Antoine Roblin

Comparison of iterative and monte carlo methods for calculation of the Aureole about a point source in the earth's atmosphere.

Point sources in the atmosphere are surrounded by aureoles because of atmospheric scattering. The properties of an aureole were calculated by use of a Monte Carlo approach and an iterative method for an isotropic source and an axially symmetric emission source inside an infinite homogeneous atmosphere. The influence of single-scattering albedo, optical depth between source and observer, and source intensity anisotropy were studied from both approaches. For each situation, the limits and advantages of the Monte Carlo technique and the iterative method are described.

Ultraviolet light propagation under low visibility atmospheric conditions and its application to aircraft landing aid

Light scattering in the atmosphere by particles and molecules gives rise to an aureole surrounding the source image that tends to reduce the contrast of the source with respect to the background. However, UV scattering phase functions of the haze droplets present a very important forward peak. The spreading of a detected signal in the UV is not as important as in the case of a clear atmosphere where Rayleigh scattering predominates. This physical property has to be taken into account to evaluate the potential of UV radiation as an aircraft landing aid under low visibility conditions. Different results characterizing UV runway lights, simulations of UV radiation propagation in the atmosphere, and the use of a simple detection algorithm applied to one particular sensor are presented.

Solar glint from oriented crystals in cirrus clouds

Solar scattering on oriented cirrus crystals near the specular reflection direction is modeled using a mix method combining geometric optics and diffraction effects at three wavelengths in the visible and infrared domains. Different potential sources of phase function broadening around the specular direction, such as multiple scattering, solar disk, or tilt effects, are studied by means of a Monte Carlo method. The radiance detected by an airborne sensor located a few kilometers above the cirrus cloud and pointing in the specular scattering direction is calculated at four solar zenith angles showing a dramatic increase of the signal in relation to the usual assumption of random crystal orientation.

MATISSE: version 1.4 and future developments

This paper presents the MATISSE-v1.4 code whose main functionality is to compute spectral or integrated natural background radiance images. The spectral bandwidth extends from 765 to 3300 cm-1 (3 to 13 μm) with a 5 cm-1 resolution. Natural backgrounds include the atmosphere, low and high altitude clouds, sea and land. The most particular functionality of the code is to take into account atmospheric spatial variability quantities (temperatures, mixing ratio, etc) along each line of sight of the image. In addition to image generation capacity, the code computes atmospheric radiance and transmission along a line of sight with the same spectral characteristics as in imaging mode. In this case atmospheric refraction effects and radiation from high or low altitude clouds can be taken into account. A high spectral resolution mode is also available to propagate radiation from a high temperature medium in the same atmospheric state as that used for the image generation. Finally, an Application Programming Interface (API) is included to facilitate its use in conjunction with external codes. This paper describes the range of functionalities of MATISSE-v1.4 whose release is planned for April 2006. Future developments are also presented.

Experimental and theoretical studies of the aureole about a point source that is due to atmospheric scattering in the middle ultraviolet.

In the atmosphere, pointlike sources are surrounded by aureoles because of molecular and aerosol scattering. In various meteorological conditions, this variance field can be a nonnegligible part of the signal detected by a large-field-of-view sensor. A model based on a Monte Carlo technique has been developed to simulate the propagation of radiation coming from a UV point source. The model was validated with an experimental comparison by a photon-counting technique, and good agreement between experimental and theoretical results was found.

