Radiometric Calibration Approach Research Articles

Assessing radiometric calibration methods for multispectral UAV imagery and the influence of illumination, flight altitude and flight time on reflectance, vegetation index and inversion of winter wheat AGB and LAI

Radiometric correction is essential to unmanned aerial vehicle (UAV) precision agriculture applications, especially for crop multi-period analysis and quantitative inversion of phenotypic and physiological parameters. In this study, we investigated the performance of four radiometric correction methods: “Camera only” (A), “Camera and sun irradiance” (B) and “Camera, sun irradiance and sun angle” (C) methods provided by Pix4d Mapper and an irradiance sensor-based method (D) implemented with the R software. UAV campaigns were conducted at flight altitudes of 30 m, 60 m and 90 m under clear and cloudy skies at different times over an experimental field of winter wheat. The influences of illumination, flight altitude and flight time on reflectance and vegetation indexes derived from different radiometric calibration methods were analyzed. Moreover, we further assessed the impact of reflectance and vegetation index variations on the above-ground biomass (AGB) and leaf area index (LAI) estimations of winter wheat. Results showed that method D is a promising approach for radiometric calibration, which produced consistent reflectance with method A at noon (e.g., 10:44:41 AM March 17 and 11:39:37 AM April 7) and mid-afternoon (e.g., 15:30:02 PM May 1) and more reliable reflectance in the morning (e.g., 7:15:44 AM March 17). Methods B and C seriously underestimated the reflectance at 10:44:41 AM March 17 and 11:39:37 AM April 7. This was caused by the incorrect attitude angles of the irradiance sensor and further miscalculation of the solar irradiance by Pix4d Mapper. Interestingly, these errors were eliminated in the calculation of the ratio type of vegetation indexes. Methods B and C yielded vegetation indexes that were almost the same as those of methods A and D. Under cloudy sky, when direct sunlight is not blocked by clouds, method D was effective; with cloud occlusion, results on April 24 indicated that the method failed to estimate the total sunlight and direct sunlight ratio. Reflectance and vegetation indexes obtained from different radiometric correction methods were all obviously affected by flight altitude. Furthermore, the variations of NDVI with flight altitude increasing were different with reflectance. Changes in reflectance and vegetation indexes with flight altitude had little effect on the correlation coefficients yielded with AGB, but showed a significant influence on those of LAI. Through analyzing the variation of observed solar radiation at different time intervals, caution is recommended when using the assumption that solar radiation is stable during the UAV flight. The longer the flight, the greater the variation in solar radiation, especially at sunrise and sunset. Even at noon, when the flight duration is 30 min, the change in solar radiation may exceed 10 % (e.g., 16.5 % on March 20).

Absolute radiometric calibration with lunar surface reflectance as reference for earth-observing VNIR camera

The absolute radiometric accuracy of earth-observing camera is crucial for the applications of natural resources, environment, agriculture and other industries. To continue the progress in this filed, a lunar surface reflectance based radiometric calibration approach is given in this paper. We chose IIM, M3, SP lunar models as references and GF-4 VNIR camera as sensor under calibrating. The lunar calibration sites were MS-2 site, Apollo-16 site and CE-3 site. The equivalent reflectance models of lunar were retrieved by Multiplying and integrating with the spectral response function of VNIR camera. Absolute radiometric calibrations with the equivalent reflectance models of lunar were carried out for 520–590nm, 630–690nm and 770–890nm spectral bands. The ground-based validation experiments were conducted with low, medium and high reflectance targets. The calibration accuracy was evaluated by comparing the relative errors of derived radiance after radiometric calibration with the benchmarks of TOA radiance transferred by in-situ measured reflectance. Lunar-based calibration models, lab and on-orbit filed-based models were used to compare the relative errors between proposed method and traditional way. The results showed that using IIM lunar model had better radiometric accuracy than other models, and SP model had the similar performance with traditional on-orbit filed-based model. The results indicated that using lunar to calibrate the earth-observing camera had the capability to improve the radiometric calibration accuracy.

Pixel-wise radiometric calibration approach for infrared focal plane arrays using multivariate polynomial correction

Advances in infrared focal plane arrays allow thermal cameras to operate without active cooling equipment, however, the outputs of uncooled thermal cameras are closely related to their own temperatures, which degrades the temperature retrieval accuracy when ambient environment varies. In this paper, a novel radiometric calibration approach is proposed to decrease the errors derived from ambient temperature drift and internal heating. Firstly, a multivariate polynomial correction plus a second-order polynomial correction are implemented to stabilize the camera output derived from temporal non-uniformity. Afterwards, multi-point correction is applied to remove the fixed pattern noise caused by spatial non-uniformity. Finally, object temperature is calculated from thermal image gray values using the Planck-like approximation function. Experimental results demonstrated that the proposed method outperforms existing shutterless and shutter-based methods especially when uncooled thermal cameras are in unstable status.

