In the marine boundary layer, air-sea interaction processes have an impact on radar and infrared propagation. Range performance near the sea surface depends on the meteorological conditions and sea surface roughness. Strong gradients of humidity and temperature close to the air-water interface are most often the reason for abnormal propagation effects such as ducting or mirage. For ship borne radars the evaporation duct is the dominant propagation mechanism affecting the maximum detection range of horizon-search radars. Ducting can also increase sea clutter return within and beyond the geometric horizon. Surface-based ducts can enhance land clutter return from extended ranges. During a sea trial in the Baltic Sea in 2005, FWG characterized the environmental boundary layer. In-situ measurements included recordings of atmospheric and sea surface parameters. Simultaneous investigations were carried out at the land based test site and on board two ships. Based on FWG-buoy measurements and radiosoundings the sea surface and meteorological conditions were analyzed to study refractive variability within the maritime boundary layer. We compared measurement results with predictions of the mesoscale meteorological Local Model (LM), developed by German Weather Service. Radar propagation was measured in addition to atmospheric conditions. A research vessel was illuminated by radar operating at X-band on outbound and inbound runs. The radar system was located at the pier of the land based test site. Radar propagation characteristics were measured on board the ship with two omni directional antennas mounted in 5.5 m and 16.8 m height above mean sea level. Results of refractive variability are presented in conjunction with radar propagation data and model outputs.

Bulk Monin-Obhukov modeling of atmospheric profiles is extensively employed in conjunction with ray-tracing to account for refraction-induced ray-bending in sensor simulation studies. Last year, the accuracy of such methods was assessed on a large diversity of conditions by consolidating data obtained from two different measurement campaigns. Model predictions of path deviations were found to agree with measurements to within plus or minus 0.1 mrad in most cases, except for air-sea temperature differences greater than about 2 °C where much larger discrepancies were obtained. In this paper, bulk models of temperature and humidity profiles under stable conditions are discussed. Calculations using two modeling approaches found in the literature are compared against measurements, and suggestions are made for model improvement.

The performances of Electro-Optical (EO) systems such as visible or infrared cameras, lasers, operating within the Marine Surface Boundary Layer (MSBL), i.e. at heights up to a few tens of meters above the sea surface, are disturbed by various propagation mechanisms: molecular attenuation, aerosol extinction, refraction and turbulence. Refraction is responsible for focusing and defocusing of rays, detection range limitations, mirage formation and angular deviation. The refractive index depends on atmospheric pressure, air temperature and air humidity. Within the optical transmission bands, it also depends on the wavelength. In this paper, the results provided by two different formulations of the refractive index associated with the same ray tracing program are compared and discussed.

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 lμm 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 Suerendorf 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 detection and tracking of missiles flying at a low altitude above the sea surface is one of the most urgent problems in ship self defense. These tasks are mainly managed by IR-Search and Track systems in the mid and long wave IR and by RADAR Systems. Both systems suffer severe limitations. The range efficiency of IR-systems is limited by atmospheric effects in the marine boundary layer. The NATO AC/323 SET-56/RTG32 on Integration of Radar and Infrared for Ship Self Defense has investigated the radar and infrared synergism with respect to propagation in a coastal environment. In spring 2004, the members have held the "VAlidation Measurements for Propagation in the Infrared and RAdar" (VAMPIRA). To have a direct comparison of RF and EO behavior, several systems were set up at the same altitude above sea level (approx. 19 m). This paper deals with the results of the IR-measurements. To simulate point-like targets at low altitudes, hot sources at different temperatures were installed onboard a small boat. Numerous mid and long-wave IR sensors made simultaneous measurements on the boat to analyse extinction versus range, maximum detection ranges and refraction effects. One efficient propagation models for the IR is IRBLEM (IR Boundary Layer Effects Model), developed by DRDC. The measurements of the boat runs were compared to the model predictions. Most of the measured data analyzed in this paper were gained by DDRE, DK. Based on the Danish results on the signal variations with range, a comparison with a Thermal Range Model for Point Target Detection (TRP) was done. TRP is an analytic model which had been developed at FGAN-FOM to estimate the range performance of a point target detection system working in the infrared.

