Dennis L. Helder

Electrical Engineering

25.02

Publications

  • Remote Sensing 02/2015; 7(2):1962-1980. DOI:10.3390/rs70201962 · 3.18 Impact Factor
  • Frank Pesta · Suman Bhatta · Dennis Helder · Nischal Mishra
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    ABSTRACT: Landsat 8 is the first satellite in the Landsat mission to acquire spectral imagery of the Earth using pushbroom sensor instruments. As a result, there are almost 70,000 unique detectors on the Operational Land Imager (OLI) alone to monitor. Due to minute variations in manufacturing and temporal degradation, every detector will exhibit a different behavior when exposed to uniform radiance, causing a noticeable striping artifact in collected imagery. Solar collects using the OLI's on-board solar diffuser panels are the primary method of characterizing detector level non-uniformity. This paper reports on an approach for using a side-slither maneuver to estimate relative detector gains within each individual focal plane module (FPM) in the OLI. A method to characterize cirrus band detector-level non-uniformity using deep convective clouds (DCCs) is also presented. These approaches are discussed, and then, correction results are compared with the diffuser-based method. Detector relative gain stability is assessed using the side-slither technique. Side-slither relative gains were found to correct streaking in test imagery with quality comparable to diffuser-based gains (within 0.005% for VNIR/PAN; 0.01% for SWIR) and identified a 0.5% temporal drift over a year. The DCC technique provided relative gains that visually decreased striping over the operational calibration in many images.
    Remote Sensing 12/2014; 7(1). DOI:10.3390/rs70100430 · 3.18 Impact Factor
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    ABSTRACT: This study evaluates the radiometric consistency between Landsat-8 Operational Land Imager (OLI) and Landsat 7 Enhanced Thematic Mapper Plus (ETM+) using cross calibration techniques. Two approaches are used, one based on cross calibration between the two sensors using simultaneous image pairs, acquired during an underfly event on 29-30 March 2013. The other approach is based on using time series of image statistics acquired by these two sensors over the Libya 4 pseudo invariant calibration site (PICS) (+28.55 degrees N, + 23.39 degrees E). Analyses from these approaches show that the reflectance calibration of OLI is generally within +/- 3% of the ETM+ radiance calibration for all the reflective bands from visible to short wave infrared regions when the ChKur solar spectrum is used to convert the ETM+ radiance to reflectance. Similar results are obtained comparing the OLI radiance calibration directly with the ETM+ radiance calibration and the results in these two different physical units (radiance and reflectance) agree to within +/- 2% for all the analogous bands. These results will also be useful to tie all the Landsat heritage sensors from Landsat 1 MultiSpectral Scanner (MSS) through Landsat-8 OLI to a consistent radiometric scale.
    Canadian Journal of Remote Sensing 12/2014; 6(12). DOI:10.3390/rs61212619
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    ABSTRACT: Abstract- This study evaluates the radiometric consistency of Landsat-8 Operational Land Imager (OLI) data by cross calibrating it to Landsat 7 Enhanced Thematic Mapper Plus (ETM+). Two approaches are used, one based on cross calibration between the two sensors using simultaneous image pairs acquired during an underfly event on March 29-30, 2013. The other approach is based on using time series of image statistics acquired by these two sensors over the Libya 4 pseudo invariant calibration site (PICS) (+28.55o N, +23.39o E). Analysis from these approaches show that the reflectance calibration of OLI is generally within ±3% of ETM+ radiance calibration for all the reflective bands from visible to short wave infrared regions when the ChKur solar spectrum is used to convert the ETM+ radiance to reflectance. Similar results are obtained comparing the OLI radiance calibration directly with the ETM+ radiance calibration and the results in these two different physical units (radiance and reflectance) agree to within ±2% for all the analogous bands. These results will also be useful to tie all the Landsat heritage sensors from Landsat 1 Multi-Spectral Scanner (MSS) thorough Landsat-8 OLI to a consistent radiometric scale.
