We show how to simulate realistic turbulent imagery using only two scalar fields, from which we derive a Gaussian and
non-isoplanatic Point-Spread Function (PSF). The first field controls mainly scintillation effects, while the second
principally controls image displacements. The model is designed for weak turbulence and is based on the first-order
Rytov theory for propagation through turbulence. We explain the physical principles behind the model and justify them
using empirical evidence.
A method is developed to study the synergism that can exist between horizon search radar and IR search and track (IRST) systems onboard ship under various meteorological conditions within the marine surface layer (<50 m). The method shows that four operational regions can be defined through the effect of the air-sea temperature difference and the air-sea water vapor pressure difference to produce sub- or superrefractive IR and rf propagation. It is also shown that no conditions can exist such that both IR and rf have subrefractive propagation. Applying the method to many meteorological observations shows that the method works quite well; however, it also indicates that there is not necessarily a detection range advantage to having both an optical and a radar system. However, the advantages to having an optical system are not solely dependent on its range performance. The precision with which an optical system can provide target track parameters, its ability to maintain track when radar systems cannot, and its ability to identify targets are synergisms that are quite significant.
A multinational campaign was organized by the NATO SET56 Group to assess transmission and propagation in coastal
environments: the VAlidation Measurements of Propagation in IR and RAdar (VAMPIRA) experiment. VAMPIRA was
conducted in the Baltic, near Surendorf, Germany, from 27 March to 4 April 2004. During VAMPIRA, transmission
was measured in the IR and the visible using a diversity of techniques. Among these, transmission was deduced from
point-target tracking using blackbodies on board a boat. In this paper, VAMPIRA transmission measurements in the IR
are compared with model predictions. We use MODTRAN for the calculation of gaseous attenuation in conjunction with
aerosol extinction models currently available, namely: NAM (as in MODTRAN), WKDAERX (as in IRBLEM), ANAM3 and MEDEX. The various models are presented and put in their historical contexts. We found that under most stable situations encountered at VAMPIRA, the 3-mode models, NAM and WKD, provide better prediction than the 4-mode models ANAM3 and MEDEX.
A multinational campaign was organized by the NATO SET56 Group to assess transmission in coastal environments:
the VAlidation Measurements of Propagation in IR and RAdar (VAMPIRA) experiment. VAMPIRA was conducted in
the Baltic Sea, near Surendorf, Germany, from 27 March to 4 April 2004. During VAMPIRA, transmission was
measured in the IR and the visible using a diversity of techniques. Transmissometers were installed across Eckernfoerde
Bay, while aerosol measurements were made on the pier using Particle Measurement Systems (PMS), and
visibilitymeters were deployed onshore and on a boat. Furthermore, VAMPIRA included point-target tracking
experiments using blackbodies mounted on a boat. Some VAMPIRA measurements have already been presented at
various symposiums. The purpose of this paper is to compare VAMPIRA transmission measurements and make
comparisons with transmission estimates that can be deduced from the blackbody tracking sessions.
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
Ray-bending resulting from atmospheric refraction in the maritime environment has been shown to potentially produce significant effects on electro-optical target detection and imaging. Positive bending makes possible detection beyond the horizon while negative bending reduces the maximum inter-vision range (MIVR) and is likely to produce severe image distortion or mirages. It has been shown by many authors that these phenomena can be efficiently described using ray-tracing in conjunction with bulk estimations of the refractivity profiles based on the Monin-Obhukov theory. In this paper, the accuracy of bulk methods to describe ray bending is assessed by examining angular deviations of apparent target elevations with respect to the meteorological conditions. Prediction accuracy is shown for a large spectrum of conditions, characterized by the air-sea temperature difference, by combining measurements collected in the North Sea and in the Baltic Sea. Moreover, the use of bulk profiles as opposed to profiles measured at sea by using a buoy is discussed.
