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This paper discusses the use of smoke obscurants as countermeasures against high energy lasers (HEL). Potential success of the smoke does not depend only the performance of the smoke. The transmission loss in the smoke is part of a chain of system components, including warning sensors, smoke launchers, etc.. The core of the paper deals with experimental work on the following research questions: - Does smoke attenuate an incoming beam of a HEL? - Does the HEL affect the smoke itself? The experimental set-up with the TNO 30kW HEL and the scale model for the smoke transmission path will be shown. Selected experimental results will be shown and discussed. Finally we will compare the results to theoretical calculations and we will analyze the properties of an ideal HEL attenuation smoke.

We report the analysis of radio soundings launched at our permanent measurement site in North-western Germany. The data of potential temperature and wind speed are used to derive vertical profiles of the gradient Richardson number 𝑅𝑖𝑔 in the free troposphere, and subsequently, the strength of optical turbulence 𝐶𝑛 2. These values are compared to a numerical framework, which derives 𝐶𝑛 2 from mesoscale weather prediction data by two schemes based on the gradient Richardson number and Monin-Obukhov similarity theory, respectively.

Military operations in arid regions of the world are becoming more and more regular. The atmospheric conditions in these regions impose severe restrictions on the performance of optical systems. In contrast to regions, where many airports are located and therefore the monitoring network of ground stations is very dense, only few ground measurements are available for arid regions. To a certain extent, measurements can be collected and generalized with large-scale measurement campaigns, but they are very cost-intensive and partly not achievable due to the political situation. Another possibility to close this gap of data is provided by satellite measurements. For various measurement parameters such as humidity, wind, solar radiation and aerosols, this works quite well with some limitations. For this reason, models are a good complement to fill the lack of data in these regions. The study is concerned with identifying the turbulence in Western Sahara. The models used WRF (Weather Research and Forecasting Model) and ICON (Icosahedral Nonhydrostatic Model) have been sufficiently tested in different regions of the world. As there are no turbulence measurements in the Sahara, this is the first test to estimate the magnitude of the turbulence in order to discuss the need for an extensive measurement campaign. The models can be validated with previous trials of IOSB such as White Sands Missile Range (WSMR) in the USA, (New Mexico).

Our aim is to characterize the optical turbulence over the urban areas. Since it is difficult to measure 𝐶2/𝑛 (see PDF) continuously over an urban area, we explore the possibility of using a mesoscale weather prediction model to predict 𝐶2/𝑛 (see PDF) over the urban area. To this end, the output of the Weather Research and Forecast model (WRF) was coupled with a micrometeorological parametrization, which allowed calculation of 𝐶2/𝑛 (see PDF) at each numerical grid point in the surface layer. Numerical results are compared to data of path-averaged measurements of optical turbulence performed with a large aperture scintillometer (BLS900) over the city of Ettlingen (southwestern Germany) during two time periods in Spring and Summer 2013. Effects of the heat island effect are revealed by high turbulence values, observed at night-time.

Military operations on target practice and wind farms demand on knowledge of the turbulent state and wind speed in the surrounding atmosphere. However, the definition of turbulence is completely different in both cases. In the military context we speak about optical turbulence in the wind industry, kinetic turbulence is essential. In the following we want to present an approach that combines both turbulences. In the vicinity of a military test range and 120 m high wind turbines a new Doppler LIDAR system (WINDCUBETM v200s, Leosphere, France) was operated to investigate the vertical distribution of turbulent kinetic energy and the spatial distribution of the radial wind speed in the atmospheric boundary layer. Deployments in different modes were carried out to get best estimations of turbulence. Vertical profile measurements obtained with two ultrasonic anemometers at discrete levels on an 80 m high mast close to the position of the LIDAR were used for comparison. We present an adapted algorithm to calculate the TKE from a low-frequency signal (vertical profile every 10 s) and a approach to calculate C2n. The first preliminary results show a good agreement. Results on diurnal cycles of the TKE in different stability regimes and local effects will be discussed.

Infrared imaging of the sea surface is used for many purposes, such as remote sensing of large oceanographic structures, environmental monitoring, surveillance applications and platform signature research. Many of these studies rely on determining the contrast of a target feature with its background and therefore benefit from accurately predicting the signature of the underlying sea surface background. We here present a model that synthesizes infrared spectral images of sea surfaces. This model traces explicitly the behaviour of the sea wave structure and light propagation. To self-consistently treat spatial and temporal correlations of the clutter, geometrical realizations of sea surfaces are built based on realistic sea wave spectra and their temporal behaviour is subsequently followed. A camera model and a ray tracer are used to determine which parts of the sea surface are observable by individual camera pixels. Atmospheric input elements of the model, being sky dome, path radiance and transmission, are computed with MODTRAN for a chosen atmosphere.