The performance of sensors operating in coastal environments is severely influenced by the actual atmospheric conditions and the sea surface. Propagation models are in existence, which cope with the varying environment and allow a performance prediction for sensors in different bands of the electromagnetic spectrum. Model calculations give evidence for a complementary performance of sensors operating in the IR region and at radar frequencies ranging from X- to W-band. To validate existing radar propagation models like TERPEM and to compare IR with mm-wave propagation over sea under various atmospherically conditions, joint experiments with IR- and radar sensors were conducted over transmission ranges well beyond the horizon. For the measurements a naval vessel was moving on outbound and inbound courses ranging from the sensor site over the horizon, carrying corner reflectors acting as point targets at different heights above sea. This allowed a thorough investigation of duct propagation at different heights above the sea surface. The measurements were accompanied by a detailed environmental characterization of the sea surface and the atmosphere. The paper describes the experimental approach and gives representative results for measurement and simulation. The implications on performance especially for a multispectral (IR/mmW) approach are discussed.

This paper concerns the presentation of the MATISSE-v1.4 code whose main functionality is computation of spectral or integrated natural background radiance images. The spectral bandwidth ranges 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. Moreover, an Application Programming Interface (API) is included to facilitate its use in conjunction with external codes. In comparison with the previous version, the main improvement of MATISSE-v1.4 concerns the line of sight mode, the possibility to use a user atmospheric profile and computations in the Maritime Boundary Layer. This paper describes the range of functionalities of MATISSE-v1.4 as well as future developments.

This paper presents the basic theoretical equations needed for the measurement of turbulence characteristics from observations of the jitter of astronomic images from onboard a flying aircraft. The method allows us to record the integral turbulence intensity (integral of the structure characteristic of refractive index fluctuations C²_n along the optical path). The variance of the jitter of astronomic images is a measured characteristic. This characteristic is thoroughly studied and stable to measurement errors. Two optical detectors: monostatic and bistatic differential ones, were used as recording devices.

In this work a new model of density fluctuation for turbulent shear layers is presented. The density fluctuations are calculated by new numerical and analytical models based on CFD and are compared to those calculated by existing models. Some comparisons to experimental results are presented as well. I also use realization method of the turbulence field in order to calculate beam wander directly.

FGAN-FOM carried out a long-term experiment to measure C_n² over sea in littoral area in moderate climate, Central Europe. A Boundary Layer Scintillometer was installed along a 1.7 km path crossing a bay of the Baltic Sea (Eckernfoerder Bucht) at a height of 4.7 m above water level. Meteorological parameters were measured simultaneously. One of the main parameters, which effects C_n², is the temperature difference between air and ground. In general a larger temperature difference causes stronger turbulence. Over sea, the air-sea temperature difference, ASTD, is generally smaller than the air-ground temperature difference over land, which implicates smaller C_n² values. Turbulence over sea differs significantly from turbulence over land. The diurnal run of C_n² does not show generally the characteristic maximum at midday, C_n² values measured during night are not generally smaller than those measured at midday, and C_n² values measured in the daytime in summer are not generally larger than those measured in winter season. Since C_n² usually changes with environmental conditions, its influence on the effectiveness of electro-optical systems can normally only be expressed in a statistical way. We worked out a statistical database for atmospheric turbulence over sea accordingly to our turbulence statistics over land. The cumulative frequency of occurrence was calculated for a period of one month for a two-hour time interval during daytime and during night. Even though the meteorological conditions in Central Europe show a large variability, the cumulative frequencies of occurrence derived for 2003 and 2004 indicate the same seasonal devolution. We applied the LWKD model of the Defence Research and Development, DRDC Valcartier, Canada, to calculate C_n² as a function of ASTD for the measured meteorological parameters. The measurements indicate larger C_n² values than the calculations.

Atmospheric turbulence may have strong impact on astronomical imaging, aerial surveying, terrestrial geodesy, optical ranging, and wireless optical communication. Major effects are beam broadening, irradiance fluctuations (scintillation), and angle-of-arrival (AA) fluctuations. The interesting effects of atmospheric turbulence for optical propagation studies are the variation (gradient and fluctuations) of refractive index. The corresponding refractive- index structure constant, C_n², is the parameter most commonly used to describe the strength of atmospheric turbulence. Besides, the Modulation Transfer Function of the atmosphere is measurable by C_n². Good image quality requires C_n² being as small as possible. In this work we present an easily applicable and accurate method, based on moire technique, for the measurement of C_n² and its profile in the ground level atmosphere. In this method from a low frequency sinusoidal amplitude grating, installed at certain distance from a telescope, successive images are recorded and stored in a computer. By rotating one of the image by +θ, say 4°, and multiplying its transmission function by the transmission functions of the other images which have been rotated by -θ, a large number of moire patterns are produced. By finding the traces of the moire fringes in the patterns, the fluctuations of the image grating lines are obtained. Which correspond to AA fluctuations distribution. From the AA fluctuations distribution in successive patterns, C_n² and its profile in vertical direction are deduced. This technique renders to measure some other atmospheric parameters which are discussed in the report.

