Ground-based Spectroradiometric Measurements Research Articles

Retrieving LAI, chlorophyll and nitrogen contents in sugar beet crops from multi-angular optical remote sensing: Comparison of vegetation indices and PROSAIL inversion for field phenotyping

Remote sensing has gained much attention for agronomic applications such as crop management or yield estimation. Crop phenotyping under field conditions has recently become another important application that requires specific needs: the considered remote-sensing method must be (1) as accurate as possible so that slight differences in phenotype can be detected and related to genotype, and (2) robust so that thousands of cultivars potentially quite different in terms of plant architecture can be characterized with a similar accuracy over different years and soil and weather conditions. In this study, the potential of nadir and off-nadir ground-based spectro-radiometric measurements to remotely sense five plant traits relevant for field phenotyping, namely, the leaf area index (LAI), leaf chlorophyll and nitrogen contents, and canopy chlorophyll and nitrogen contents, was evaluated over fourteen sugar beet (Beta vulgaris L.) cultivars, two years and three study sites. Among the diversity of existing remote-sensing methods, two popular approaches based on various selected Vegetation Indices (VI) and PROSAIL inversion were compared, especially in the perspective of using them for phenotyping applications.Overall, both approaches are promising to remotely estimate LAI and canopy chlorophyll content (RMSE≤10%). In addition, VIs show a great potential to retrieve canopy nitrogen content (RMSE=10%). On the other hand, the estimation of leaf-level quantities is less accurate, the best accuracy being obtained for leaf chlorophyll content estimation based on VIs (RMSE=17%). As expected when observing the relationship between leaf chlorophyll and nitrogen contents, poor correlations are found between VIs and mass-based or area-based leaf nitrogen content. Importantly, the estimation accuracy is strongly dependent on sun-sensor geometry, the structural and biochemical plant traits being generally better estimated based on nadir and off-nadir observations, respectively. Ultimately, a preliminary comparison tends to indicate that, providing that enough samples are included in the calibration set, (1) VIs provide slightly more accurate performances than PROSAIL inversion, (2) VIs and PROSAIL inversion do not show significant differences in robustness across the different cultivars and years. Even if more data are still necessary to draw definitive conclusions, the results obtained with VIs are promising in the perspective of high-throughput phenotyping using UAV-embedded multispectral cameras, with which only a few wavebands are available.

Determining the Most Appropriate Classification Methods for Water Quality

Assessing water resources’ quality and also monitoring them have attracted lots of attention in the recent years. Remote sensing has been growing widely in the last decade and its resources are very usable when it comes to water resources management. In this study, by using remote sensing technology, satellite images that have 350 to 1050 nanometres wavelength band sensors are used to determine the quality of the Kizilirmak River's water. Through the river's resources, ground based spectral measurements are made to identify the quality differences of the water at the test spots that have been determined before. In this context at Imranli, where the river contacts civilization for the first time, which is located in Sivas city of Turkey, samples are gathered in order to do ground based spectroradiometer measurements. These samples are gathered simultaneously with the image acquiring time of CHRIS Proba satellite. Spectral signatures that are obtained from ground measurements are used as reference data in order to classify CHRIS Proba satellite's hyperspectral images over the study area. Satellite images are classified based on Chemical Oxygen Demand (COD), Turbidity and Electrical Conductivity (EC) attributes. As a result, interpretations obtained from classified CHRIS Proba satellite hyperspectral images of the study area are presented. Spectras are readied for Matched Filtering and Spectral Angle Mapper methods for determining the best classification method.

Determining and Monitoring the Water Quality of Kizilirmak River of Turkey: First Results