High frequency fluctuations in the refractive index are the main atmospheric influence on wave propagation in the atmosphere. They are caused mainly from variations in atmospheric temperature. Ultrasonic anemometers have been established as common tools in performing in-situ measurements of turbulence. They were used as fixed point instruments from towers, for the characterization of local turbulence or from mobile platforms. Besides wind velocity fluctuations, high frequency variations of the sonic temperature are analyzed. Time series of temperature data are analyzed to derive the structure function of temperature CT 2, that should be proportional the strength of optical turbulence Cn 2. The application of the Fast Fourier transformation, temporal resolution and averaging time are addressed to show the applicability of ultrasonic anemometers. Also the influence of humidity fluctuations on the determination of Cn 2 in applications over land and over the sea is discussed. Errors in the applications from fixed points as well as from mobile platforms are estimated for different types of anemometers.

Using seasonally averaged meteorological and spectrally resolved aerosol profiles extracted from a maritime environment, this paper investigated how the resolution of the vertical profiles influences the 3-5μm and 8-12μm average transmittance and integrated path radiance computations conducted by MODTRAN in high-elevation scenarios. First, the minimum altitude to which the atmosphere should be defined in order to accurately determine the transmittance and path radiance along vertical and slant paths was investigated by recursively removing vertical layers until the relative changes in the transmittance and path radiance became smaller than those due to instrument uncertainty. Once this minimum height was found, the vertical resolution in the atmosphere below the minimum altitude was systematically varied. The suitability of several gradient-based criteria has been investigated to determine the optimal discretization of the vertical profiles. The results indicate that, depending on the quantity to be calculated, vertical discretizations based on the gradient in either the pressure, temperature or humidity serve as optimal discretizations in maritime high-elevation scenarios. Moreover, the followed methodology demonstrates how to adaptively implement a vertical resolution in a generic atmosphere, which generates crucial knowledge in supporting signature and sensor performance modelling for high-elevation scenarios.

Electro-optical and laser systems are operated world-wide. Their performance in the outside atmosphere is mainly governed by the strength of optical turbulence Cn2. The predictability of Cn2 using weather-forecast models is investigated by performing simulations with the Weather Research and Forecast Model (WRF). The WRF output data were combined with a micrometeorological parametrization to derive Cn2. Simulation runs were performed for locations and times included in our worldwide data set of Cn2 obtained in several field trials over land and over the sea. Experimental data of point and integrated path measurements in the surface layer were compared to model calculations of Cn2. The regions include different climatic conditions from South Africa, the US, as well as Central and Northern Europe. The applicability of WRF to predict Cn2 at the different locations will be discussed. It will be shown that WRF in a 1.1-km resolution is adequate to provide a first estimate of Cn2 © 2019 SPIE.

The atmospheric influence on wave propagation was investigated during the First European South African Transmission ExpeRiment from June 2015 to February 2016. The focus in this article was set on optical turbulence, the main atmospheric factor affecting the position and strength of Laser beams, the performance of electro-optical systems and imaging. Measurements were performed continuously during the campaign on three sites over the northwestern part of False Bay. The optical turbulence measurements include in situ measurements using an ultrasonic anemometer at the Roman Rock Island. Integrated optical turbulence measurements were performed at two sites, over a path of 1.8 km and a long distance path of 8.6 km. The sites may be affected by local effects of the coastal environment. For comparison, the optical turbulence was modeled using micrometeorological parameterization. Additionally, the optical turbulence was determined by simulations using the weather research and forecast model WRF. Simulation results were compared to measurements considering seasonal and meteorological variations. The representativeness of the measurements locations for offshore measurements will be discussed. © 2018 SPIE.

Atmospheric turbulence impacts on the propagation of electro-optical radiation. Typical manifestations of optical turbulence are scintillation (intensity fluctuations), beam wander and (for laser systems) reduction of beam quality. For longer propagation channels, it is important to characterize the vertical and horizontal distribution (inhomogeneity) of the optical turbulence. In the framework of the First European South African Transmission ExpeRiment (FESTER) optical turbulence was measured between June 2015 and February 2016 on a 2 km over-water link over False Bay. The link ran from the Institute of Maritime Technology (IMT) in Simons Town to the lighthouse at Roman Rock Island. Three Boundary layer scintillometers (BLS900) allowed assessing the vertical distribution of optical turbulence at three different heights between 5 and 12 m above the water surface. The expected decrease of Cn 2 with height is not always found. These results are analyzed in terms of the meteorological scenarios, and a comparison is made with a fourth optical link providing optical turbulence data over a 8.7 km path from IMT to Kalk Bay, roughly 36° to the north of the three 2 km paths. The results are related to the inhomogeneous meteorological conditions over the Bay as assessed with the numerical weather prediction tool, the Weather Forecast and Research model WRF. © Copyright 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

A field trial was performed in the arid scrub area of White Sands, NM /USA in October 2017 investigating the atmospheric influence on imaging and sensor performance. In this paper we focus on the strength of optical turbulence. Optical turbulence is described by the structure function parameter of the refractive index Cn 2. It is responsible for beam wander, blurring. and scintillation. "Ground truth" measurements of optical turbulence were carried out using a Boundary Layer Scintillometer . The measurements were taken along an optical path of 3.64 km and a height of 1.5 m. Additionally, height dependency of Cn 2 is explored in the surface layer using 4 ultrasonic anemometers at discrete heights between 1 and 10 m. Power spectra of temperature were determined from time series of the ultrasonic anemometer data, which were inspected for the height dependency of spectral characteristics. The effect of the arid scrubs area on the applicability of Kolmogorov turbulence was investigated and discussed in this paper. © 2018 SPIE.