    Remote Sensing 08/2014; · 3.18 Impact Factor
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    ABSTRACT: This paper presents the vicarious calibration results of Landsat 8 OLI that were obtained using on-site personnel at test sites in Nevada, California, Arizona, and South Dakota, USA, and also by using the Radiometric Calibration Test Site (RadCaTS), which is located at Railroad Valley, Nevada, USA. The results for the top-of-atmosphere spectral radiance show an average difference of –2.9, –1.0, 1.4, 2.2, 0.1, 3.9, 6.1, and 0.8% in OLI bands 1–8 as compared to an average of all of the ground-based measurements, while the top-of-atmosphere reflectance shows an average difference of 1.4, 1.2, 2.0, 2.0, 1.1, 2.5, 3.5, and 2.2% from the average of all of the ground-based results. Except for OLI band 7, the spectral radiance results are generally within the ±5% design specification, and the reflectance results are generally within the ±3% design specification. 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 spectral radiance, and to within ±4% in reflectance.
    Remote Sensing 08/2014; DOI:10.3390/rs70100600 · 3.18 Impact Factor
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    ABSTRACT: The Operational Land Imager (OLI) on Landsat-8 has been collecting imagery on orbit for 17 months. The radiometric performance of the OLI is monitored using on-board systems (lamps and solar diffusers) as well as by reference to lunar and ground measurements and other satellite systems. Over this nearly 1½ years of operation the OLI has been extremely radiometrically stable in all of its 9 spectral bands. Only the shortest wavelength band, centered at 443 nm, which has degraded about 0.8%, has changed by more than the variability among the measurements (~0.2%). This consistency between the lamps, diffusers, moon, and ground measurements lends high confidence to these statements, which is unusual for a system so early in its lifetime. Comparisons to other satellite systems and ground measurements show that the OLI is calibrated to within requirements and generally better than 3% in both radiance and reflectance.
    SPIE Annual Meeting, Earth Observing Systems XIX, San Diego, California; 08/2014
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    ABSTRACT: The objective of this paper is to report the improvements in an empirical absolute calibration model developed at South Dakota State University using Libya 4 (+28.55 degrees, +23.39 degrees)pseudo invariant calibration site (PICS). The approach was based on use of the Terra MODIS as the radiometer to develop an absolute calibration model for the spectral channels covered by this instrument from visible to shortwave infrared. Earth Observing One (EO-1) Hyperion, with a spectral resolution of 10 nm, was used to extend the model to cover visible and near-infrared regions. A simple Bidirectional Reflectance Distribution function (BRDF) model was generated using Terra Moderate Resolution Imaging Spectroradiometer (MODIS) observations over Libya 4 and the resulting model was validated with nadir data acquired from satellite sensors such as Aqua MODIS and Landsat 7 (L7) Enhanced Thematic Mapper (ETM+). The improvements in the absolute calibration model to account for the BRDF due to off-nadir measurements and annual variations in the atmosphere are summarized. BRDF models due to off-nadir viewing angles have been derived using the measurements from EO-1 Hyperion. In addition to L7 ETM+, measurements from other sensors such as Aqua MODIS, UK-2 Disaster Monitoring Constellation (DMC), ENVISAT Medium Resolution Imaging Spectrometer (MERIS) and Operational Land Imager (OLI) onboard Landsat 8 (L8), which was launched in February 2013, were employed to validate the model. These satellite sensors differ in terms of the width of their spectral bandpasses, overpass time, off-nadir-viewing capabilities, spatial resolution and temporal revisit time, etc. The results demonstrate that the proposed empirical calibration model has accuracy of the order of 3% with an uncertainty of about 2% for the sensors used in the study.
