Laser Beam Atmospheric Propagation Modelling for Aerospace LIDAR Applications

Atmospheric effects have a significant impact on the performance of airborne and space laser systems. Traditional methods used to predict propagation effects rely heavily on simplified assumptions of the atmospheric properties and of the interactions with the laser systems. These models need to be continually improved to develop high-resolution predictors of laser performance for applications including LIDAR (light detection and ranging), free-space optical communications, remote sensing, etc. The underlying causes of laser beam attenuation in the atmosphere are examined with particular focus on dominant linear effects: absorption, scattering, turbulence, and non-linear thermal effects such as blooming, kinetic cooling, and bleaching. These phenomena are quantitatively analyzed, highlighting the key assumptions made in the empirical modelling. Absorption and scattering, as the dominant causes of attenuation, are generally well applied in models, but the impact of non-linear phenomena is less well captured and applied as it tends to be application specific. Atmospheric radiative transfer codes, such as MODTRAN, ARTS, etc., and the associated spectral databases, such as HITRAN, are the effective implementation of the total propagative effects on the laser systems. These codes are powerful, widely used tools to analyze performance. However, atmospheric radiative transfer codes make several assumptions that reduce accuracy in favor of faster processing. The key atmospheric radiative transfer models are reviewed highlighting the associated methodologies, assumptions, and application. Empirical models are found to offer a robust analysis of atmospheric propagation, which is particularly well-suited for design, development, test and evaluation (DDT&E) purposes. As such, empirical, semi-empirical, and ensemble methodologies are suggested to compliment and augment the existing atmospheric radiative transfer codes. There is scope to evolve the numerical codes and empirical approaches to better suit aerospace applications, where fast analysis is required over a range of slant paths, incidence angles, altitudes, and atmospheric properties, which are not exhaustively captured in current quantitative performance assessment methods.

Optomechanical Design and Application of Solar-Skylight Spectroradiometer

Using a solar radiometer is an effective approach for improving the remote sensing of solar irradiance distribution and atmospheric composition. Long-term development of a solar scanning radiometer enables frequent and reliable measurement of atmospheric parameters such as the water vapor column and aerosol optical properties. However, the discrete wavelength radiometer has encountered a bottleneck with respect to its insufficient spectral resolution and limited observation waveband, and it has been unable to satisfy the needs of refined and intelligent on-site experiments. This study proposes a solar-skylight spectroradiometer for obtaining visible and near-IR fine spectrum with two types of measurement: direct-sun irradiance and diffuse-sky radiance. The instrument adopts distributed control architecture composed of the ARM-Linux embedded platform and sensor networks. The detailed design of the measuring light-path, two-axis turntable, and master control system will be addressed in this study. To determine all coefficients needed to convert instrument outputs to physical quantities, integrating sphere and Langley extrapolation methods are introduced for diffuse-sky and direct-sun calibration, respectively. Finally, the agreement of experimental results between spectroradiometers and measuring benchmarks (DTF sun-photometer, microwave radiometer, and Combined Atmospheric Radiative Transfer simulation) verifies the feasibility of the spectroradiometer system, and the radiation information of feature wavelengths can be used to retrieve the characteristics of atmospheric optics.

Can the Earth–Moon distance influence the accuracy of lunar irradiance with the plane-parallel assumption in atmospheric radiative transfer at night?

The plane-parallel atmosphere as an underlying assumption in physics is appropriately used in the rigorous numerical simulation of the atmospheric radiative transfer model (RTM) with incident solar light. The Solar irradiance is a constant with the plane-parallel assumption, which is attributed to the small difference in the distance between any point on the Earth’s surface to the Sun. However, at night, atmospheric RTMs use the Moon as a unique incident light source in the sky. The Earth–Moon distance is approximately 1/400 of the Earth–Sun distance. Thus, the varying Earth–Moon distance on the Earth’s surface can influence the top of atmosphere (TOA) lunar irradiance for the plane-parallel atmosphere assumption. In this investigation, we observe that the maximum biases in Earth–Moon distance and day/night band lunar irradiance at the TOA are ±1.7% and ±3.3%, respectively, with the plane-parallel assumption. According to our calculations, this bias effect on the Earth–Moon distance and lunar irradiance shows a noticeable spatio-temporal variation on a global scale that can impact the computational accuracy of an RTM at night. In addition, we also developed a fast and portable correction algorithm for the Earth–Moon distance within a maximum bias of 18 km or ±0.05%, because of the relatively low computational efficiency and the large storage space necessary for the standard ephemeris computational software. This novel correction algorithm can be easily used or integrated into the atmospheric RTM at night.

Optical Characteristics and Radiative Properties of Aerosols in Harbin, Heilongjiang Province during 2017

This study considers aerosol optical properties and direct radiative forcing over Harbin (126.63° E, 45.75° N), the highest latitude city in Northeast China, during 2017. Observations based on the CE-318 sun-photometer show that the annual mean values of the aerosol optical depth (AOD) at 500 nm and the Angstrom exponent (AE) at 440–870 nm over Harbin are respectively 0.26 ± 0.20 and 1.36 ± 0.26. Aerosol loading is the highest in the spring followed by winter, and the lowest loading is in autumn. AE440–870 is the highest in summer, second highest in winter, and lowest in autumn. The Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model is used to estimate the shortwave aerosol radiative forcing at the top of the atmosphere, on the Earth’s surface and in the atmosphere, and the annual mean values are −16.36 ± 18.42 Wm−2, −71.01 ± 27.37 Wm−2 and 54.65 ± 30.62 Wm−2, respectively, which indicate that aerosols cause climate effects of cooling the earth-atmosphere system, cooling the earth’s surface and heating the atmosphere. Four main aerosol types in Harbin are classified via AOD and AE. Specifically, clean continental, mixed type, biomass burning and urban industry, and desert dust aerosols accounted for 51%, 38%, 9%, and 2% of the total, respectively. Aerosol radiative forcing varies greatly in different seasons, and the aerosol load and type from different emission sources have an important influence on the seasonal variation of radiative forcing.