We present results from trials of the LUCIE 2 (Laser Underwater Camera Image Enhancer) conducted in Halifax Harbor, Nova Scotia, Canada and Esquimalt Harbor, Victoria, British Columbia, Canada. LUCIE 2 is a new compact laser range gated camera (10 inches in diameter, 24 inches in length, and neutrally buoyant in water) originally designed to improve search and recovery operations under eye safe restrictions. The flexibility and eye safety of this second generation LUCIE makes it a tool for improved hull searches and force protection operations when divers are in the water attempting to identify bottom lying objects. The camera is equipped with a full image geo-positioning system. To cover various environmental and targets size conditions, the gate-delay, gate width, polarization and viewing and illuminating angles can be varied as well. We present an analysis on the performance of the system in various water conditions using several target types and a comparison with diver and camera identification. Coincident in-situ optical properties of absorption and scattering were taken to help resolve the environmental information contained in the LUCIE image. Several new capabilities are currently being designed and tested, among them a differential polarization imaging system, a stabilized line of sight system with step-stare capability for high resolution mosaic area coverage, a precision dimensioning system and a diver guided and operated version.
The effects of wind on scintillation decorrelation times are well known only for winds transverse to the propagation path. Therefore, we have investigated such effects using laser scintillation and sonic anemometer data obtained at an outdoor test site at DRDC-Valcartier. The data were taken during both day and night periods in May and June 2003. Meteorological data were also taken over these periods. Scintillation decorrelation times were then compared with the times deduced using the sonic anemometer wind data and Clifford's theory, which only uses the wind component transverse to the propagation path of the laser. Substantial differences were observed, because Clifford's theory does not take into account the effects of the wind's longitudinal component. A simple theoretical model has been developed to include the longitudinal wind. The model includes a free parameter that must be fitted to the data. Once this is done, the model allows us to predict, with reasonable accuracy, the scintillation decorrelation time scale given the inner scale and the wind at any angle. Residual discrepancies may be due to the internal evolution of the turbulence, which has not been included in our model and causes a decorrelation independent of the wind.
We provide a description of the general meteorological conditions observed during the POLLEX trial, the effects of turbulence on imaging systems, the measurement of the effects of turbulence on a specially constructed image target, and the determination of the refractive index structure parameter using the results from two different scintillometers, from a fast sonic anemometer and hygrometer, and from the bulk meteorology.
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.
Near the sea surface, atmospheric refraction and turbulence affect both IR transmission and image quality. This produces an impact on both the detection and classification/identification of targets. With the financial participation of the U.S. Office of Naval Research (ONR), Canada's Defence Research Establishment Valcartier (DREV) is developing PRIME (Propagation Resources In the Maritime Environment), a computer model aimed at describing the overall atmospheric effects on IR imagery systems in the marine surface layer. PRIME can be used as a complement to MODTRAN to compute the effective transmittance in the marine surface layer, taking into account the lens effects caused by refraction. It also provides information on image degradation caused by both refraction and turbulence. This paper reviews the refraction phenomena that take place in the surface layer and discusses their effects on target detection and identification. We then show how PRIME can benefit detection studies and image degradation simulations.
Extensive measurements were carried out of the optical properties of seawater off the East and West Coasts of Canada using a full-spectrum near-forward angle nephelometer (NEARSCAT). Using a new phase function adapted specifically to scattering from water borne particles, we analyze the data from coastal waters in the Straits of Juan de Fuca and in the Gulf of St. Lawrence. We show explicitly how the spectral properties can be combined with the angular properties to more reliably extract the Junge exponent of the particle size distribution and the mean index of refraction of the particles. We obtain a simple analytic expression for the normalized cumulative phase function that can be used to compute the backscatter ratio, and its explicit wavelength dependence. Accurate estimation of this wavelength dependence is required for accurate hyperspectral image prediction.