Operation and design of electro-optical systems are affected by atmospheric turbulence, quantified by refractive structure function parameter Cn 2. The long-term VerTurM experiment at a rural site in Northwestern Germany served to collect an extended dataset of vertical profiles of Cn 2 values up to 64 meters, completed by a characterization of the surface layer meteorology. A micrometeorological model in terms of the Monin-Obukhov similarity theory (MOST) was developed to predict the values of Cn 2 and its results were compared to the measurements. © Copyright 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

This paper reviews three phenomena that affect the propagation of electro-optical radiation through the atmosphere: absorption and scattering, refraction and turbulence. The net effect on imaging or laser systems is a net reduction of the effective range, or a degradation of the information contained in the electro-optical radiation. Recent advances in techniques to assess the concentration and composition of atmospheric aerosols, and to assess the strength of optical turbulence are discussed in more detail. © 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

The knowledge of the optical contrast of an oil layer on the sea under various surface roughness conditions is of great interest for oil slick monitoring techniques. This paper presents a 3D simulation of a dynamic sea surface contaminated by a floating oil film. The simulation considers the damping influence of oil on the ocean waves and its physical properties. It calculates the radiance contrast of the sea surface polluted by the oil film in relation to a clean sea surface for the SWIR spectral band. Our computer simulation combines the 3D simulation of a maritime scene (open clear sea/clear sky) with an oil film at the sea surface. The basic geometry of a clean sea surface is modeled by a composition of smooth wind driven gravity waves. Oil on the sea surface attenuates the capillary and short gravity waves modulating the wave power density spectrum of these waves. The radiance of the maritime scene is calculated in the SWIR spectral band with the emitted sea surface radiance and the specularly reflected sky radiance as components. Wave hiding and shadowing, especially occurring at low viewing angles, are considered. The specular reflection of the sky radiance at the clean sea surface is modeled by an analytical statistical bidirectional reflectance distribution function (BRDF) of the sea surface. For oil at the sea surface, a specific BRDF is used influenced by the reduced surface roughness, i.e., the modulated wave density spectrum. The radiance contrast of an oil film in relation to the clean sea surface is calculated for different viewing angles, wind speeds, and oil types characterized by their specific physical properties. © 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Electro-optical and laser systems are presently deployed in naval operations around the world. The performance of these systems is negatively affected by optical turbulence in the atmosphere, quantified by the parameter Cn 2. The strength of the integrated optical turbulence Cn 2 was investigated for several coastal locations in different climatic conditions: False Bay (South Africa), the Baltic Sea (Bay of Eckernförde, Germany), the Mediterranean Sea (Crete, Greece), the Gulf of Mexico (Dauphin Island, Alabama, US), and the Arabian Gulf. The over-water, near-surface turbulence was characterized along paths that typically spanned 1.5 - 8.7 km using large aperture scintillometers. The dependency of Cn 2 on the air-sea surface temperature difference and wind speed is discussed, and the results for the five geographic regions are compared and discussed in terms of environmental conditions and climate. © Copyright 2017 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

A transmission experiment has been performed over an optical path of 1.53 km at a rural test site in Meppen, Northwest Germany. Direct transmission measurements were made by a 7-wavelength transmissometer. Transmission was further estimated from the average voltage received by a BLS2000 scintillometer, and evaluated with Mie theory from in-situ aerosol measurements near the optical path. Furthermore, the transmission was modeled with MODTRAN, driven with local meteorology, visibility and the rural aerosol model. For a central wavelength of 0.88μm, the transmissometer, the BLS200 and MODTRAN agree well. Remaining differences may be due to water transmission and continuum around 0.95μ;m that is picked up by the transmissometer and not by the narrow-banded BLS2000 and MODTRAN calculations. When MODTRAN is run without an aerosol model, or when this model is driven by a "default" visibility, the overlap with the measurements is extremely poor. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

