Changping Huang

Laboratory Calibration of a Field Imaging Spectrometer System

A new Field Imaging Spectrometer System (FISS) based on a cooling area CCD was developed. This paper describes the imaging principle, structural design, and main parameters of the FISS sensor. The FISS was spectrally calibrated with a double grating monochromator to determine the center wavelength and FWHM of each band. Calibration results showed that the spectral range of the FISS system is 437–902 nm, the number of channels is 344 and the spectral resolution of each channel is better than 5 nm. An integrating sphere was used to achieve absolute radiometric calibration of the FISS with less than 5% calibration error for each band. There are 215 channels with signal to noise ratios (SNRs) greater than 500 (62.5% of the bands). The results demonstrated that the FISS has achieved high performance that assures the feasibility of its practical use in various fields.

Monitoring and Assessing the 2012 Drought in the Great Plains: Analyzing Satellite-Retrieved Solar-Induced Chlorophyll Fluorescence, Drought Indices, and Gross Primary Production

We examined the relationship between satellite measurements of solar-induced chlorophyll fluorescence (SIF) and several meteorological drought indices, including the multi-time-scale standard precipitation index (SPI) and the Palmer drought severity index (PDSI), to evaluate the potential of using SIF to monitor and assess drought. We found significant positive relationships between SIF and drought indices during the growing season (from June to September). SIF was found to be more sensitive to short-term SPIs (one or two months) and less sensitive to long-term SPI (three months) than were the normalized difference vegetation index (NDVI) or the normalized difference water index (NDWI). Significant correlations were found between SIF and PDSI during the growing season for the Great Plains. We found good consistency between SIF and flux-estimated gross primary production (GPP) for the years studied, and synchronous declines of SIF and GPP in an extreme drought year (2012). We used SIF to monitor and assess the drought that occurred in the Great Plains during the summer of 2012, and found that although a meteorological drought was experienced throughout the Great Plains from June to September, the western area experienced more agricultural drought than the eastern area. Meanwhile, SIF declined more significantly than NDVI during the peak growing season. Yet for senescence, during which time the reduction of NDVI still went on, the reduction of SIF was eased. Our work provides an alternative to traditional reflectance-based vegetation or drought indices for monitoring and assessing agricultural drought.

Comparison of the Continuity of Vegetation Indices Derived from Landsat 8 OLI and Landsat 7 ETM+ Data among Different Vegetation Types

Landsat 8, the most recently launched satellite of the series, promises to maintain the continuity of Landsat 7. However, in addition to subtle differences in sensor characteristics and vegetation index (VI) generation algorithms, VIs respond differently to the seasonality of the various types of vegetation cover. The purpose of this study was to elucidate the effects of these variations on VIs between Operational Land Imager (OLI) and Enhanced Thematic Mapper Plus (ETM+). Ground spectral data for vegetation were used to simulate the Landsat at-senor broadband reflectance, with consideration of sensor band-pass differences. Three band-geometric VIs (Normalized Difference Vegetation Index (NDVI), Soil-Adjusted Vegetation Index (SAVI), Enhanced Vegetation Index (EVI)) and two band-transformation VIs (Vegetation Index based on the Universal Pattern Decomposition method (VIUPD), Tasseled Cap Transformation Greenness (TCG)) were tested to evaluate the performance of various VI generation algorithms in relation to multi-sensor continuity. Six vegetation types were included to evaluate the continuity in different vegetation types. Four pairs of data during four seasons were selected to evaluate continuity with respect to seasonal variation. The simulated data showed that OLI largely inherits the band-pass characteristics of ETM+. Overall, the continuity of band-transformation derived VIs was higher than band-geometry derived VIs. VI continuity was higher in the three forest types and the shrubs in the relatively rapid growth periods of summer and autumn, but lower for the other two non-forest types (grassland and crops) during the same periods.

A Radiometric Calibration Model for the Field Imaging Spectrometer System

Using the field imaging spectrometer system (FISS) recently developed by us, a new operational radiometric calibration (RC) model that takes into account three main adjustable sensor system settings, including the integration time (t), the aperture (F), and the detector temperature (T), is proposed. To better understand the influence of a single setting on the RC model, controlled experiments with one variable and two fixed settings were conducted and analyzed using a well-calibrated integrating sphere. Subsequently, a new variable was constructed with the ratio of t and F2 to determine the system-setting-based RC model, where the radiometric offset was derived from system noise estimated by keeping the FISS entrance slit from a light source in a dark environment. Finally, the model was evaluated using experimental calibration results from the integrating-sphere data and real vegetation data. The results indicated that standard and calculated radiances were consistent over most spectral wavelengths. The proposed RC model could be effectively applied not only for the FISS and other ground-based sensors but also for future Chinese-developed intelligent remote sensing satellite systems that can automatically modify imaging settings in line with specific requirements.

Retrieval of Sun-Induced Chlorophyll Fluorescence Using Statistical Method Without Synchronous Irradiance Data

Remote sensing of top-of-canopy (TOC) long-term sun-induced chlorophyll fluorescence (SIF) is necessary to better understand the SIF-photosynthesis relationship. Statistical methods provide an alternative to TOC SIF retrieval, as they are independent of synchronous irradiance measurements and may better describe actual irradiance. This letter aims to evaluate the feasibility of using statistical methods for time series TOC SIF retrieval in the absence of synchronous irradiance measurements. Results show that the training set should include nonfluorescent radiance spectra under a variety of solar zenith angles, and that water vapor is an important contributor of spectral variation within 717–745 nm. On the diurnal scale, atmospheric features trained from irradiance spectra can be used to retrieve SIF values from high-frequency upwelling radiance spectra. Features independently trained from nonfluorescent radiance spectra measured on one day can be used for SIF retrieval on a different day within a relatively short period. Our results show that statistical methods have the potential to simplify ground-based SIF measurements and data processing.