Abstract. Water resources are getting more and more important with each passing day in case of survival of humanity. For this reason, assessing water resources’ quality and also monitoring them have attracted lots of attention in the recent years. Remote sensing has been growing widely in the last decade and its resources are very usable when it comes to water resources management. In this study, by using remote sensing technology, satellite images that have 350 to 1050 nanometres wavelength band sensors (e.g. CHRIS Proba) are used to determine the quality of the Kizilirmak River’s water. Kizilirmak River is born and also pours out to sea in country limits of Turkey. It is the longest river of the country by the length of 1355 kilometres. Through the river’s resources, ground based spectral measurements are made to identify the quality differences of the water at the test spots that have been determined before. In this context at Imranli, where the river contacts civilization for the first time, which is located in Sivas city of Turkey, samples are gathered in order to do ground based spectroradiometer measurements. These samples are gathered simultaneously with the image acquiring time of CHRIS Proba satellite. Spectral signatures that are obtained from ground measurements are used as reference data in order to classify CHRIS Proba satellite’s hyperspectral images over the study area. Satellite images are classified based on Chemical Oxygen Demand (COD), Turbidity and Electrical Conductivity (EC) attributes. As a result, interpretations obtained from classified CHRIS Proba satellite hyperspectral images of the study area are presented.

Validation of total ozone column derived from OMPS using ground-based spectroradiometer measurements

The total ozone column (TOC) observations derived from the Ozone Mapping and Profiler Suit (OMPS) on-board the Suomi National Polar-orbiting Partnership (Suomi NPP) spacecraft were recently released for monitoring ozone in the stratosphere. Two kinds of TOC data which are derived using the NPOESS algorithm (OMPS-NPOESS) and the TOMS Version 7 algorithm (OMPS-TOMS) are provided. So far, few studies have been conducted to validate the accuracy of the OMPS TOC data. In this letter, we validate a 1 year OMPS-derived TOC data set (from March 2012 to February 2013) by comparing them with ground-based spectroradiometer data. We also examine the difference of the data derived using OMPS-NPOESS and OMPS-TOMS algorithms. Our results show a moderate correlation between OMPS-derived TOC and ground-based data, and the average relative difference between the two data sets (OMPS data minus the ground-based data) is negative, which indicates an underestimation of the TOC in the OMPS data. Such relative difference is not globally consistent: smaller values (around –10% to 0%) are found in low latitudinal areas (from 30° S to 30° N), whereas comparatively larger differences (around –25% to –15%) are detected in data on the mid-high latitudinal areas. A comparison of the two different algorithms suggests that the OMPS-TOMS-derived data have a generally better accuracy than those from OMPS-NPOESS.

Using field spectroscopy to assess the potential of statistical approaches for the retrieval of sun-induced chlorophyll fluorescence from ground and space

Sun-induced chlorophyll fluorescence (Fs) is an electromagnetic signal emitted in the 650–800nm spectral window by the chlorophyll-a of green leaves. Previous studies demonstrated the retrieval of Fs on a global scale using high spectral resolution measurements by the Fourier Transform Spectrometer (FTS) on board the greenhouse gases observing satellite (GOSAT). The retrieval of Fs from GOSAT-FTS data is based on the modeling of the in-filling of solar Fraunhofer lines by Fs. The first Fs retrieval methods for GOSAT-FTS measurements were based on physical formulations of the radiative transfer between the atmosphere, the surface and the instrument including the Fs emission. As an alternative, a statistical method was also successfully applied to GOSAT data. This method is based on a singular vector decomposition (SVD) technique producing a basis of spectral functions able to model the contribution of the reflected solar radiation to the top-of-atmosphere measurement in a linear way. The Fs signal is included in the forward model as an extra parameter adding to the reflected solar radiation. Here, we use field spectroscopy measurements to provide further experimental evidence on the retrieval of Fs with statistical approaches in both Fraunhofer lines and atmospheric oxygen and water vapor bands. The statistical retrieval method used with GOSAT-FTS data has been adapted to a set of ground-based spectro-radiometer measurements in the 717–780nm range. Retrieval results in the 745–759nm window, which contains only Fraunhofer lines, support the overall approach of estimating Fs from space measurements in that spectral window. Furthermore, the application of the method to broader fitting windows including both Fraunhofer lines and and (oxygen and water vapor) atmospheric bands atmospheric bands has been proven to be very effective to reduce the retrieval noise and has also shown a good comparison with reference O2A-based retrievals. This allows consideration of statistical methods as a powerful option for Fs retrieval from broad-band space-based measurements in the near-infrared.