The First European South African Experiment (FESTER) was conducted over about a 10 month period at the Institute of Maritime Technology (IMT) in False Bay, South Africa. One of the important goals was to validate atmospheric refraction and turbulence models. To achieve this goal it was required to measure the vertical profile of meteorological parameters and compare this to model predictions. A special helium kite balloon (Helikite) was used as lifting device for weather and temperature sensors to obtain a measured vertical air profile. This measurement was conducted in the middle of the atmospheric path for the principal electro-optic transmission link monitoring equipment (i.e. scintillometer and multi-spectral radiometer-transmissometer system). First results will focus on the vertical air temperature profile shape as function of general environmental conditions and the comparison to model predictions. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Optical turbulence for over-water conditions was investigated in a long-term experiment over False Bay near Cape Town, South Africa. A sonic anemometer and two boundary-layer scintillometers were deployed to access in-situ turbulence as well as the integrated turbulence over two 1.8 and 8.7 km paths. Statistical analysis reveals spatial temporal variations of the turbulence conditions over False Bay, which might be related to differences in the atmospheric conditions and/or the surface (water) temperatures. An analysis in terms of mechanical and thermal forcing reveals that the latter factor is more dominant in determining the turbulence strength. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

An overview is given of the First European - South African Transmission ExpeRiment (FESTER), which took place in South Africa, over the False Bay area, centered around Simon's Town. The experiment lasted from April 2015 through February 2016 and involved continuous observations as well as periodic observations that took place during four Intensive Observation Periods (IOPs) of 2 weeks each, which were spread over the year. The continuous observations aimed at a characterization of the electro-optical propagation environment, and included standard meteorology, aerosol, refraction and turbulence measurements. The periodic observations aimed at assessing the performance of electro-optical sensors in VIS / SWIR / MWIR and LWIR wavebands by following a boat sailing outbound and inbound tracks. In addition, dynamic aspects of electro-optical signatures, i.e., the changes induced by variations in the environment and/or target orientation, were studied. The present paper provides an overview of the trial, and presents a few first results. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

The First European South African Experiment (FESTER) was conducted over about a 10 month period at the Institute of Maritime Technology (IMT) in False Bay, South Africa. One of the important goals was the establishment of the air-sea temperature difference (ASTD) homogeneity along the main propagation link atmospheric path since it is a basic assumption for most of the atmospheric turbulence models (caused by refractive index variations). The ASTD was measured from a small scientific work boat (called Sea Lab) moving along a straight in- and outbound track along the main propagation link path. The air temperature on-board was measured using standard weather sensors, while the sea surface temperature was measured using a long wavelength infrared radiometer, which was compared to the bulk sea temperature half a meter below the sea surface. This was obtained by an under water temperature sensor mounted on a â€surfboard' that was towed alongside Sea Lab. Vertical water temperature profiles were also measured along the main propagation path in order to determine the depth of the surface mixed layer and thermocline using a Conductivity Temperature Depth profiler (CTD). First results investigated the ASTD variation along the horizontal line-of-sight path used by the principal electro-optic transmission link monitoring equipment (i.e. scintillometer and multi-spectral radiometer-transmissometer system). © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

A long term field trial called FESTER (First European South African Transmission Experiment) has been conducted by an international collaboration of research organizations during the course of almost one year at False Bay, South Africa. Main objectives of the experiment are a better insight into atmospherical effects on propagation of optical radiation, a deeper understanding of the effects of (marine) aerosols on transmission, and the connection of the mentioned effects to the general meteorological and oceanographic conditions/parameters. Modelling of wakes and possible infrared-radar synergy effects are further points of interest. The duration of one year ensures the coverage of most of the relevant meteorological conditions during the different seasons. While some measurements have been performed by permanent installations, others have been performed during intensive observation periods (IOP). These IOPs took place every two to three months to ensure seasonal changes. The IOPs lasted two weeks. We will give an overview of the general layout of the experiment and report on first results. An outlook on the planned analysis of the acquired data, which includes linkage to the Weather Research and Forecasting model (WRF), will be given. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

The experiment FESTER (First European South African Transmission ExpeRiment) took place in 2015 to investigate the atmospheric influence on electro-optical systems performance across False Bay / South Africa on a long term basis. Several permanent stations for monitoring electro-optical propagation and atmospheric parameters were set up around the Bay. Additional intensive observation periods (IOPs) allowed for boat runs to assess the inhomogeneous atmospheric propagation conditions over water. In this paper we focus on the distribution of optical turbulence over the Bay. The different impact of water masses originating from the Indian Ocean and the Benguela current on the development of optical turbulence is discussed. The seasonal behavior of optical turbulence is presented and its effect on electro-optical system performance examined. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

A comparison of aerosol data acquired at five different sites around the globe is presented. All data has been acquired with the same instrumentation and representative size distributions for marine air masses at 10 m/s wind speed have been selected for comparison. Differences in the concentrations of larger and smaller aerosols at the various sites are explained in terms of fetch, trade winds, shielding, pollution, seawater temperature and phytoplankton bloom. The differences in size distribution induce significant differences in the extinction coefficients from the VIS to the LWIR at the various sites. Consequently, the transmission over a specific range also varies significantly. This suggests that a detailed analysis of the conditions at each site is necessary in order to understand the exact aerosol behavior and to correctly predict electro-optical propagation effects due to aerosols. © 2016 SPIE. The Society of Photo-Optical Instrumentation Engineers (SPIE)

A recent field trial in the Northern German littoral area of the Baltic Sea yielded a dataset of visibility, meteorological parameters, aerosol size distributions, as well as transmission over a horizontal path of 1344 m. The experimental results are compared to simulations using the MODTRAN (moderate resolution atmospheric transmission) model, that was run with the rural and Navy Aerosol Model, (NAM) in various configurations. Best results were obtained when MODTRAN was tuned with the measured visibility values. When NAM was used without visibility tuning, MODTRAN tended to overestimate the transmission in low-visibility conditions, which was attributed to the presence of a non-maritime aerosol fraction. © 2015 SPIE.

