Operation and design of electro-optical systems are affected by atmospheric optical turbulence quantified by the
refractive index parameter Cn2. Regarding wave propagation in the visible and infrared (IR), Cn2 is a function of height,
dependant on temperature, pressure, and the structure temperature function parameter Cn2. The long-term experiment
VerTurM (vertical turbulence measurements) was designed to characterize the vertical variations of optical turbulence up
to 250 m in the lower atmospheric boundary layer for a moderate typical central European climate. Since May 2009 three
independent measurement systems have been operated in a flat pasture site in north-western Germany. In the atmospheric
surface layer at a tall tower sonic anemometer measurements are performed on four discrete heights between 4 and 64 m
providing information about atmospheric stability and turbulence. Cn2 is derived. From 30 to 250 m a SODAR-RASS
system (Sound Detection and Ranging - Radio acoustic sounding system) yields every half an hour profiles of Cn2. Additional direct measurements of Cn2 have been performed near the ground using a scintillometer. First results of the
three measurement systems are presented. Vertical profiles and stability dependence are analysed in respect of Monin-
Obukhov-similarity theory (MOST). Differences in the measurement systems and the expected height variations are
discussed.
Electro-optical sensors are affected by the atmospheric turbulence, as quantified by the refractive index structure
parameter. The present study introduces a method to predict the meteorological-scale variations of this quantity near the
surface. The predictions are evaluated against long-term scintillometry measurements. The essential aspects of the
meteorological variability of the optical turbulence rate are captured. The method is illustrated to provide a global and
predictive assessment of the optical turbulence rate. It can also be used to analyze the corresponding climatological
distributions. Existing relationships can further be incorporated to form predictions of the mean optical sensing
performance.
Within the scope of the investigation of material for camouflage, concealment, and deception (CC&D material) in
desert environment, it is necessary to evaluate the impact of turbulence on thermal imagers in the MWIR and in the
LWIR. Turbulence decreases the effectiveness of electro-optical systems. It causes a reduction of the spatial resolution
of thermal imagers, which is characterized by the turbulence modulation transfer function MTF. Turbulence MTF
depends on atmospheric parameters, e.g. the strength of atmospheric turbulence as described by the structure parameter
of the refractive index fluctuations, Cn2, the atmospheric path length, the cross-wind velocity, and on the sensor
parameters, i.e. the wavelength and the aperture diameter of the front optics. The total MTF is the product of turbulence
MTF and sensor MTF, which in turn is the product of detector MTF and optics MTF. As a figure of merit for the spatial
resolution, we used the area under the total MTF (MTFA). Based on our turbulence measurement that was taken in arid
climate in Negev desert, Middle East, we calculated the total MTFA for a dual-band thermal imager (MWIR and
LWIR) with two available optics under diverse turbulence conditions and for different path lengths up to 5 km. The
selected Cn2 values are representative for the diurnal run of Cn2 in arid summer or different times of day, respectively.
We defined a turbulence degradation factor X to estimate the impact of turbulence on the image quality as a function of
time of day and path length. Resulting MTFAs and the corresponding turbulence degradation factors will be discussed
in details.
FGAN-FOM carried out a long-term experiment to measure Cn2 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 Cn2, 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 Cn2 values. Turbulence over sea differs significantly from turbulence over land. The diurnal run of Cn2 does not show generally the characteristic maximum at midday, Cn2 values measured during night are not generally smaller than those measured at midday, and Cn2 values measured in the daytime in summer are not generally larger than those measured in winter season. Since Cn2 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 Cn2 as a function of ASTD for the measured meteorological parameters. The measurements indicate larger Cn2 values than the calculations.
The refractive-index structure parameter Cn2 is the parameter most commonly used to describe the optically active turbulence. In the past, FGAN-FOM carried out long-term experiments in moderate climate (Central Europe, Germany), arid (summer), and semiarid (winter) climate (Middle East, Israel). Since Cn2 usually changes as a function of time of day and of season its influence on electro-optical systems should be expressed in a statistical way. We composed a statistical data base of Cn2 values. The cumulative frequency of occurrence was calculated for a time interval of two hours around noon (time of strongest turbulence), at night, and around sunrise (time of weakest turbulence) for an arbitrarily selected period of one month in summer and in winter. In October 2004 we extended our long-term turbulence experiments to subarctic climate (North Europe, Norway). First results of our turbulence measurement over snow-covered terrain indicate Cn2 values which are similar or even higher than measured values in Central European winter. The statistical data base was used to calculate the expected turbulence-induced aperture-averaged scintillation index for free-space optical systems (FSO system) in different climates. The calculations were performed for commercially available FSO systems with wavelength of 785 nm and 1.55 µm respectively and with aperture diameters of the receiver of 60 mm and 150 mm for horizontal path at two heights, 2.3 m and 10 m above ground.
