Atmospheric Compensation Techniques Research Articles

Oblique In-Scene Atmospheric Compensation

This research introduces a novel oblique longwave infrared atmospheric compensation technique for hyperspectral imagery, Oblique In-Scene Atmospheric Compensation (OISAC). Current atmospheric compensation algorithms have been developed for nadir-viewing geometries which assume that every pixel in the scene is affected by the atmosphere in nearly the same manner. However, this assumption is violated in oblique imaging conditions where the transmission and path radiance vary continuously as a function of object-sensor range, negatively impacting current algorithms’ ability to compensate for the atmosphere. The technique presented here leverages the changing viewing conditions to improve rather than hinder atmospheric compensation performance. Initial analysis of OISAC’s ability to perform atmospheric compensation for both synthetic and measured hyperspectral images suggests improved performance in oblique viewing conditions compared to standard techniques. OISAC is an extension of ISAC, a technique that has been used extensively for longwave infrared atmospheric compensation applications for over 20 years. OISAC has been developed to incorporate the range-dependence of atmospheric transmission and path radiance in identification of the atmospheric state. Similar to the full implementation of ISAC, OISAC requires the existence of near blackbody-like materials over the 11.73 micrometer water band.

Wavelength Calibration Correction Technique for Improved Emissivity Retrieval

Accurate retrieval of surface emissivity from long-wave infrared hyperspectral imaging data is necessary for many scientific and defense applications. Emissivity retrieval requires an atmospheric model for the scene and consists of two interwoven steps: atmospheric compensation (AC) and temperature–emissivity separation (TES). AC converts the at-aperture radiance to ground radiance, and TES uses the ground radiance to produce a temperature and emissivity estimate. TES assumes that emissivity spectra for solids are smooth, compared to atmospheric features. Model-based techniques find an atmospheric model, which produces the smoothest emissivity estimates. The high-resolution model must be band averaged to the sensor's spectral response function (SRF), which is difficult to characterize and maintain. Any errors in the SRF cause errors in the atmospheric spectra and roughness in the emissivity estimates. We propose a technique that improves the quality of the retrieved emissivity by correcting band-averaging errors of the model from the SRF. An in-scene AC (ISAC) technique is used to find pixels containing a high emissivity material. Atmospheric model bands far from absorption features and a material library are then used to identify the material and estimate the pixel temperatures. At-aperture radiance spectra are generated for the pixels, instead of the ground radiance spectra generated by ISAC. The linear relationship between the generated and measured at-aperture radiances is used to determine model correction factors. Using simulated data, we demonstrate the capability of this technique to substantially reduce calibration error effects in the corrected atmospheric spectra and retrieved emissivities.

Flexible atmospheric compensation technique (FACT): a 6S based atmospheric correction scheme for remote sensing data

Atmospheric correction is an important pre-processing step in various spatio-temporal and multi-sensor remote sensing data analyzes based applications. Absolute atmospheric corrections are carried out using physical based models, generally known as radiative transfer (RT) codes such as MODerate resolution atmospheric TRANsmission (MODTRAN), Second Simulation of the Satellite Signal in the Solar Spectrum (6S) etc. Most of the available atmospheric correction schemes are commercially off-the-shelf and use patented RT codes. The objective of the present work is to develop an open-end atmospheric correction scheme, named as Flexible Atmospheric Compensation Technique (FACT), based on open source 6S RT code. The proposed FACT scheme utilizes look-up architecture for simulating the outputs of 6S RT code for various input parameters’ combination. Input parameters such as initial visibility, columnar water vapour are estimated using the dark object and the Continuum Interpolated Band Ratio (CIBR) methods respectively. The proposed FACT scheme has been evaluated exhaustively using spatio-spectral statistical error measures such as Spatial-Root Mean Square Error (S-RMSE), Spatial-Mean Absolute Error (S-MAE) and spectral-RMSE by comparing the performance with widely used Fast Line-of-sight Atmospheric Analysis of Hypercubes (FLAASH) and the customized NASA JPL’s atmospheric correction scheme. Datasets from hyperspectral (AVIRIS-NG and Hyperion) and multispectral (LANDSAT-8 OLI and WorldView-3) remote sensors were chosen for comparative analysis of the developed atmospheric correction scheme against other atmospheric correction schemes. Results confirm that the proposed FACT scheme offers accuracy of about 95% for hyperspectral imaging sensors and close to 98% for multispectral imaging sensors when compared with FLAASH. Despite marginal disagreements for certain land cover features at the water vapour absorbing spectral regions, we find the proposed FACT scheme a plausible option for carrying out absolute atmospheric correction of various operational remote imaging sensors.

Capacity of coherent free-space optical links using atmospheric compensation techniques

We analyze the ergodic capacity and epsilon-outage capacity of coherent optical links through the turbulent atmosphere. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise, for both passive receivers and those employing active modal compensation of wavefront phase distortion. We study the effect of various parameters, including the ratio of receiver aperture diameter to wavefront coherence diameter, the strength of the scintillation index, and the number of modes compensated.

Performance of synchronous optical receivers using atmospheric compensation techniques

We model the impact of atmospheric turbulence-induced phase and amplitude fluctuations on free-space optical links using synchronous detection. We derive exact expressions for the probability density function of the signal-to-noise ratio in the presence of turbulence. We consider the effects of log-normal amplitude fluctuations and Gaussian phase fluctuations, in addition to local oscillator shot noise, for both passive receivers and those employing active modal compensation of wave-front phase distortion. We compute error probabilities for M-ary phase-shift keying, and evaluate the impact of various parameters, including the ratio of receiver aperture diameter to the wave-front coherence diameter, and the number of modes compensated.