Our world is constantly changing, and this has its effect on worldwide military operations. For example, there is a change from conventional warfare into a domain that contains asymmetric threats as well. The availability of high-quality imaging information from Electro-Optical (EO) sensors is of high importance, for instance for timely detection and identification of small threatening vessels in an environment with a large amount of neutral vessels. Furthermore, Rules of Engagement often require a visual identification before action is allowed. The challenge in these operations is to detect, classify and identify a target at a reasonable range, while avoiding too many false alarms or missed detections. Current sensor technology is not able to cope with the performance requirements under all circumstances. For example, environmental conditions can reduce the sensor range in such a way that the operational task becomes challenging or even impossible. Further, limitations in automatic detection algorithms occur, e.g. due to the effects of sun glints and spray which are not yet well-modelled in the detection filters. For these reasons, Tactical Decision Aids will become an important factor in future operations to select the best moment to act. In this paper, we describe current research within The Netherlands on this topic. The Defence Research and Development Programme “Multifunctional Electro-Optical Sensor Suite (MEOSS)” aims at the development of knowledge necessary for optimal employment of Electro-Optical systems on board of current and future ships of the Royal Netherlands Navy, in order to carry out present and future maritime operations in various environments and weather conditions.

Knowledge on the marine boundary layer is of importance for the prediction of the optical image quality obtained from long range targets. One property of the boundary layer, that can be studied rather easily by means of optical refraction measurements, is the vertical temperature profile. This profile can be compared with the profile, as predicted by the generally accepted Monin-Obukhov (M-O) similarity theory, such as applied in the EOSTAR model, developed at TNO. This model also predicts the atmospheric turbulence profile, for which a validation can be done by means of scintillation measurements. Along these lines we explored the data from the year-round FATMOSE experiment, arranged over the False Bay (South-Africa). Because of the large amount of refraction and scintillation data, supported by extensive data from various local weather stations, we could select the conditions for which the M-O theory is valid and determine the particular conditions where this theory is failing. In the paper model predictions (including Angle of Arrival calculations in non-homogeneous conditions along the 15.7 km path) and associated refraction and scintillation measurements are shown for a representative variety of conditions. © 2011 SPIE.

The FATMOSE trial (False Bay Atmospheric Experiment) is a continuation of the cooperative work between TNO and IMT on atmospheric propagation and point target detection and identification in a maritime environment (South Africa). The atmospheric transmission, being of major importance for target detection, was measured with the MSRT multiband optical/IR transmissometer over a path of 15.7 km over sea. Simultaneously a set of instruments was installed on a midpath lighthouse for collection of local meteorological data, including turbulence, scintillation, sea surface temperature and visibility. The multiband transmission data allow the retrieval of the size distribution (PSD) of the particles (aerosols) in the transmission path. The retrieved PSD’s can be correlated with the weather data such as windspeed, wind direction, relative humidity and visibility. This knowledge will lead to better atmospheric propagation models. The measurement period covered nearly a full year, starting in November 2009 and ending in October 2010. The False Bay site is ideal for studies on propagation effects over sea because of the large variety of weather conditions, including high windspeed, expected from the South East with maritime air masses, as well as Northerly winds, expected to bring warm and dry air from the continent. From an operational point of view the False Bay area is interesting, being representative for the scenery around the African coast with warships in an active protecting role in the battle against piracy. The yearround transmission data are an important input for range performance calculations of electro-optical sensors against maritime targets. The data support the choice of the proper spectral band and contain statistical information about the detection ranges to be expected. In this paper details on the instrumentation will be explained as well as the methods of calibration and PSD retrieval. Data are presented for various weather conditions, showing correlations between different parameters and including statistical behaviour over the year. Examples will be shown of special conditions such as refractive gain, gravity waves and showers.