During long-term experiments FGAN-FOM measured Cn2 values over land with identical scintillometers in two different climates, in moderate climate in mid-Europe and in arid climate. Since Cn2 usually changes as a function of time-of-day and of season its influence on electro-optical systems can only be expressed in a statistical way. The cumulative frequencies of occurrence were calculated for a time period of one month for different times of the day.
The statistical analysis was applied to calculate the effects of atmospheric turbulence on sensor performance like turbulence MTF, the resolution limit due to turbulence and intensity fluctuation. The calculations were performed for a SWIR sensor (active imaging system) and for typical MWIR and LWIR warning sensors. Turbulence MTF were calculated for a slant path of 5 km from the ground to a height of 100 m for upward and downward looking cases. For horizontal paths at a height of 2 m and 30 m the resolution limits due to turbulence were compared with the corresponding diffraction-limited ones. Calculations of the normalized intensity fluctuations were carried out for two slant propagation paths (zenith angle β = 30° and 80°).
NATO Task group TG16 is cooperating on topics related to ship self-defence. One of these topics is related to IR Search and Track sensors, which are in development for detection of low altitude air targets. In particular the group is working on models to predict the range performance of these sensors. Newly developed models include marine boundary layer effects such as refraction due to temperature gradients, scintillation due to turbulence and particle size distributions. TG16 organized in May 2001a trial in the Mediterranean Sea near Livorno, Italy, called POLLEX to further validate these models. Seven nations particulated with complementary instruments for measurements of the target signatures and environmental characteristics. Three targets were provided, a series of small visual/IR sources at a fixed distance, visual/IR soruces on a ship moving in and out up-to and beyond the horizon and a helicopter. The weather conditions during the measurement period showed interesting variations in Air to Sea Temperature DIfference and atmospheric turbulence. Data have been analzyed and samples of the results, as collected and/or analyzed by the participants, are discussed in this paper.
Atmospheric turbulence may have a strong impact on the imaging quality of long range warning sensors and other electro-optical systems. Major effects are beam broadening, intensity fluctuations (or scintillation) and angle-of- arrival fluctuations. The structure constant of refractive index fluctuations, Cn2, is the parameter most commonly used to describe the strength of atmospheric turbulence. FGAN-FOM measured Cn2 values in two different climates, moderate climate in mid-Europe, Germany and arid climate in Israel. The measurements in arid climate were carried out in cooperation with the EORD (Electro-Optics Research & Development Foundation Ltd.), TECHNION, Haifa, Israel. The measurements were performed with identical laser scintillometers along a horizontal optical path of about 100 m, above grassland in mid-Europe, and above stony ground without vegetation in Israel. The data were collected continuously for a time period of at least one year at a time resolution of 5 minutes. For both climates examples of the diurnal cycle of Cn2 are given. Since Cn2 usually changes as a function of time-of-day and of season its influence on electro-optical systems can only be expressed in a statistical way. Therefore the cumulative frequencies of occurrence of Cn2 were calculated for a time period of one month for both climates. These results were used to calculate the corresponding turbulence modulation transfer function (MTF) and point spread function (PSF) for a typical IR sensor with a Cadmium Mercury Telluride detector (CMT) and a UV sensor.
The development of IR sensors with better spatial, temporal and thermal resolution pushes the necessity to revise and quantify the different atmospheric effects on sensor performance. In this paper, a high resolution IR sensor for point target detection is used as an example to show theoretically and experimentally the relevant atmospheric limitations. Special emphasis is put on the signal-to-noise ratio of point like objects in IR image sequences, on track evaluation of such objects, on the impact of optical turbulence, polarization and refraction in the track analysis.
The FGAN-FfO participated in the LAPTEX trial in Crete in July 1996. Irradiance fluctuations or scintillation of visible point targets were measured with a digital camera at frame rates up to 1000 Hz during different atmospheric turbulence conditions. One optical path with a length of 2560 m was directed parallel to the coast line crossing water as well as land. The strength of atmospheric turbulence was determined by measuring the structure constant of refractive index, Cn2, and the inner scale of turbulence, l0, with a laser scintillometer over irregular coastal rock structures. The analysis of the recorded image sequences was performed with the interactive data language IDL. The irradiance fluctuations of the target-to-background differences show a log-normal frequency of occurrence distribution. The corresponding variances in angle-of-arrival fluctuation ((alpha) 2) were determined. Calculations of the variance in angle-of-arrival fluctuation were carried out using the path-weighted structure constant of refractive index, Cn2. Theoretical values of ((alpha) 2) indicate a variation, which is an order-of- magnitude larger than the variation in the experimental values.