MATISSE-v2.0: new functionalities and comparison with MODIS satellite images

MATISSE (Advanced Modeling of the Earth for Environment and Scenes Simulation) is an infrared background scene generator developed for computing natural background spectral radiance images. The code also provides atmospheric radiatives quantities along lines of sight. Spectral bandwidth ranges from 0.4 to 14 μm. Natural backgrounds include atmosphere, sea, land and high and low altitude clouds. The new version MATISSE-v2.0, released this year, has been designed to treat spatial multi resolution in the generated images in order to be able to reach metric spatial variability in pixels footprints. Moreover, MATISSE-v2.0 includes a new sea surface radiance model (water waves and surface optical properties) which depends on wind speed, wind direction and fetch value. Preliminary validations using radiometric measurements have been conducted concerning sea radiances and give promising results. In order to go further in the validation process of MATISSE-v2.0, comparisons with MODIS satellite images have been led. The results of comparing the simulated MATISSE images radiances with the MODIS observations show that the code is performing well. This paper gives a description of MATISSE-v2.0 new functionalities and focus on first results on comparison between MATISSE/MODIS images radiances.

Simulation comparison of aircraft landing performance in foggy conditions aided by different UV sensors.

In the atmosphere pointlike sources are surrounded by an aureole due to molecular and aerosol scattering. UV phase functions of haze droplets have a very important forward peak that limits signal angular spreading in relation to the clear atmosphere case where Rayleigh scattering predominates. This specific property can be exploited using solar blind UV source detection as an aircraft landing aid under foggy conditions. Two methods have been used to compute UV light propagation, based on the Monte Carlo technique and a semi-empirical approach. Results obtained after addition of three types of sensor and UV runway light models show that an important improvement in landing conditions during foggy weather could be achieved by use of a solar blind UV intensified CCD camera with two stages of microchannel plates.

Solar-blind UV imaging photon detector with automatic gain control

We describe a solar-blind UV imaging photon detector with an extended dynamic range which can be used to record very faint UV sources. This detector includes an RbCsTe photocathode, microchannel plates (MCPs) and a two-dimensional resistive anode. It is coupled with an objective lens and a UV filter which rejects daylight with an optical density higher than 12 for experiments performed outdoors at sea level. In order to increase the relatively small dynamic range of the detector due to local saturation in the MCP, a method based on control of the proximity voltage between the photocathode and the MCP has been developed. Control is performed remotely by a TTL signal. It regulates the detected event rate within a pre-defined interval. The system spatial and temporal performance has been studied in detail. A comparison between the results obtained with and without gating has been made and confirms the interest of our new acquisition system for measuring radiometric quantities of low light level sources over an extended dynamic range.

High-Altitude Cloud Effects on Airborne Electro-Optical Sensor Performance

Abstract A modeling approach is proposed to predict the impact of high-altitude clouds on the detection performance of an airborne electro-optical surveillance sensor. The sensor is dedicated to target detection, and it may be disturbed by cloud presence along the line of sight. The targets are supposed to be dim artificial sources in the atmosphere or stars, and the viewing conditions lead to lines of sight close to horizontal paths. The model is suitable to determine sensor performance for various locations in the earth’s atmosphere and for different seasons. It is based on cloud statistics data determined by the inversion of NOAA satellite observations. Computations of clear line-of-sight probabilities are based on geometric and statistical approaches, and the major hypothesis used here is that all clouds can be considered opaque. Seasonal and diurnal/nocturnal cloud cover variations strongly change the probability of a clear line of sight for a fixed viewing geometry, and the maximal range without clo...

UV missile-plume signature model

A new 3D radiative code is used to solve the radiative transfer equation in the UV spectral domain for a nonequilibrium and axisymmetric media such as a rocket plume composed of hot reactive gases and metallic oxide particles like alumina. Calculations take into account the dominant chemiluminescence radiation mechanism and multiple scattering effects produced by alumina particles. Plume radiative properties are studied by using a simple cylindrical media of finite length, deduced from different aerothermochemical real rocket plume afterburning zones. Assumed a log-normal size distribution of alumina particles, optical properties are calculated by using Mie theory. Due to large uncertainties of particles properties, systematic tests have been performed in order to evaluate the influence of the different input data (refractive index, particle mean geometric radius) upon the radiance field. These computations will help us to define the set of parameters which need to be known accurately in order to compare computations with radiance measurements obtained during field experiments.