The FATMOSE trial (FAlse-bay ATMOSpheric Experiment) running over a period from November 2009 to July 2010,was a continuation of the cooperation between TNO and IMT on atmospheric propagation and point target detection and identification in a maritime environment. Instruments were installed for measuring scintillation, blurring- and refraction effects over a 15.7 km path over sea. Simultaneously, a set of instruments was installed on a mid-path lighthouse for collecting local meteorological data, including scintillation, sea surface temperature and visibility. The measurements covered summer and winter conditions with a prevailing high wind speed from the South East, bringing in maritime air masses. The weather conditions included variations in the Air-Sea Temperature Difference (ASTD), that may affect the vertical temperature gradient in the atmospheric boundary layer, causing refraction effects in the lightpath. This was measured with a theodolite camera, providing absolute Angles of Arrival (AOA). Blur data were collected with a high resolution camera system with 10 bits dynamic range. Specially designed image analysis software allows determination of the atmospheric blur, while simultaneously providing information on the Scintillation Index (S.I.). This S.I. was also measured by using the Multiband Spectral Radiometer Transmissometer (MSRT). The ratio of the transmission levels of this instrument contains information on the size distribution of the aerosols along the path. In the paper, experimental details on the set-up and the instrumentation are given as well as the methods of analysis. Preliminary results are shown, including a comparison of measured blur and scintillation data with Cn 2 data from the scintillometer, correlation between AOA and ASTD and comparison of transmission data with data from the visibility meter. Blur and scintillation data are compared with predictions from standard turbulence model predictions, using Cn 2. In future studies the data will be used for validation of propagation models such as EOSTAR.

The FATMOSE trial (FAlse-bay ATMOSpheric Experiment) running over a period from November 2009 to July 2010, was a continuation of the cooperation between TNO and IMT on atmospheric propagation and point target detection and identification in a maritime environment. Instruments were installed for measuring scintillation, blurring- and refraction effects over a 15.7 km path over sea. Simultaneously, a set of instruments was installed on a mid-path lighthouse for collecting local meteorological data, including scintillation, sea surface temperature and visibility. The measurements covered summer and winter conditions with a prevailing high wind speed from the South East, bringing in maritime air masses. The weather conditions included variations in the Air-Sea Temperature Difference (ASTD), that may affect the vertical temperature gradient in the atmospheric boundary layer, causing refraction effects in the lightpath. This was measured with a theodolite camera, providing absolute Angles of Arrival (AOA). Blur data were collected with a high resolution camera system with 10 bits dynamic range. Specially designed image analysis software allows determination of the atmospheric blur, while simultaneously providing information on the Scintillation Index (S.I.). This S.I. was also measured by using the Multiband Spectral Radiometer Transmissometer (MSRT). The ratio of the transmission levels of this instrument contains information on the size distribution of the aerosols along the path. In the paper, experimental details on the set-up and the instrumentation are given as well as the methods of analysis. Preliminary results are shown, including a comparison of measured blur and scintillation data with Cn2 data from the scintillometer, correlation between AOA and ASTD and comparison of transmission data with data from the visibility meter. Blur and scintillation data are compared with predictions from standard turbulence model predictions, using Cn2. In future studies the data will be used for validation of propagation models such as EOSTAR.

Atmospheric turbulence can cause severe blurring and scintillation in images. These distortions result in less detailed images. Using only image enhancement techniques can improve the quality of the images to some extend, but usually not enough to see all details. Turbulence correction techniques, such as post-processing techniques using a deconvolution, are developed and show good results. A more profound understanding of atmospheric turbulence, and especially the impact of turbulence on the image quality, might help to improve the quality of these images even further. To this end, we present a simple method of estimating the shape and size of the point spread function. The method is tested using images from an ongoing trial in South-Africa.

The Advanced Navy Aerosol model (ANAM), being a modified version of the Navy Aerosol Model (NAM), is a wellknown engineering tool providing a quick and reasonable estimate of the aerosol extinction in the marine near-surface environment on the basis of simple meteorological input data. The original NAM consists of 3 lognormal distributions, which describe freshly produced marine aerosols, aged marine aerosols (produced elsewhere and advected to the measurement site) and a background concentration of marine aerosols. The ANAM adds a 4th lognormal mode to NAM to account for the largest marine particles. To account for non-marine particles, a special lognormal mode, called “dust mode” was included in NAM. The relative importance of the dust mode versus the marine background concentration is governed by a special input parameter known as the air mass parameter (AMP). Unfortunately, the AMP is ill-defined and the NAM user community has found it difficult to attribute a proper value to the AMP. This inconvenience became even more stressing when NAM was used for assessing aerosol extinction in the coastal zone. To overcome this inconvenience, a new approach is suggested which involves replacement of the AMP by the Angstrom coefficient. The advantage is that the latter parameter can be directly measured and has a physical relation to the aerosol size distribution. When the particle size distribution is dominated by small particles, usually associated with pollution, the Angstrom coefficients are high; in clear conditions they are usually low. Therefore this parameter is a good tracer of the aerosols originated over land and hence a good replacement for the AMP.