Retrieval of atmospheric optical parameters from ground-based sun-photometer measurements for Zanjan, Iran

Abstract. We are reporting the results of ground-based spectroradiometric measurements on aerosols and water vapor in the atmosphere of Zanjan for the period of October 2006 to September 2008 using a CIMEL CE318-2 sun-photometer. Zanjan is a city in Northwest Iran, located at 36.70° N, 48.51° E, and at an altitude of 1800 m a.m.s.l. (above mean sea level). The spectral aerosol optical depth, Ångström exponent, and columnar water vapor have been calculated using the data recorded by the sun-photometer through the direct measurements on the sun radiance (sun-mode). The average values of aerosol optical depth at 440 nm, columnar water vapor, and the Ångström exponent, α, during the mentioned period are measured as, 0.28 ± 0.14, 0.57 ± 0.37 cm and 0.73 ± 0.30, respectively. The maximum (minimum) value of the aerosol optical depth was recorded in May 2007 (November 2007), and that of columnar water vapor, in July 2007 (January 2008). Using the least-squares method, the Ångström exponent was calculated in the spectral interval 440–870 nm along with α1 and α2, the coefficients of a second order polynomial fit to the plotted logarithm of aerosol optical depth versus the logarithm of wavelength. The coefficient α2 shows that most of the aerosols in the Zanjan area have dimensions larger than 1 micron. The calculated values for α2 − α1 indicate that 80 % of the aerosols are in the coarse-mode (>1 μm) and 20 % of them are in the fine-mode (<1 μm). Comparison of α2 − α1 for the atmosphere over Zanjan with other regions indicates dust particles are the most dominant aerosols in the region.

An Evaluation of the Use of Atmospheric and BRDF Correction to Standardize Landsat Data

Normalizing for atmospheric and land surface bidirectional reflectance distribution function (BRDF) effects is essential in satellite data processing. It is important both for a single scene when the combination of land covers, sun, and view angles create anisotropy and for multiple scenes in which the sun angle changes. As a consequence, it is important for inter-sensor calibration and comparison. Procedures based on physics-based models have been applied successfully with the Moderate Resolution Imaging Spectroradiometer (MODIS) data. For Landsat and other higher resolution data, similar options exist. However, the estimation of BRDF models using internal fitting is not available due to the smaller variation of view and solar angles and infrequent revisits. In this paper, we explore the potential for developing operational procedures to correct Landsat data using coupled physics-based atmospheric and BRDF models. The process was realized using BRDF shape functions derived from MODIS with the MODTRAN 4 radiative transfer model. The atmospheric and BRDF correction algorithm was tested for reflectance factor estimation using Landsat data for two sites with different land covers in Australia. The Landsat reflectance values had a good agreement with ground based spectroradiometer measurements. In addition, overlapping images from adjacent paths in Queensland, Australia, were also used to validate the BRDF correction. The results clearly show that the algorithm can remove most of the BRDF effect without empirical adjustment. The comparison between normalized Landsat and MODIS reflectance factor also shows a good relationship, indicating that cross calibration between the two sensors is achievable.

Spectral investigation of the diffuse-to-direct solar beam irradiances ratio (UV–VIS) in the urban Athens atmosphere

This study explores the influence of air gaseous pollutants–aerosols and solar zenith angle (SZA) on the spectral diffuse-to-direct beam E dλ/E bλ irradiances ratio. It does so using ground-based spectroradiometric measurements taken over the Athens atmosphere during May 1995. It was found that the spectral E dλ/E bλ ratio decreases rapidly with increasing wavelength and regression curves of the form E dλ/E bλ = aλ−b fitted the experimental data. These curves are strongly modified by aerosols–air pollutants, aerosol optical properties, and SZA. The log–log plot of E dλ/E bλ versus λ reveals a significant departure from linearity, which is likely to be associated with aerosol physical properties and SZA effects. The effect of atmospheric turbidity, as expressed through the aerosol optical at 500 nm and SZA on the spectral E dλ/E bλ ratio, is investigated in detail for two discernible atmospheric conditions observed in the urban Athens atmosphere. The first case includes different atmospheric turbidity levels under the same SZA, while the second corresponds to different SZA values under the same turbidity levels. It was found that the correlation between E dλ/E bλ and spectral aerosol optical depth can be a useful tool in determining the aerosol optical properties and aerosol types composition.