A lidar-transmissometer intercomparison was made during an international experiment held in the German Alps to characterize the vertical structure of aerosols and clouds. The transmission path was 2325-m long and inclined at 30 degrees along the slope of a steep mountain ridge. the transmissometer consisted of a Nd:YAG and a CO2 laser located in the valley and a large-mirror receiver that captured the full beams on the mountain top. Two lidars, one at 1.06 micrometer and one at 1.054 micrometer, were operated with their axes approximately parallel to the transmissometer axis but separated by a horizontal distance on the order of 20 - 40 m. The first one was operated in retroreflector mode and the relative transmittance was determined from the reflection off the mountain ridge above the cloud layer. The second one had a special receiver designed to make simultaneous recordings at four fields of view. The range-resolved scattering coefficient and effective cloud droplet radius are calculated from these four-field-of-view measurements by solving a simplified model (Appl. Opt. 34, 6959-6975, 1995) of the multiply scattered returns. The two simultaneous solutions for the scattering coefficient and effective droplet size make possible extrapolation at wavelengths other than the lidar wavelength of 1.054 micrometer. The main measurement event analyzed in this paper lasted 1.5 hours and produced transmittances ranging from less than 5% to more then 90%. The comparisons show good correlation between the transmissometer data and all lidar solutions including extrapolation at 10.59 micrometer.
The influence of atmospheric vertical structure variations on infrared propagation and imaging has been analyzed based on measurements taken during the VAST92 experiment in the German Alps. A blackbody source at a distance of 2.3 km and an altitude of 1.8 km was detected against various sky backgrounds by a scanning DUWIR camera from GEC and a staring PtSi-camera from Mitsubishi. The propagation media has been characterized in detail using supporting atmospheric parameter measurements from lidars, a transmissometer, a spectral radiometer, aerosol counters and various meteorology stations at different elevations. The analysis of these measurements is presented. Two different weather conditions have been selected for analysis in this report: a dynamic cloud layer between sensor and point source, and a clear and stable situation with blue sky in the background. The recorded image sequences of the point source have been analyzed with respect to spatial and temporal fluctuations. Emphasis has been placed on the comparison of the two wavebands (3 - 5 micrometer and 8 - 12 micrometer) and on comparison of different sensing techniques. The results are currently being used to improve and validate point detection algorithms.
The vertical structure of the atmospheric aerosols was investigated during cable car ascents along a steep mountain slope in the German Alps. Aerosol size distributions were measured by operating two particle spectrometer probes with diameter ranges of 0.15-3 micrometers and 1-95 micrometers . Aerosol and hydrometeorits were sampled under the meteorological conditions of clear atmosphere, haze, and thick clouds. For each atmospheric layer the phase function P((theta) ) and the extinction coefficient (sigma) ext were calculated for the visual ((lambda) equals 0.55 micrometers ), the near IR with Nd:YAG laser radiation ((lambda) equals 1.064 micrometers ) and iodine laser radiation ((lambda) equals 1.315 micrometers ) and the middle IR with CO2 laser radiation ((lambda) equals 10.59 micrometers ) using the computer code AGAUS of the EOSAEL program library of the US Army Research Laboratory. In the visual and the near IR the aerosol extinction coefficient is not sensitive to variations of the complex refractive index. However, for the CO2 laser radiation, the uncertainty in the values of optical constants causes large deviations in calculated aerosol extinction coefficients.
A long-range laser transmissometer to measure atmospheric extinction at different laser wavelengths simultaneously from near to thermal infrared, (1.06 and 10.6 micron) was designed, constructed, and operated under different atmospheric conditions over a distance of 8.6 km in hilly terrain near Tuebingen, Germany. Beam extinction was obtained by measuring the ratio of the total transmitted laser radiation to the total received radiation as collected by a mirror and focused onto a pyroelectrical detector array. Measured values of Nd:YAG and CO2 laser extinction are compared with model predictions (FASCODE 3P) based on simultaneously measured meteorological data as model input parameters. The agreement between measurement and calculation is better than expected. The corresponding extinction coefficients cover the range 0.05/km to 0.17/km for Nd:YAG laser radiation and 0.07/km to 0.25/km for CO2 laser radiation.
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