    Remote Sensing 02/2014; 6(6):1327-1346. DOI:10.3390/rs6021327 · 3.18 Impact Factor
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    ABSTRACT: Recently, the Moderate Resolution Imaging Spectroradiometer (MODIS) Characterization Support Team (MCST) has worked closely with the science team members to make significant improvements in both Terra and Aqua MODIS sensors. These refinements are included in the newly released Collection 6 (C6) Level 1B (L1B) products, and they primarily mitigate the long-term drifts observed in the short-wavelength bands of the MODIS sensors. This letter focuses particularly on evaluating the improvement in the long-term on-orbit radiometric calibration stability of the Terra MODIS sensor by comparing the trends with the Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+) sensor. In this study, Terra MODIS L1B products from both Collection 5 (C5) and C6 were used to compare the long-term top-of-atmosphere (TOA) reflectance trending of the Terra MODIS reflective solar bands (RSB) with spectrally matching bands of the L7 ETM+ sensors over the Committee on Earth Observation Satellites (CEOS) reference pseudo-invariant calibration sites (PICS). In addition, intensive statistical tests were performed to support the assessment of the observed long-term drifts. The results from the newly processed Terra MODIS C6 L1B products clearly show the excellent calibration stability (long-term drift within 2%) of the MODIS sensor with the multi-year drifts within the specified calibration uncertainty.
    Remote Sensing Letters 09/2013; 4(9):873-881. DOI:10.1080/2150704X.2013.809496 · 1.43 Impact Factor
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    ABSTRACT: To monitor land surface processes over a wide range of temporal and spatial scales, it is critical to have coordinated observations of the Earth's surface acquired from multiple spaceborne imaging sensors. However, an integrated global observation framework requires an understanding of how land surface processes are seen differently by various sensors. This is particularly true for sensors acquiring data in spectral bands whose relative spectral responses (RSRs) are not similar and thus may produce different results while observing the same target. The intrinsic offsets between two sensors caused by RSR mismatches can be compensated by using a spectral band adjustment factor (SBAF), which takes into account the spectral profile of the target and the RSR of the two sensors. The motivation of this work comes from the need to compensate the spectral response differences of multispectral sensors in order to provide a more accurate cross-calibration between the sensors. In this paper, radiometric cross-calibration of the Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and the Terra Moderate Resolution Imaging Spectroradiometer (MODIS) sensors was performed using near-simultaneous observations over the Libya 4 pseudoinvariant calibration site in the visible and near-infrared spectral range. The RSR differences of the analogous ETM+ and MODIS spectral bands provide the opportunity to explore, understand, quantify, and compensate for the measurement differences between these two sensors. The cross-calibration was initially performed by comparing the top-of-atmosphere (TOA) reflectances between the two sensors over their lifetimes. The average percent differences in the long-term trends ranged from -5% to +6%. The RSR compensated ETM+ TOA reflectance (ETM+*) measurements were then found to agree with MODIS TOA reflectance to within 5% for all bands when Earth Observing-1 Hyperion hyperspectral data were used to produce the SBAFs. These differences were later reduced to with- n 1% for all bands (except band 2) by using Environmental Satellite Scanning Imaging Absorption Spectrometer for Atmospheric Cartography hyperspectral data to produce the SBAFs.
    IEEE Transactions on Geoscience and Remote Sensing 03/2013; 51(3):1267-1281. DOI:10.1109/TGRS.2012.2228007 · 3.51 Impact Factor
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    ABSTRACT: Pseudo invariant calibration sites (PICS) have been used for on-orbit radiometric trending of optical satellite systems for more than 15 years. This approach to vicarious calibration has demonstrated a high degree of reliability and repeatability at the level of 1-3% depending on the site, spectral channel, and imaging geometries. A variety of sensors have used this approach for trending because it is broadly applicable and easy to implement. Models to describe the surface reflectance properties, as well as the intervening atmosphere have also been developed to improve the precision of the method. However, one limiting factor of using PICS is that an absolute calibration capability has not yet been fully developed. Because of this, PICS are primarily limited to providing only long term trending information for individual sensors or cross-calibration opportunities between two sensors. This paper builds an argument that PICS can be used more extensively for absolute calibration. To illustrate this, a simple empirical model is developed for the well-known Libya 4 PICS based on observations by Terra MODIS and EO-1 Hyperion. The model is validated by comparing model predicted top-of-atmosphere reflectance values to actual measurements made by the Landsat ETM+ sensor reflective bands. Following this, an outline is presented to develop a more comprehensive and accurate PICS absolute calibration model that can be Système international d'unités (SI) traceable. These initial concepts suggest that absolute calibration using PICS is possible on a broad scale and can lead to improved on-orbit calibration capabilities for optical satellite sensors.