Advantages of a two band EVI calculated from solar and photosynthetically active radiation fluxes

A two band Enhanced Vegetation Index (EVI2) without the blue band reflectance has recently been developed as a proxy for the phenology, quantity, and activity of vegetation. We compared the ability of EVI2 and the more commonly used Normalized Difference Vegetation Index (NDVI) to resolve differences in surface greenness and Leaf Area Index (LAI) among three sites located along a burn severity gradient in arctic tundra. We calculated vegetation indices from solar and photosynthetically active radiation fluxes, and validated these calculations against vegetation indices from the Terra MODerate resolution Imaging Spectroradiometer (MODIS) and ground-based spectroradiometer measurements. EVI2 performed slightly better than NDVI when comparing tower derived vegetation indices to MODIS and spectroradiometer derived vegetation indices. Burn severity decreased albedo and resulted in differences in soil background reflectance among sites. Soil darkening had no effect on EVI2, but artificially increased NDVI, resulting in separate relationships between NDVI and Leaf Area Index for burned and unburned tundra. Our results indicate that EVI2 has several advantages over NDVI including the ability to resolve LAI differences for vegetation with different background soil reflectance.

Comparison of ground reflectance measurement with satellite derived atmospherically corrected reflectance: A case study over semi-arid landscape

Atmospheric corrections to satellite data are important for comparing multitemporal data sets over tropical regions with variable aerosol loading. In this study, we evaluated the potential of 6S radiative transfer model for atmospheric corrections of IRS-P6 AWiFS satellite data sets, in a semi-arid landscape. Ground measurements of surface reflectance representing different land use/land cover categories were conducted to relate IRS-P6 AWiFS top of atmospheric reflectance. The 6S radiative transfer model was calibrated for local conditions using ground measurements for aerosol optical depth, water vapor and ozone with a sun photometer. Surface reflectance retrieved from 6S code was compared with top of atmosphere (TOA) reflectance and ground based spectroradiometer measurements. Accurate parameterization of the 6S model using measurements of aerosol optical depth, water vapor and ozone plays an important role while comparing ground and satellite derived reflectance measurements.

Comparison of the Ångström parameters retrieval in different spectral ranges with the use of different techniques

Ground-based spectroradiometric measurements obtained in Athens have been used in order to derive the aerosol optical depth and the Angstrom parameters. Their derivation is achieved using three well-established and widely used techniques; the Volz method, the direct method and the least-squares fit to the experimental aerosol optical depth values. This study aims at investigating the ability of the different methods to derive similar Angstrom turbidity coefficients and their dependence on the spectral range used for their determination. In addition, the uncertainties revealed especially in the UV spectral band are highlighted. The various techniques lead to different Angstrom turbidity coefficients especially if narrow spectral bands at the shorter wavelengths are used. It is also established that the Angstrom turbidity coefficients derived by any of the three methods at short wavelengths are not representative of the whole spectrum, while their derivation exhibits large uncertainties especially under low turbidity conditions. Therefore, the comparison of α and β values obtained using the three above-mentioned techniques in several spectral bands is not an easy task, since these values differ significantly. From the whole analysis it is established that the least-squares method is the least imprecise, also exhibiting the least wavelength dependence.

On the characterization of aerosols using the Ångström exponent in the Athens area

Ground-based spectroradiometric measurements were taken in the atmosphere of Athens during May 1995 in order to investigate various atmospheric conditions. This study focuses on the Ångström exponent α, which is the slope of the logarithm of the aerosol optical depth (AOD) versus the logarithm of the wavelength, ln λ, and is commonly used to characterize the wavelength dependence of AOD and to provide some basic information on the aerosol size distribution. Using the Volz method, Ångström exponent values, α, were derived in five narrow spectral bands, 340–380, 380–440, 440–500, 500–670 and 670–870 nm. Also using both Volz and least-squares fit methods α values were derived for the whole spectrum 340-870 nm.The results show that α depends strongly on the wavelength interval used due to the curvature of the lnAOD versus ln λ line. Using the spectral dependence of the Ångström exponent and the relationship between α computed in different spectral bands with AOD, an attempt to investigate on the aerosol types is made. The results obtained are rather contradictory and the determination of a dominant aerosol type in the Athens area is not clear. Nevertheless, in the most cases the anthropogenic aerosols seem to be the dominant type with rather significant contributions of coarse-mode particles due to particle growing or due to mixing processes with other aerosol types.

