER-MODIS is an ideal case for intercomparison since the sensors have nearly identical views and acquisitions times and therefore can be used as a baseline of intercalibration performance of other satellite sensor pairs.
A goal of the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission is to observe highaccuracy, long-term climate change trends over decadal time scales. The key to such a goal is to improving the accuracy of SI traceable absolute calibration across infrared and reflected solar wavelengths allowing climate change to be separated from the limit of natural variability. The advances required to reach on-orbit absolute accuracy to allow climate change observations to survive data gaps exist at NIST in the laboratory, but still need demonstration that the advances can move successfully from to NASA and/or instrument vendor capabilities for spaceborne instruments. The current work describes the radiometric calibration error budget for the Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) which is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO. The goal of the CDS is to allow the testing and evaluation of calibration approaches, alternate design and/or implementation approaches and components for the CLARREO mission. SOLARIS also provides a test-bed for detector technologies, non-linearity determination and uncertainties, and application of future technology developments and suggested spacecraft instrument design modifications. The resulting SI-traceable error budget for reflectance retrieval using solar irradiance as a reference and methods for laboratory-based, absolute calibration suitable for climatequality data collections is given. Key components in the error budget are geometry differences between the solar and earth views, knowledge of attenuator behavior when viewing the sun, and sensor behavior such as detector linearity and noise behavior. Methods for demonstrating this error budget are also presented.
The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission addresses the need to observe highaccuracy, long-term climate change trends and to use decadal change observations as the most critical method to determine the accuracy of climate change. One of the major objectives of CLARREO is to advance the accuracy of SI traceable absolute calibration at infrared and reflected solar wavelengths. This advance is required to reach the on-orbit absolute accuracy required to allow climate change observations to survive data gaps while remaining sufficiently accurate to observe climate change to within the uncertainty of the limit of natural variability. While these capabilities exist at NIST in the laboratory, there is a need to demonstrate that it can move successfully from NIST to NASA and/or instrument vendor capabilities for future spaceborne instruments. The current work describes the test plan for the Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) which is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO. The goal of the CDS is to allow the testing and evaluation of calibration approaches, alternate design and/or implementation approaches and components for the CLARREO mission. SOLARIS also provides a test-bed for detector technologies, non-linearity determination and uncertainties, and application of future technology developments and suggested spacecraft instrument design modifications. The end result of efforts with the SOLARIS CDS will be an SI-traceable error budget for reflectance retrieval using solar irradiance as a reference and methods for laboratory-based, absolute calibration suitable for climate-quality data collections.
Many inter-consistency efforts force empirical agreement between sensors viewing a source nearly coincident in time
and geometry that ensures consistency between sensors rather than obtain an SI-traceable calibration with documented
error budgets. The method described here provides inter-consistency via absolute radiometric calibration with defensible
error budget avoiding systematic errors through prediction of at-sensor radiance for a site viewed by multiple sensors but
not necessarily viewed at coincident times. The method predicts spectral radiance over a given surface site for arbitrary
view and illumination angles and for any date dominated by clear-sky conditions. The foundation is a model-based, SItraceable
prediction of at-sensor radiance over selected sites based on physical understanding of the surface and
atmosphere. The calibration of the ground site will include spatial, spectral, and sun-view geometric effects based on
satellite and ground-based data. The result is an interconsistency of hyperspectral and multispectral sensors spanning
spatial resolutions from meters to kilometers all relative to the surface site rather than a single sensor. The sourcecentric
philosophy of calibrating the site inherently accounts for footprint size mismatch, spectral band mismatch, and
temporal and spatial sampling effects. The method for characterizing the test site allows its use for SI-traceable
calibration of any sensor that can view the calibrated test site. Interconsistency is obtained through the traceability and
error budget rather than coincident views. Such an approach to inter-consistency provides better understanding of biases
between sensors as well producing more accurate results with documented SI-traceability that reduces the need for
overlapping data sets.
The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission key goals include enabling
observation of high accuracy long-term climate change trends, use of these observations to test and improve climate
forecasts, and calibration of operational and research sensors. The spaceborne instrument suites include a reflected solar
(RS) spectroradiometer, emitted infrared spectroradiometer, and radio occultation receivers. The requirement for the RS
instrument is that derived reflectance must be traceable to SI standards with an absolute uncertainty of <0.3% and the
error budget that achieves this requirement is described in previous work. This work describes the Solar/Lunar Absolute
Reflectance Imaging Spectroradiometer (SOLARIS), a calibration demonstration system for RS instrument, and presents
initial calibration and characterization methods and results. SOLARIS is an Offner spectrometer with two separate focal
planes each with its own entrance aperture and grating covering spectral ranges of 320-640, 600-2300 nm over a full
field-of-view of 10 degrees with 0.27 milliradian sampling. Results from laboratory measurements including use of
integrating spheres, transfer radiometers and spectral standards combined with field-based solar and lunar acquisitions
are presented.