    IEEE Transactions on Geoscience and Remote Sensing 03/2013; 51(3):1360-1369. DOI:10.1109/TGRS.2013.2243738 · 3.51 Impact Factor
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    ABSTRACT: Cross-calibration of satellite sensors permits the quantitative comparison of measurements obtained from different Earth Observing (EO) systems. Cross-calibration studies usually use simultaneous or near-simultaneous observations from several spaceborne sensors to develop band-by-band relationships through regression analysis. The investigation described in this paper focuses on evaluation of the uncertainties inherent in the cross-calibration process, including contributions due to different spectral responses, spectral resolution, spectral filter shift, geometric misregistrations, and spatial resolutions. The hyperspectral data from the Environmental Satellite SCanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY and the EO-1 Hyperion, along with the relative spectral responses (RSRs) from the Landsat 7 Enhanced Thematic Mapper (TM) Plus and the Terra Moderate Resolution Imaging Spectroradiometer sensors, were used for the spectral uncertainty study. The data from Landsat 5 TM over five representative land cover types (desert, rangeland, grassland, deciduous forest, and coniferous forest) were used for the geometric misregistrations and spatial-resolution study. The spectral resolution uncertainty was found to be within 0.25%, spectral filter shift within 2.5%, geometric misregistrations within 0.35%, and spatial-resolution effects within 0.1% for the Libya 4 site. The one-sigma uncertainties presented in this paper are uncorrelated, and therefore, the uncertainties can be summed orthogonally. Furthermore, an overall total uncertainty was developed. In general, the results suggested that the spectral uncertainty is more dominant compared to other uncertainties presented in this paper. Therefore, the effect of the sensor RSR differences needs to be quantified and compensated to avoid large uncertainties in cross-calibration results.
    IEEE Transactions on Geoscience and Remote Sensing 03/2013; 51(3):1282-1296. DOI:10.1109/TGRS.2012.2228008 · 3.51 Impact Factor
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    ABSTRACT: Pre-flight radiometric calibration is critical to ensure that sensors can demonstrate that they meet their design performance specification. However, following launch it is critical that this performance is verified and in particular that the radiometric accuracy is validated or if necessary corrected. As Earth observation data becomes widely used it becomes increasingly important that data from each satellite sensor can be relied upon in the short and longer term and any potential biases between sensors can be removed. This post-launch calibration/validation is thus recognized as a key activity by all satellite operators. There are various methods available for carrying out this post-launch vicarious calibration but one of the most common and generic approaches is to use a dedicated and characterised “test site”. In such a method, ground based measurements of surface reflectance/radiance using similar solar illumination angles and sensor view angles (or at least corrected for these) is propagated to the top of the atmosphere (TOA) using a radiative transfer code. For the highest accuracy, surface measurements should be made within a few minutes of the satellite overpass and the characteristics of the atmosphere at that time also measured, particularly its optical depth.