Comparison between experimental data and modeling estimates of aerosol optical depth over Athens, Greece

Ground-based spectroradiometric measurements taken in the atmosphere of Athens during May 1995 have been used to investigate various atmospheric parameters. From the direct-beam irradiance measurements the spectral aerosol optical depths as well as the Ångström parameters are derived using the least-squares method applied to measured spectral irradiance data. In this way the errors associated with the turbidity coefficient, β, are found to be 0.0029 on average and with the wavelength exponent, α, 0.031. In this paper, a comparative study between the aerosol optical depth derived by the spectral direct-beam irradiance measurements and those estimated through the Ångström's formula is made. The correlation coefficient between the measured and computed optical depth is higher than 0.93 in most cases, while their greatest differences take place at the shorter wavelengths. A further analysis of the above differences at 500 and 1000 nm shows very low values, especially at 500 nm, indicating the accuracy of the fitting. The differences at 1000 nm are much higher due to uncertainties in the water-vapor optical depth determination. The errors in Ångström parameters as well as in the Ångström's formula fitting to the derived spectral aerosol optical depth depend strongly on atmospheric turbidity, reaching to higher values under low atmospheric turbidity conditions.

Modification of solar radiation components under different atmospheric conditions in the Greater Athens Area, Greece

The influence of the atmospheric turbidity on the spectral distribution of solar irradiance components is investigated using ground-based spectroradiometric measurements taken in Athens area during May 1995. It is found that both the diffuse-to-global and diffuse-to-direct-beam irradiance ratios exhibits a strong wavelength dependence and exponential curves associated with 99% of the variance can fit each parameter. These exponential curves are further modified as function of the solar zenith angle and atmospheric turbidity conditions. It is found that the slope of the curves strongly depends on the processes attenuating irradiance and aerosol optical characteristics in the short wavelengths. New relations are proposed, which allow the spectral distribution of diffuse irradiance to be estimated as a function of the measured broadband global and diffuse solar irradiances. The diffuse-to-direct-beam ratio, which is an indicator of the atmospheric transmittance, exhibits a strong wavelength and aerosol-loading dependence. The observed differences between turbid urban and clean rural atmospheres constitute a manifestation of contrasting air properties and influence solar irradiance spectra.

A spectral index for land degradation mapping using ASTER data: Application to a semi-arid Mediterranean catchment

Flagrant soil erosion in Morocco is an alarming sign of soil degradation. Due to the considerable costs of detailed ground surveys of this phenomenon, remote sensing is an appropriate alternative for analyzing and evaluating the risks of the expansion of soil degradation. In this paper, we characterize the state of land degradation in a small Mediterranean watershed using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data and ground-based spectroradiometric measurements. The two visible, the near-infrared and six shortwave infrared bands of the above sensor were calibrated using ground measurements of the spectral reflectance. Field measurements were carried out in the Saboun experimental basin located in the marl soil region of the Moroccan western Rif. The study leads to the development and evaluation of a new spectral approach to express land degradation. This index called Land degradation index (LDI) is based on the concept of the soil line derived from spectroradiometric ground measurements. In this study, we compare LDI and the spectral angle mapping (SAM) approaches to assess and map land degradation. Results show that LDI provides more accurate results for mapping land degradation (Kappa=0.79) when compared to the SAM method (Kappa=0.61). Validation and evaluation of the results are based on the thematic maps derived from the ground data (organic matter, clay, silt and sand) by kriging, DEM, slope gradient and photointerpretation.