The National Ecological Observatory Network (NEON) is a planned facility of the National Science Foundation with the
mission to enable understanding and forecasting of the impacts of climate change, land use change and invasive species
on continental-scale ecology. Airborne remote sensing plays a critical role by providing measurements at the scale of
individual shrubs and larger plants over hundreds of square kilometers. The NEON Airborne Observation Platform is
designed to bridge scales from organism and stand scales, as captured by plot and tower observations, to the scale of
satellite based remote sensing. Fused airborne spectroscopy and waveform LiDAR is used to quantify vegetation
composition and structure. Panchromatic photography at better than 30 cm resolution will retrieve fine-scale information
on land use, roads, impervious surfaces, and built structures. NEON will build three airborne systems to allow for
regular coverage of NEON sites and the capacity to respond to investigator requests for specific projects. The system
design achieves a balance between performance and development cost and risk, taking full advantage of existing
commercial airborne LiDAR and camera components. To reduce risk during NEON construction, an imaging
spectrometer design verification unit is being developed at the Jet Propulsion Laboratory to demonstrate that operational
and performance requirements can be met. As part of this effort, NEON is also focusing on science algorithm
development, computing hardware prototyping and early airborne test flights with similar technologies. This paper
presents an overview of the development status of the NEON airborne instrumentation in the context of the NEON
mission.
Airborne remote sensing measurements provide the capability to quantitatively measure biochemical and biophysical
properties of vegetation at regional scales, therefore complementing surface and satellite measurements. The National
Ecological Observatory Network (NEON) will build three airborne systems to allow for routine coverage of NEON sites
(60 sites nationally) and the capacity to respond to investigator requests for specific projects. Each airborne system will
consist of an imaging spectrometer, waveform lidar and high-resolution digital camera. Remote sensing data gathered
with this instrumentation needs to be quantitative and accurate in order to derive meaningful information about
ecosystem properties and processes. Also, comprehensive and long-term ecological studies require these data to be
comparable over time, between coexisting sensors and between generations of follow-on sensors. NEON's calibration
plan for the airborne instrument suite relies on intensive laboratory, on-board, ground-based characterization as well as
inter-sensor comparisons. As part of these efforts, NEON organized a pathfinder mission in September 2010 to test
prototype techniques and procedures for field sampling and sensor validation. Imaging spectroscopy data from AVIRIS
and waveform lidar data were acquired in addition to ecological field sampling at the Ordway-Swisher Biological
Station near Gainesville, Florida. This paper presents NEON's capabilities for validation of at-sensor radiance of
airborne and space-based sensors and shows results from the September 2010 pathfinder mission.
The National Ecological Observatory Network (NEON) will be the first observatory network of its kind designed to
detect and enable forecasting of ecological change at continental scales over multiple decades. NEON will collect data at
sites distributed at 20 ecoclimatic domains across the United States on the impacts of climate change, land use change,
and invasive species on natural resources and biodiversity. The NEON Airborne Observation Platform (AOP) is an
aircraft platform carrying remote sensing instrumentation designed to achieve sub-meter to meter scale ground
resolution, bridging the scales from organisms and individual stands to satellite-based remote sensing. AOP
instrumentation consists of a VIS/SWIR imaging spectrometer, a scanning small-footprint waveform LiDAR, and a high
resolution airborne digital camera. AOP data will provide quantitative information on land use change and changes in
ecological structure and chemistry including the presence and effects of invasive species. A Pathfinder Flight Campaign
was conducted over a two week period during late August to early September 2010 in order to collect representative
AOP data over one NEON domain site. NASA JPL flew the AVIRIS imaging spectrometer and NCALM flew an Optech
Gemini waveform LiDAR over the University of Florida Ordway-Swisher Biological Station and Donaldson tree
plantation near Gainesville Florida. The pathfinder data are discussed in detail along with how the data are being used
for early algorithm and product development prototyping activities. The data collected during the campaign and
prototype products are openly available to scientists to become more familiar with representative NEON AOP data.
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.
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.
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 RSG's 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 Globe's 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.
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.
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%.
KEYWORDS: Calibration, Short wave infrared radiation, Sensors, Reflectivity, Radiometry, Radio optics, Space telescopes, Telescopes, Spectroscopy, Space operations
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.
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