A Spectrum Extension Approach for Radiometric Calibration of the Advanced Hyperspectral Imager Aboard the Gaofen-5 Satellite

The advanced hyperspectral imager (AHSI) is one of the sensors aboard the Chinese Gaofen-5 (GF-5) satellite, possessing characteristics of high spatial and spectral resolution, as well as width swath. To better understand the radiometric performance of GF-5/AHIS after its launch, this article presents an on-orbit radiometric calibration approach for AHSI visible and near-infrared (VNIR) and shortwave infrared (SWIR) sensors from field automatic observations with a field spectrometer in the absence of SWIR measurements. A spectrum extension method was proposed to extend the retrieved surface hyperspectral reflectance in the VNIR spectral ranges to SWIR by incorporating the historical hyperspectral reflectance library. The radiometric calibration coefficients of GF-5/AHSI were calculated by linear fitting of the observed digital number (DN) values with GF-5/AHSI and predicted at-sensor radiances with MODTRAN 5 based on extended hyperspectral surface reflectance. Comparisons with onboard calibration results were also performed, and the averaged relative differences were within 5% with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\delta $ </tex-math></inline-formula> standard deviations less than 10% for most bands, except for those in the atmospheric absorption and low signal-to-noise ratio bands. The comparison results indicate that the on-site radiometric calibration results are consistent with the onboard results, and the operational on-orbit radiometric calibration approach is reliable in the case that there are no measurements in the SWIR spectra range. The on-orbit radiometric performance of GF-5/AHSI rapidly degraded during the first several months after its launch and then tended to be relatively stable.

Assessing the 3-D Structure of Bamboo Forests Using an Advanced Pseudo-Vertical Waveform Approach Based on Airborne Full-Waveform LiDAR Data

Remote-sensing-assisted estimates of Moso bamboo forest structure are imperative for supporting sustainable forest management (SFM). Passive sensors have limited usages in characterizing continuous vertical structures of Moso bamboos since they have difficulty in penetrating dense upper canopies. Airborne full-waveform (FWF) Light Detection and Ranging (LiDAR) technology has shown significant potentials in providing an improved representation of 3-D structures in much denser canopies, while the capability of FWF to assess the structure of the unique Moso bamboo forests is less known. Many previous studies commonly assume perfectly vertical waveforms and do not account for slanted penetrations of waveforms in canopies. This study presented a framework for investigating the availability of FWF to assess subtropical Moso bamboo forest structures. We used a physical-based approach for radiometric calibration and an advanced voxel-based pseudo-vertical waveform approach to assign waveforms into voxels and reconstruct composite waveforms for extracting FWF metrics. Second, the differences in FWF-derived metrics in response to canopy structures across various management strategies were compared. Finally, the capability of FWF metrics for estimating bamboo structural parameters was evaluated by parametric and nonparametric models. The results showed that the FWF metrics varied significantly (p-values < 0.05) in response to different management strategies. In general, nonparametric approaches (locally weighted linear regression and k-nearest neighbor) outperformed parametric approaches (multiple linear regression). This study demonstrated that the pseudo-vertical waveform approach could improve the performances of FWF metrics for characterizing 3-D Bamboo forest structure and provide new insights on accurately estimating bamboo structural parameters across different management strategies.

RAPID RADIOMETRIC CALIBRATION OF MULTIPLE CAMERA ARRAY USING IN-SITU DATA FOR UAV MULTISPECTRAL PHOTOGRAMMETRY

Abstract. This study compares two possible radiometric calibration approaches of Tetracam μMCA Snap multispectral camera using the Dark offset subtraction and Look-Up Table (LUT) methods. A laboratory-based calibration using correction images taken under the controlled conditions was compared with a rapid in-situ based calibration featuring correction images taken during the field campaign. The hypothesis was that the accuracy of in-situ calibration could be comparable with the laboratory calibration, and thus could replace it and simplify the radiometric calibration process. The accuracy of calibration approaches was assessed by comparison of three validation targets reflectance values extracted from corrected UAV images based on laboratory and in-situ calibration with a reference spectroscopy measurement. The results of the field experiment showed that both calibration approaches led to significant accuracy improvement compared to raw data. The vignetting correction using resulted in a significant reduction of the Coefficient of variation by half in all bands and overall equalizing the DNs on the selected diagonal profile. The NRMSEs after processing all corrections ranged from 0.24 to 3.40%. Although the statistical testing revealed slightly better agreement of laboratory calibrated reflectance with reference data, the accuracy of in-situ calibration is sufficient, because the accuracy improvement quantified by the NRMSE is 2–10 times better using both calibration approaches compared to raw data than the NRMSE differences between them. These findings make the proposed in-situ approach usable for various environmental studies featuring UAV multispectral photogrammetry.

An advanced radiometric calibration approach for uncooled thermal cameras

AbstractUncooled thermal cameras are increasingly used in thermography applications due to their lower cost and size. However, there are two significant limiting constraints which must be taken into consideration in a radiometric calibration before the actual application: (i) temporal non‐uniformity (a temperature‐dependent drift problem); and (ii) spatial non‐uniformity (fixed‐pattern noise – FPN). Conventional temporal non‐uniformity corrections (NUC) take advantage of an internal reference source but such methods are not valid for sequential images when the focal‐plane array (FPA) temperature is changing rapidly. A novel shutterless correction method is proposed to stabilise the camera's response. Moreover, instead of implementing the spatial NUC first, multi‐point correction is leveraged to remove FPN after the temporal NUC. Finally, a Planck curve is applied to convert thermal image values into object temperatures. Experimental results show that the proposed method is more effective for images with rapidly changing FPA temperatures than conventional shutterless methods and traditional shutter‐based methods.