    IGARSS 2012; 07/2012
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    Brian L. Markham · Dennis L. Helder
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    ABSTRACT: Sensors on Landsat satellites have been collecting images of the Earth's surface for nearly 40 years. These images have been invaluable for characterizing and detecting changes in the land cover and land use of the world. Although initially conceived as primarily picture generating sensors, even the early sensors were radiometrically calibrated and spectrally characterized prior to launch and incorporated some capabilities to monitor their radiometric calibration once on orbit. Recently, as the focus of studies has shifted to monitoring Earth surface parameters over significant periods of time, serious attention has been focused toward bringing the data from all these sensors onto a common radiometric scale over this 40-year period. This effort started with the most recent systems and then was extended back in time. Landsat-7 Enhanced Thematic Mapper (ETM)+, the best-characterized sensor of the series prior to launch and once on orbit, and the most stable system to date, was chosen to serve as the reference. The Landsat-7 project was the first of the series to build an image assessment system into its ground system, allowing systematic characterization of its sensors and data. Second, the Landsat-5 Thematic Mapper (TM) (still operating at the time of the Landsat-7 launch and continues to operate) calibration history was reconstructed based on its internal calibrator, vicarious calibrations, pseudo-invariant sites and a tie to Landsat-7 ETM + at the time of the commissioning of Landsat-7. This process was performed in two iterations: the earlier one relied primarily on the TM internal calibrator. When this was found to have some deficiencies, a revised calibration was based more on pseudo-invariant sites, though the internal calibrator was still used to establish the short-term variations in response due to contaminant build up on the cold focal plane. As time progressed, a capability to monitor the Landsat-5 TM was added to the image assessment system. The Landsat-4 TM, which operated from 1982 to 1992, was the third system to which the radiometric scale was extended. The limited and broken use of the Landsat-4 TM made this analysis more difficult. Eight-day separated image pairs from Landsat-5 combined with analysis of pseudo invariant sites established this history. The fourth and most challenging effort was making the Landsat 1–5 Multi-Spectral Scanner (MSS) sensors' data internally radiometrically consistent. This effort was particularly complicated by the age of the MSS data, varying formats and processing levels in the archive, limited datasets, and limited documentation available. Ultimately, pseudo-invariant sites were identified in North America and used for this effort. Note that most of the Landsat MSS archived data had already been calibrated using the MSS internal calibrators, so this processing was embedded in the result. The final effort was developing an absolute scale for Landsat MSS similar to what was already established for the “TM” sensors. This was accomplished by using simultaneous data from Landsat-5 MSS and Landsat-5 TM, accounting for spectral differences between the sensors using EO-1 Hyperion data. The recalibrated history of the Landsat data and implications to users are discussed. The key result from this work is a consistently calibrated Landsat data archive that spans nearly 40 years with total uncertainties on the order of 10% or less for most sensors and bands.
    Remote Sensing of Environment 07/2012; 122:30–40. DOI:10.1016/j.rse.2011.06.026 · 6.39 Impact Factor
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    ABSTRACT: Multispectral remote sensing of the Earth using Landsat sensors was ushered on July 23, 1972, with the launch of Landsat-1. Following that success, four more Landsat satellites were launched, and each of these carried the Multispectral Scanner System (MSS). These five sensors provided the only consistent multispectral space-based imagery of the Earth's surface from 1972 to 1982. This work focuses on developing both a consistent and absolute radiometric calibration of this sensor system. Cross-calibration of the MSS was performed through the use of pseudoinvariant calibration sites (PICSs). Since these sites have been shown to be stable for long periods of time, changes in MSS observations of these sites were attributed to changes in the sensors themselves. In addition, simultaneous data collections were available for some MSS sensor pairs, and these were also used for cross-calibration. Results indicated substantial differences existed between instruments, up to 16%, and these were reduced to 5% or less across all MSS sensors and bands. Lastly, this paper takes the calibration through the final step and places the MSS sensors on an absolute radiometric scale. The methodology used to achieve this was based on simultaneous data collections by the Landsat-5 MSS and Thematic Mapper (TM) instruments. Through analysis of image data from a PICS location and through compensating for the spectral differences between the two instruments, the Landsat-5 MSS sensor was placed on an absolute radiometric scale based on the Landsat-5 TM sensor. Uncertainties associated with this calibration are considered to be less than 5%.
    IEEE Transactions on Geoscience and Remote Sensing 06/2012; 50(6):2380-2399. DOI:10.1109/TGRS.2011.2171351 · 3.51 Impact Factor
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    ABSTRACT: A significant problem facing the optical satellite calibration community is limited knowledge of the uncertainties associated with fundamental measurements, such as surface reflectance, used to derive satellite radiometric calibration estimates. In addition, it is difficult to compare the capabilities of calibration teams around the globe, which leads to differences in the estimated calibration of optical satellite sensors. This paper reports on two recent field campaigns that were designed to isolate common uncertainties within and across calibration groups, particularly with respect to ground-based surface reflectance measurements. Initial results from these efforts suggest the uncertainties can be as low as 1.5% to 2.5%. In addition, methods for improving the cross-comparison of calibration teams are suggested that can potentially reduce the differences in the calibration estimates of optical satellite sensors.