Numerical Simulations to Explore Deviations from the Beer-Lambert-Bouguer Law in a Correlated Random Medium

<p>The Beer-Lambert-Bouguer Law of exponential attenuation is ubiquitous in the study of atmospheric radiative transfer. However, previous work has shown that adherence to the classical Beer-Lambert-Bouguer law requires the scatterers in the medium to be spatially uncorrelated. As particulates in the atmosphere are often statistically correlated/clustered, it is useful to identify the magnitude of the deviation from the classical expectation under different degrees of spatial clustering.</p><p>Measuring this deviation is difficult in an experimental setting both because it is challenging to measure the spatial clustering and the deviations from the classical expectation are expected to be modest. Thus, we approach this question through a simplified “ballistic-photon” computational simulation.</p><p>Here, we use a simplified numerical model to track the extinction of a collimated light source through correlated random media. The geometry is taken to mimic a sub-volume of the Michigan Technological University Pi Chamber, and the scatterers (cloud droplets) are explicitly resolved using a variety of increasingly realistic techniques for a frozen-field representation of the particle positions.</p><p>We report on the anticipated deviations from the classical Beer-Lambert-Bouguer law through examination of the resulting intensity of the illumination leaving through different walls of the simulation domain.</p>

INTRODUCTION OF CLOUDS IN DART MODEL

Clouds cover around two thirds of the Earth’s surface. Most of them are thick enough to influence the radiative budget of our planet: they increase the top of atmosphere (TOA) exitance and they alter the bottom of atmosphere (BOA) direct and diffuse irradiance. However, most radiative transfer models dedicated to Earth surfaces, such as DART (Discrete Anisotropic Radiative Transfer), simulate only cloudless atmospheres. We recently introduced clouds in DART in order to improve the modelling of weather for remote sensing simulations. In this implementation, clouds were characterized with user specified optical properties and vertical distribution. They were modelled as layered one-dimensional medium that coexists with gases and aerosols. The atmospheric radiative transfer modelling relies on the discrete ordinate method already in DART. In addition, an iterative inversion procedure was designed to test this improvement with field measurements during two cloudy days at Lamasquère meteorological station (France). Specifically, it derives time-series of atmosphere parameters from time-series of BOA solar irradiance measurements. These inversed atmospheric parameters were used to simulate total and diffuse BOA irradiance in PAR (Photosynthetically Active Radiation) domain. The comparison of time-series of measured and DART simulated PAR irradiance lead to very encouraging results (mean relative error ∼8% for total irradiance and ∼20% for diffuse irradiance). It stresses the potential of DART to accurately simulate irradiance in cloudy days.

Comparative analysis of atmospheric radiative transfer models using the Atmospheric Look-up table Generator (ALG) toolbox (version 2.0)

Atmospheric radiative transfer models (RTMs) are software tools that help researchers in understanding the radiative processes occurring in the Earth's atmosphere. Given their importance in remote sensing applications, the intercomparison of atmospheric RTMs is therefore one of the main tasks used to evaluate model performance and identify the characteristics that differ between models. This can be a tedious tasks that requires good knowledge of the model inputs/outputs and the generation of large databases of consistent simulations. With the evolution of these software tools, their increase in complexity bears implications for their use in practical applications and model intercomparison. Existing RTM-specific graphical user interfaces are not optimized for performing intercomparison studies of a wide variety of atmospheric RTMs. In this paper, we present the Atmospheric Look-up table Generator (ALG) version 2.0, a new software tool that facilitates generating large databases for a variety of atmospheric RTMs. ALG facilitates consistent and intuitive user interaction to enable the running of model executions and storing of RTM data for any spectral configuration in the optical domain. We demonstrate the utility of ALG in performing intercomparison studies of radiance simulations from broadly used atmospheric RTMs (6SV, MODTRAN, and libRadtran) through global sensitivity analysis. We expect that providing ALG to the research community will facilitate the usage of atmospheric RTMs to a wide range of applications in Earth observation.

Clear Night Sky Polarization Patterns Under the Super Blue Blood Moon

Investigating celestial polarization patterns in the case of different environments is important for exploring the atmospheric radiative transfer mechanism. Although intensive studies on clear sky, foggy sky, and even total solar eclipse sky have been conducted, the polarization distribution generated by the moonlight has not been well investigated. This study analyzes celestial polarization patterns generated by the Super Blue Blood Moon (SBBM) through several comparative studies. The polarization patterns under the SBBM are collected, analyzed, and compared with both those generated by the ideal single-scattering Rayleigh model and those in the normal sky. From the analysis of the relative variation of the celestial polarization characteristics including the Degree of Polarization (DoP) and Angle of Polarization (AoP), the changes of the extremum, frequency, symmetric line, and neutral points are discussed. As a result, SBBM polarization patterns change at the beginning of the partial eclipse, and the neutral points vary from traditional neutral points. The value of DoP gradually decreases as the obscuration ratio of the Moon increases. The AoP is no longer symmetrical about the celestial meridian. As a conclusion, it is suggested that the variation of the polarized skylight during the SBBM should be considered in atmospheric model calculation for nocturnal biological activity and navigation information computation.