The problem of compensation for turbulent distortions of images is investigated.The full system of the Navier-Stokes equations was used for modeling of turbulent fluctuations. The problem of the convergence of the proposed mathematical procedure and the focusing of the laser beam in non-uniform atmosphere are also discussed. Results may be applied to the compensation for distortions of images from modern optical telescopes without using deformable mirrors, to indication targeting problems and even to the control of laser beam focusing in the case of the turbulent atmosphere.

Several approaches to the formation of a laser guide star for large-aperture telescopes are considered. Limiting capabilities of the optical arrangements considered are studies from the viewpoint of the wave front tilt correction.

The turbulence of the atmosphere puts an upper limit on the quality of the image of a ground object obtained by long-exposure photography from low or high altitudes in the atmosphere or in the space. By using good optics and high resolution film or CCD and a stable platform, this limit could be approached but not exceeded. A useful quantity for indicating the magnitude of this limit is the integral of the modulation transfer function (MTF) associated with the turbulence. In this work, we introduce a new method for measuring the MTF of the atmosphere in the surface layer, based on moire technique. In this technique, from a low frequency Ronchi grating, installed at a certain distance from a digital camera equipped with a tele lens, successive images are recorded and then transferred to a PC. By rotating each image by θ/2 and -θ/2, say ±3°, and superimposing them, a large number moire patterns are produced. The average transmittance function of the superimposed image gratings is measured in a moire fringe interval. The latter function is measured by scanning the moire pattern by a slit parallel to moire fringes. It is shown theoretically that from the Fourier transform of the latter function the MTF of the atmosphere can be deduced, if the MTFs of the imaging system and the grating are given or their effects are negligible. The atmospheric MTFs have been measured at different turbulence conditions. Also, we have studied the behavior of the atmospheric MTF respect to exposure time.

Atmospheric turbulence induces random delay fluctuations to any optical signal transmitted through the air. These fluctuations can influence for example the measurement precision of laser rangefinders. We have found an appropriate theoretical model based on geometrical optics that allows us to predict the amplitude of the random delay fluctuations for different observing conditions. We have successfully proved the applicability of this model by a series of experiments, directly determining the amplitude of the turbulence-induced pulse delay fluctuations by analysis of a high precision laser ranging data. Moreover, we have also shown that a standard theoretical approach based on diffractive propagation of light through inhomogeneous media and implemented using the GLAD software is not suitable for modeling of the optical signal delay fluctuations caused by the atmosphere. These models based on diffractive propagation predict the turbulence-induced optical path length fluctuations of the order of micrometers, whereas the fluctuations predicted by the geometrical optics model (in agreement with our experimental data) are generally larger by two orders of magnitude, i.e. in the submillimeter range. The reason of this discrepancy is a subject to discussion.

The beam spot will break up into some cracks when laser propagates through the turbulent atmosphere. The characteristics of the cracked beam spot are studied statistically for different apertures and beam qualities, and with different turbulent strengths. It is shown that, with the degeneration of the beam quality and the turbulence being stronger, the total size of the beam spots and the numbers of the fragments will increase; meanwhile the scale of the fragments will keep invariable almost. Four special situations are analyzed and the physical hypostasis of Fourier transformation is discussed to understand these results.

During recent years measurement campaigns have been conducted to result in a data base representative for propagation in the marine boundary layer at radar bands ranging from X- to W-band, which is valid for ocean areas typical for Europe. Very often a shortcoming of these measurements has been the lack of sufficient environmental data or a too small variation of environmental conditions during the measurement period. In the context of the CEPA 1, radar measurements have been conducted at the Baltic Sea with an excellent characterization of the atmosphere and the sea surface. In the framework of a cooperative program between Singapore and Germany new radar measurements have been done in the sea area around Singapore, which is a typical tropical environment. The data have been analyzed and propagation models have been tested using the relevant environmental information. It turned out that atmospheric conditions exist, which are considerably different from the known Baltic Sea situations. The paper describes the experimental approach and compares typical results for European and tropical conditions.