    Metrologia 04/2012; 49(2). DOI:10.1088/0026-1394/49/2/S21 · 2.04 Impact Factor
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    ABSTRACT: The Landsat 4 Thematic Mapper (TM) collected imagery of the Earth's surface from 1982 to 1993. Although largely overshadowed by Landsat 5 which was launched in 1984, Landsat 4 TM imagery extends the TM-based record of the Earth back to 1982 and also substantially supplements the image archive collected by Landsat 5. To provide a consistent calibration record for the TM instruments, Landsat 4 TM was cross-calibrated to Landsat 5 using nearly simultaneous overpass imagery of pseudo-invariant calibration sites (PICS) in the time period of 1988–1990. To determine if the radiometric gain of Landsat 4 had changed over its lifetime, time series from two PICS locations (a Saharan site known as Libya 4 and a site in southwest North America, commonly referred to as the Sonoran Desert site) were developed. The results indicated that Landsat 4 had been very stable over its lifetime, with no discernible degradation in sensor performance in all reflective bands except band 1. In contrast, band 1 exhibited a 12% decay in responsivity over the lifetime of the instrument. Results from this paper have been implemented at USGS EROS, which enables users of Landsat TM data sets to obtain consistently calibrated data from Landsat 4 and 5 TM as well as Landsat 7 ETM+ instruments.
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    ABSTRACT: The Landsat-7 ETM+ sensor has been operating on orbit for more than 12 years, and characterizations of its performance have been ongoing over this period. In general, the radiometric performance of the instrument has been remarkably stable: 1) noise performance has degraded by 2% or less overall, with a few detectors displaying step changes in noise of 2% or less; 2) coherent noise frequencies and magnitudes have generally been stable, though the within-scan amplitude variation of the 20 kHz noise in bands 1 and 8 disappeared with the failure of the scan line corrector and a new similar frequency noise (now about 18 kHz) has appeared in two detectors in band 5 and increased in magnitude with time; 3) bias stability has been better than 0.25 DN out of a normal value of 15 DN in high gain; 4) relative gains, the differences in response between the detectors in the band, have generally changed by 0.1% or less over the mission, with the exception of a few detectors with a step response change of 1% or less; and 5) gain stability averaged across all detectors in a band, which is related to the stability of the absolute calibration, has been more stable than the techniques used to measure it. Due to the inability to confirm changes in the gain (beyond a few detectors that have been corrected back to the band average), ETM+ reflective band data continues to be calibrated with the prelaunch measured gains. In the worst case, some bands may have changed as much as 2% in uncompensated absolute calibration over the 12 years.
    IEEE Transactions on Geoscience and Remote Sensing 01/2012; 50(5):2056-2062. · 3.51 Impact Factor
  • 01/2012; XXXIX-B1:145-148. DOI:10.5194/isprsarchives-XXXIX-B1-145-2012
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    ABSTRACT: A field campaign had been organized in August 2010 on Tüz Gölü salt lake, Turkey, with the aim of characterizing the site for satellite optical sensor vicarious calibration, and of comparing different methodologies of surface reflectance factor characterization. Several teams have made groundbased reflectance measurements with a field spectrometer on different areas of the salt lake of 100 m x 300 m and 1km x 1 km size. Different types of sampling strategies and measurements methods have been used by the participants, and are described in this paper. Preliminary results on one area are presented, that show a good agreement between the different measurements.
    IGARSS; 01/2012
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    ABSTRACT: A field campaign had been organized in August 2010 on Tüz Gölü salt lake, Turkey, with the aim of characterizing the site for satellite optical sensor vicarious calibration, and of comparing different methodologies of surface reflectance factor characterization. Several teams have made groundbased reflectance measurements with a field spectrometer on different areas of the salt lake of 100 m x 300 m and 1km x 1 km size. Different types of sampling strategies and measurements methods have been used by the participants, and are described in this paper. Preliminary results on one area are presented, that show a good agreement between the different measurements.
    2011 IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2011, Vancouver, BC, Canada, July 24-29, 2011; 01/2011

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