Comparison between in situ and MODIS-derived spectral reflectances of snow and sea ice in the Amundsen Sea, Antarctica

The spectral albedo and directional reflectance of snow and sea ice were measured on sea ice of various types, including nilas, grey ice, pancake ice, multi-year pack ice, and land-fast ice in the Ross, Amundsen and Bellingshausen seas during a summer cruise in February through March 2000. Measurements were made using a spectroradiometer that has 512 channels in the visible and near-infrared (VNIR) region in which 16 of the 36 bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are covered. Directional reflectance is also retrieved from the MODIS radiometrically calibrated data (Level 1B) concurrently acquired from the first National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) satellite, Terra. The locations of the ground ice stations are identified accurately on the MODIS images, and the spectral albedo and directional reflectance values at the 16 VNIR MODIS bands are extracted for those pixel locations. MODIS-derived reflectance is then corrected for the intervening atmosphere whose parameters are retrieved from the MODIS atmospheric profiles product (MOD07_L2) for the same granule. The corresponding spectral albedo and directional reflectance with the same viewing geometry as MODIS are derived from our ground-based spectroradiometer measurements. Because the footprint of the ground spectroradiometer is much smaller than the pixel sizes of MODIS images, the averaged spectral reflectance and albedo in the vicinity of each ice station are simulated for the corresponding MODIS pixel from the ground spectral measurements by weighting over different surface types (various ice types and open water). An accurate determination of ice concentration is important in deriving ground reflectance of a simulated pixel from in situ measurements. The best agreement between the in situ and MODIS measurements was found when the ground had 10/10 ice concentration (discrepancy range 0.2–11.69%, average 4.8%) or was oneice-type dominant (discrepancy range 0.8–16.9%, average 6.2%). The more homogeneous the ground surface and the less variable the ground topography, the more comparable between the in situ and satellite-derived reflectance is expected.

Spectral Solar Irradiance And Some Optical Properties For Various Polluted Atmospheres

Using ground-based spectroradiometric measurements taken over the Athens atmosphere during May 1995, the influence of gaseous pollutants and aerosol on the spectral radiant energy distribution was investigated. It was found that spectral measurements exhibited variations based on various polluted urban atmospheric conditions as determined via gaseous pollutants record analysis. The relative attenuations caused by gaseous pollutants and aerosol can exceed 27%, 17% and 16% in the global ultraviolet, visible and near-infrared portions of the solar spectrum respectively, as compared to “background” values. In contrast, an enhancement of the near-infrared diffuse component by 66%, was observed, while in visible and ultraviolet bands the relative increases reached 54% and 21% respectively. Experimental total Rayleigh-corrected and spectral aerosol optical depths were retrieved, representing differences in polluted air over the Athens atmosphere. The diffuse component accounts for more than 80% of the total radiation field under high polluted atmosphere. The observed differences of solar radiation between the Athens center and at a nearby suburban site are a manifestation of contrasting air properties provided mainly by automotive traffic.

On dynamics of irradiation distribution field and its impact on hemispherical canopy reflectance

Hemispherical canopy reflectance is not only influenced by the optical properties of canopy, soil background effects and the geometrical arrangement of the constituent elements but it also varies with the differing distributions of the irradiation over the upper hemisphere. The present paper is addressing this problem through ground-based spectroradiometric measurements of bidirectional reflectance distribution functions in case of two crops wheat and gram which differ significantly in their canopies. It was found that the variability of the reflected radiation field due to the anistropic irradiation distribution was maximum in the case of gram than wheat at wavelengths 670 and 860 nm respectively. In addition, the angular anisotropy of the reflectivity at both the wavelengths 670 and 860 nm was found to have the higher values towards off-nadir angles than the values at nadir Moreover, the variability was more pronounced in forward and backward scattering directions.

Assessment of Atmospheric Limitations on the Determination of the Solar Spectral Constant from Ground-Based Spectroradiometer Measurements

Ground-based solar radiometer measurements have long been used to investigate various properties of both the Earth's atmosphere and the sun. This paper addresses the problem of attempting to measure the solar spectral irradiance with heretofore unachieved levels of accuracy and precision (~0.5 and ~0.1 percent, respectively) via spectroradiometer measurements made at high-altitude ground stations. Instrumentation and calibration approaches are discussed, but attention is primarily directed toward assessing limitations imposed by the atmosphere. Assessments of factors such as diffuse light contributions, uncertainty in airmass determination, variability in atmospheric optical depth, spectroradiometer bandwidth, and data analysis methods are included in the paper.