The environment is nowadays one of the most limiting factors for reliable detection, clear imagery and thus a successful classification of potential threats by electro-optical (EO) sensors. However, the characterization of the environment and the assessment of its impact on sensor performance remains a difficult issue. Measurements of meteorological parameters are not always easy and cannot always be reliable. It becomes more and more interesting to extract the information the environment by new methods. In this paper, the initial steps and the methodology of an inverse scheme that retrieve valuable information about the EO propagation conditions from infrared (IR) camera images is proposed. The use of the method under subrefractive conditions shows that features of the medium can be derived through a thorough analysis of sensor images. By an original use of EO propagation modeling, it is possible to partially reconstruct sensor images that were deformed by a refractive atmosphere

The knowledge of the atmospheric aerosol characteristics is of great importance for the range performance of Infrared and Electro-Optical sensor systems. The composition, the concentration and the size distribution of aerosols determine their scattering behavior as function of wavelength and thus their attenuation of light beams. When studies are made on this attenuation, it is considered to be very useful to incorporate the spatial variation of the aerosol characteristics along the measurement path, such as found during a previous campaign, carried out over the San Diego Bay (August 2005 [1]). In a more recent experiment (November 2006) a trial was set-up at Scripps pier of the Institute of Oceanography near La Jolla (US-West Coast). This place is known for the high frequency of occurrence of hazy conditions, not only variable with location, but also in time. TNO did participate in this experiment with two Particle Measurement Systems (PMS), one on the shore and one at the end of the pier, a weather station and the Multi Spectral Radiometer Transmissometer (MSRT), used in previous trials ([1], [2], [3]). It was shown before, that the use of multi-band transmission data allows a more detailed analysis of the aerosol characteristics in the measurement path ([4]). The advantage of the MSRT concerns a larger measurement volume, thus providing a higher signal to noise (S/N) ratio and a shorter response time, compared to the in-situ PMS systems. A similar method, based on the spectral dependence of the scattering coefficient, has been used during the analysis of the data, collected in November 2006. In this paper representative data samples are presented, showing the variability of the transmission in each of the spectral bands. By using the weather data, a separation has been made between extinction by molecules and aerosols. A brief description is given of the retrieval method, just mentioned. The retrieved particle characteristics are compared to the data from the PMS systems, taking into account the fact, that part of the measurement path, having a length of 6.68 km, was grazing the shore-line. In general the particle density, as found by the PMS systems was less than the retrieved value, especially when the wind was from off-shore directions. At certain occasions, sinusoidal variations in the transmission level, with a period of about 5 minutes, were found, probably due to oscillatory motions of the air mass in the measurement path. It was found, that the value of the Junge exponent of the Particle Size Distribution (PSD) was rather frequently of the order of -3, showing that the concentration of bigger particles (> 3 μm) compared to the smaller particles (< 0.5 μm), was greater than the value, found in experiments at other locations. It is investigated how accurate the transmission levels of the IR bands can be predicted by using the retrieved PSD's and the absolute humidity, obtained from the weather data. Due to the low wind speed during the trial (< 4 m/s), it was not possible to find any particular effect from aerosols, potentially created in the surf zone at higher wind speeds.

The variation of the extinction coefficient with wavelength can be presented as a power law function with a constant (related to the power factor) known as the Angstrom coefficient. When the particle size distribution is dominated by small particles, usually associated with pollution, the Angstrom coefficients are high; in clear conditions they are usually low. Long residence time of air masses over land and in particular the passage over large urban areas cause high concentrations of fine particles and thus high values of the Angstrom coefficients. The opposite effect can be observed over water. The longer the time that the air masses spent over water the more evident is a change in the aerosol size distribution caused by the deposition of continental aerosols. As a result of this process the measured Angstrom coefficient values become much smaller. Therefore this parameter is a good tracer for the concentration of aerosols originated over land. The relation between the Angstrom coefficient and TOS (time over sea) is demonstrated on three data sets. The first data set includes measurements collected at the Irish Atlantic coast in 1994 and 1995, the second one, data collected within the Rough Evaporation Duct (RED) experiment that took place off Oahu, Hawaii in 2001. The third one represents data collected at the Baltic Sea during cruises in 1997 and 1998.

In an earlier paper [1], data from our Multi-Band Radiometer Transmissometer (MSRT) were used to compare the ratio of extinction coefficients in different spectral bands during periods of changing visibility conditions. This ratio is an indication of the characteristics and origin (eg rural or maritime) of the haze- or fog particles, present in the measurement path. In this paper we will analyze the VAMPIRA transmission data in more detail by separating the contributions due to molecular extinction, scattering and (potentially) refraction. In our analysis we take the contribution due to scattering in order to obtain the characteristics of the Particle Size Distribution (PSD). For this purpose we take the average value and the slope of the measured transmission level in two neighboring spectral bands. Via a special simulation tool, developed for Junge-type PSD's, the slope of the PSD (defined: Junge exponent) and its value at a particle diameter of 1 μm (Junge coefficient) can be determined via a set of retrieval steps. Reference is made to a similar approach [2] where in stead of a Junge distribution, three contiguous lognormal distributions are taken. The associated procedure for the Junge-type PSD is explained in detail in this paper and applied to the VAMPIRA transmission data. The versatility of the new retrieval method is demonstrated, especially when wavelengths around 1 μm are chosen (a somewhat higher number than the diameter of the majority of the particles, so that most of the scattering is in the so-called Rayleigh regime). It is obvious, that the method fails in conditions of dense fog, when the transmission levels (average value and slope) over the 8.6 km path approach zero. The results are compared with in-situ PSD measurements, carried out simultaneously with a PMS (Particle Measurement System) probe at the pier near the Surendorf shore station. In many conditions different results appear due to the fact that the MSRT system delivers path integrated data, while the PMS probe measures locally in a small volume. The MSRT data, collected over an overseas path, are more relevant to be used in the data analysis of the shore based sensor systems [3], measuring simultaneously signal values of distant point targets. The MSRT system has a higher signal to noise ratio and due to the shorter time constant, rapid fluctuations in particle characteristics are observed, not measured by the PMS probe. The availability of reliable aerosol characteristics (i.e. Junge exponent and -coefficient) allows a more precise interpretation of the data from the surveillance systems.