Development and application of automated vicarious calibration system

In order to improve the frequency and precision of radiometric calibration, the automated vicarious calibration system (AVCS) is developed and deployed at the Dunhuang test site to perform vicarious calibration without the in situ manned measurements. The surface and atmospheric parameters are automatically collected by AVCS. An absolute radiometric calibration approach based on AVCS is proposed. Six successful calibrations of the Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) are conducted. The results are in good agreement with the on-board calibration system with all the relative differences less than 4%. It enables us to monitor the change of a sensor over long time scales.

Practical example for use of the supervised vicarious calibration (SVC) method on multisource hyperspectral imagery data – ValCalHyp airborne hyperspectral campaign under the EUFAR framework

Abstract. A novel approach for radiometric calibration and atmospheric correction of airborne hyperspectral (HRS) data, termed supervised vicarious calibration (SVC) was proposed by Brook and Ben-Dor in 2010. The present study was aimed at validating this SVC approach by simultaneously using several different airborne HSR sensors that acquired HSR data over several selected sites at the same time. The general goal of this study was to apply a cross-calibration approach to examine the capability and stability of the SVC method and to examine its validity. This paper reports the result of the multi sensors campaign took place over Salon de Provenance, France on behalf of the ValCalHyp project took place in 2011. The SVC method enabled the rectification of the radiometric drift of each sensor and improves their performance significantly. The flight direction of the SVC targets was found to be a critical issue for such correction and recommendations have been set for future utilization of this novel method. The results of the SVC method were examined by comparing ground-truth spectra of several selected validation targets with the image spectra as well as by comparing the classified water quality images generated from all sensors over selected water bodies.

A Novel Adaptive Scheme for Evaluating Spectral Similarity in High-Resolution Urban Scenes

The analysis of high-spatial-resolution urban images for object recognition must deal with variable illumination conditions and many spectrally similar materials in the built environment. Spectral similarity measures have the potential to contribute to the effective analysis of urban scenes, however, without readily available surface reflectance conversion, the characteristics of existing spectral measures may lead to unacceptable performance. To better account for these spectral imaging scenarios for an urban environment, a simplified in-scene radiometric calibration approach is presented to preserve data collinearity, and a novel spectral similarity measure based on the geometric characteristics of the Mahalanobis distance is developed to incorporate both spectral direction and spectral magnitude. With a minimum of human input to define representative pixels, the experimental results demonstrate through the analysis of ROC curves the potential advantages of the novel distance measure when applied to the identification of materials in urban images.

Radiometric Calibration of LIDAR Intensity With Commercially Available Reference Targets

We present a new approach for radiometric calibration of light detection and ranging (LIDAR) intensity data and demonstrate an application of this method to natural targets. The method is based on 1) using commercially available sand and gravel as reference targets and 2) the calibration of these reference targets in the laboratory conditions to know their backscatter properties. We have investigated the target properties crucial for accurate and consistent reflectance calibration and present a set of ideal targets easily available for calibration purposes. The first results from LIDAR-based brightness measurement of grass and sand show that the gravel-based calibration approach works in practice, is cost effective, and produces statistically meaningful results: Comparison of results from two separate airborne laser scanning campaigns shows that the relative calibration produces repeatable reflectance values.

Overview of the earth observing one (eo-1) mission

The Earth Observing One (EO-1) satellite, a part of National Aeronautics and Space Administration's New Millennium Program, was developed to demonstrate new technologies and strategies for improved Earth observations. It was launched from Vandenburg Air Force Base on November 21, 2000. The EO-1 satellite contains three observing instruments supported by a variety of newly developed space technologies. The Advanced Land Imager (ALI) is a prototype for a new generation of Landsat-7 Thematic Mapper. The Hyperion Imaging Spectrometer is the first high spatial resolution imaging spectrometer to orbit the Earth. The Linear Etalon Imaging Spectral Array (LEISA) Atmospheric Corrector (LAC) is a high spectral resolution wedge imaging spectrometer designed to measure atmospheric water vapor content. Instrument performances are validated and carefully monitored through a combination of radiometric calibration approaches: solar, lunar, stellar, Earth (vicarious), and atmospheric observations complemented by onboard calibration lamps and extensive prelaunch calibration. Techniques for spectral calibration of space-based sensors have been tested and validated with Hyperion. ALI and Hyperion instrument performance continue to meet or exceed predictions well beyond the planned one-year program. This paper reviews the EO-1 satellite system and provides details of the instruments and their performance as measured during the first year of operation. Calibration techniques and tradeoffs between alternative approaches are discussed. An overview of the science applications for instrument performance assessment is presented.