Polarized Spectral Measurement and Analysis of Sedum Spectabile Boreau Using a Field Imaging Spectrometer System

Polarized hyperspectral imaging is a new remote sensing method that combines the benefits of polarization and hyperspectral characteristics. Based on a new self-developed polarized field imaging spectrometer system (FISS-P), we collected polarized hyperspectral images of leaves of Sedum spectabile Boreau. Polarization analysis of the diffuse reflectance standard white plate indicated that the FISS-P produced accurate polarization measurements. Ten related polarization parameters (<i>I</i>, <i>Q</i>, <i>U</i>, DoLP, AoP, <i>R</i>₀, <i>R</i>₆₀, <i>R</i>₁₂₀, <i>R</i>, <i>R</i>_I) were analyzed in this study. The angle of polarization (AoP) spectral curves of the S. spectabile leaf had no unique spectral features. The degree of linear polarization (DoLP) spectral curves displayed distinct spectral characteristics. However, the DoLP and spectral reflectance curves of the leaf displayed contrasting trends. Different parts of the same leaf, or different S. spectabile leaves, produced different spectral curve shapes. Analysis of the five reflectance parameters demonstrated that <i>R</i>₀, <i>R</i>₆₀, <i>R</i>₁₂₀, <i>R</i>_I, and <i>R</i> were consistent for all spectral and spatial aspects.

Decomposition of volume scattering, polarized light and chlorophyll fluorescence by in-situ polarization measurement

The remotely sensed radiation of green vegetation canopy over the spectral region of 350-2500 nm typically mixes with three components of different optical properties, namely the leaf interior volume scattering, canopy surface polarized light and leaf internal emitted chlorophyll fluorescence (ChlF). They are tightly superimposed together but convey different information of vegetation. This study emphasizes the distinction of the three radiant fluxes above and disentangles them from the observed apparent radiance of Scindapsus aureus canopy by in-situ polarization measurements. Results demonstrate that the polarization measurement enables the quantitatively separation of the volume scattering, polarized light and ChlF. This study provides further understanding of light scattering properties of the vegetation canopy and particularly has the potential of allowing improvements of current reflectance-based vegetation models.

An NDVI-Based Vegetation Phenology Is Improved to be More Consistent with Photosynthesis Dynamics through Applying a Light Use Efficiency Model over Boreal High-Latitude Forests

Remote sensing of high-latitude forests phenology is essential for understanding the global carbon cycle and the response of vegetation to climate change. The normalized difference vegetation index (NDVI) has long been used to study boreal evergreen needleleaf forests (ENF) and deciduous broadleaf forests. However, the NDVI-based growing season is generally reported to be longer than that based on gross primary production (GPP), which can be attributed to the difference between greenness and photosynthesis. Instead of introducing environmental factors such as land surface or air temperature like previous studies, this study attempts to make VI-based phenology more consistent with photosynthesis dynamics through applying a light use efficiency model. NDVI (MOD13C2) was used as a proxy for both fractional of absorbed photosynthetically active radiation (APAR) and light use efficiency at seasonal time scale. Results show that VI-based phenology is improved towards tracking seasonal GPP changes more precisely after applying the light use efficiency model compared to raw NDVI or APAR, especially over ENF.

Investigating Fraunhofer line based fluorescence retrieval in O 2 -B band with hyperspectral radiative transfer simulations

The telluric O2-B and O2-A have been proved to be capable of solar-induced vegetation fluorescence (SIF) retrieval based on Fraunhofer line depth (FLD) principle. However, most FLD based algorithms mainly aim for SIF detection in O2-A, not suitable in O2-B. One of the critical reasons is that it is very difficult to model the sudden varying reflectance around O2-B band located in the red-edge spectral region (about 680–800 nm). In order to resolve this issue, this study proposes a new method based on the established inverted Gaussian reflectance model (IGM) and FLD principle using hyperspectral radiative transfer simulations with 1 nm bandwidth in 400–1000 nm range. Results show that the proposed method can better capture the spectrally non-linear characterization of the reflectance in 680–800 nm and thereby enables retrieval in O2-B, yielding much more accurate SIFs than typical FLD methods, including sFLD, 3FLD and iFLD.

Ground-based long-term remote sensing of solar-induced chlorophyll fluorescence: Methods, challenges and opportunities

Solar-induced chlorophyll fluorescence (SIF) is a direct indicator of vegetation photosynthesis. Time-series in-situ SIF data enable better understanding of SIF-photosynthesis relationship. However, continuous seasonal SIF data is extremely lack due to several technical problems for ground-based long-term SIF observation systems. One of the major problems is it is difficult to measure radiance and irradiance simultaneously, both of which are inputs of traditional SIF retrieval methods. Statistical approaches for SIF retrieval is independent of synchronous irradiance data and therefore provides opportunities for improving long-term SIF observation systems. This report reviewed the state of the art of the automated SIF observation systems and evaluated the potential of using statistical approaches to simplify the design and data processing of these systems. Results showed that reliable SIF could be retrieved using statistical method without irradiance data once good train set was provided. Opportunities and challenges for improving ground-based long-term SIF measurement systems were discussed.