The results of applying the empirical orthogonal functions (EOF) method to decomposition and approximation of aerosol size distributions are presented. A comparison was made for two aerosol data sets, representing coastal and oceanic environments. The first data set includes measurements collected at the Irish Atlantic coast in 1994 and 1995, the second one data collected during the Rough Evaporation Duct (RED) experiment that took place off Oahu, Hawaii in 2001. The main finding is that aerosol size distributions can be represented by a superposition of the mean size distribution and the first eigenvector multiplied by an amplitude function. For the two aerosol data sets the mean size distribution is very similar in the range of small particles sizes (radius 1μm). It is also reflected by the spectral shape of the eigenvector. The differences can be related to the type of aerosols present at both locations, and the amplitude function can be associated to meteorological conditions. The amplitude function also indicates the episodes with the maximum/minimum continental influence. The results of this analysis will be used in upgrades of the ANAM model.

The presence of atmospheric aerosols along the line of sight of infrared and electro-optical sensors greatly determines the range performance of these devices. On the one hand the aerosol particles scatter background (including sun) radiance into the field of view of the sensor, on the other hand they contribute to the atmospheric contrast reduction of the target. Proper knowledge of aerosol characteristics such as composition, concentration and size distribution is of vital importance for the prediction of their scattering and extinction characteristics. It is however found to be very difficult to collect accurate information on the particle size distribution (PSD) of aerosols. One of the reasons is the variation of the PSD along the path, which is likely to occur in a coastal area such as the San Diego Bay. One way to overcome these problems is the use of a multi-band transmissometer, as was done in previous measurement campaigns in the Baltic Sea [1] and in the Persian Gulf area [2]. The TNO seven-band optical/IR transmissometer system, providing path averaged transmission data for the intervening atmosphere, is operating at wavelengths between 0.4 and 14 μm,. In this spectral band, scattering in light hazy conditions is dominated by particles with a diameter of less than 4 μm. In order to simulate the transmission losses by scattering in various spectral bands a special calculation tool has been developed. This tool, described in this paper, allows detailed investigation of the possibilities of the retrieval of the PSD from multi-band transmission data. The slope in the plots of the transmission versus wavelength is directly related to the slope of the (lognormal) PSD plots (known as Junge exponent). The average transmission in a selected number of spectral bands is directly correlated to the average particle concentration (known as Junge coefficient). The principle of the methodology is illustrated with data collected during a measurement campaign, carried out over the San Diego Bay in August 2005. In this campaign we used six of the seven spectral bands, providing data over a 7.2 km over water path. It is shown that the retrieval method is very successful and the data correspond well with those, simultaneously collected with in-situ Particle Measurement Systems (PMS), located on both sides of the path. In addition to the path averaging, another advantage of the transmissometer PSD's, is the accuracy, being an order of magnitude higher than that of the PMS probes due to the fact that the measurement volume is more than a million times larger. A detailed analysis is given of the transmission data, showing peculiar effects were in the 2.3 urn band around 19.00 UTC during most of the days. These effects are especially well illustrated in plots where the transmission in one spectral band is plotted against the transmission in another band for various times of the day.

The Navy Aerosol Model (NAM, available in MODTRAN) is widely used as a tool to assess the aerosol extinction in the marine atmospheric surface layer. NAM was built as a regression model in the 1980s to represent the aerosol extinction at deck height as a function of the meteorological conditions. The recently developed Advanced Navy Aerosol Model (ANAM) utilizes additional experimental evidence to supersede NAM by correcting the underestimation of the concentration of aerosols larger than a few microns. More importantly, ANAM provides the aerosol extinction as a function of height between the surface and several tens of meters. Present-day naval surveillance and threat scenarios require detection of targets at the horizon, such as sea-skimming missiles, or small targets such as rubber boats. In either case, the propagation path from sensor to target is likely to come very close to the wave surface and in order to estimate detection ranges, an assessment of the transmission losses along the path is necessary. To answer the question posed in the title, we assess the two models using two meteorological data sets (784 cases) representative of diverse maritime conditions in regions of interest around the world.