Results of over 300 far IR and mid IR transmission measurements taken during several EOPACE intensive operational periods over the low-level 15 km transmission path across San Diego bay are presented. A thorough comparison with calculations obtained using simultaneously measured bulk meteorological parameters with the IR Boundary Layer Model, illustrate the effects that refractance, aerosol extinction and molecular extinction can have on the transmission. Discrepancies between the transmission measurements and the model's predictions are identified and investigated by varying various model parameters, and looking at available measured aerosol size distributions and refraction measurements over the path. Comparisons with the measured transmissions are reasonably good and show that the total measurements over the path. Comparison with the measured transmissions are reasonably good and show that the total transmission depends critically on all three effects, with the molecular transmittance depending upon the water vapor density and the characteristics of the IR source and detector, the aerosol transmittance upon the visibly, and the refractive effects on the stability of the marine boundary layer or the virtual potential air-sea temperature difference.
An analysis is presented showing the effects of molecules and aerosols on atmospheric transmission data obtained during the
Electro-Optical Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in San Diego during
March and April, 1996. Mid wave infrared transmission was measured over San Diego Bay along a 14.9 km path and a 7.0
km path at heights less than 4 meters above the water. Simultaneous meteorological measurements were obtained from two
buoys placed at the mid-points of each path. An aerosol spectrometer was used to measure the aerosol size distribution over
each transmission path. Data were analyzed with MODTRAN and Mie theory. The conclusion of this and the next two
papers is that low altitude infrared transmission is a complex phenomenon whose mean value may be controlled either by
molecular absorption, aerosol scattering, or refractive focusing, and whose fluctuating value is controlled by scintillation.
An analysis is presented showing the effects of refraction, aerosol extinction, and molecular extinction on transmission
measurements obtained during the EO Propagation Assessment in Coastal Environments (EOPACE) campaign carried out in
San Diego during March and April 1996. Infrared transmission measurements were made over both a 7 km path (mid IR) and
a 15 km path (mid JR and far IR) at heights below 10 m above sea level. The average difference between all the measured
transmissions and aerosol transmittances over the two paths with results obtained using the JR Boundary Layer Effects Model
(IRBLEM) were found to be relatively small, even though the difference for individual measurements can be significant. The
effect of molecular transmittance, as calculated using MODTRAN, is found to reduce the transmission by about 35% forthe 7 km
path, 72% for the mid JR over the 15 km path, and between 70% and 90% for the far JR over the 15 km path.The effect of
aerosol transmittance, as calculated using a variation of the Navy Aerosol Model (NAM), is found to reduce the transmission
from 10% to 90% for the mid JR over both the 7 and 15 km paths, and from 10% to 60% for the far JR over the 15 km path. The
effect of refractance, the focussing and defocussing of radiation due to atmospheric refraction, on the predicted transmissions
is found to account for gains and losses up to 20% for the 7 km path, and gains and losses up to 100% for the 15 km path.
Consequently, any JR transmission model for the marine boundary layer (MBL) must properly take into account the effects on
the transmission due to molecular extinction, aerosol extinction, and refractance.
The most recent version (Ver. 6.22) of the marine boundary layer (MBL) profile program, LWWKD, now allows the creation
of a single file containing refractivity profiles for a multiple number of horizontal ranges. This file can then be used in the most
recent version (Ver. 5.3) ofthe ray-tracing program, REFRACT, so as to simulate the effects of a horizontally non-homogeneous
refractivity profile on predictions of maximum intervision range (MIVR) and minimum mirage range (MMR). Using range
dependent meteorological measurements obtained from the Hr.Ms. Tydeman during the Marine Aerosol Properties and Thermal
Imager Performance (MAPTIP) campaign, this model shows that under certain conditions, a single horizontally homogeneous
profile can produce similar MIVR and MMR predictions to those resulting from using the range dependent profile. In general,
a single homogeneous profile can give excellent results if it corresponds to the profile at the range for which the light rays are
closest to the surface of the water (the observed horizon) and the refractivity gradient is strongest.
Several marine boundary layer (MBL) models have been developed over the last few years to predict the propagation behavior of electromagnetic radiation near the sea surface. Originally, they were developed to model the refractive effects of the MBL on radar systems, but in recent years they have been extended for use at visible and infrared wavelengths. Three of the more advanced models are DREV's L(W)WKD model (Canada), CELAR's PIRAM model (France) and the LKB model developed in the U.S. Only very limited comparative studies have been performed between them. This study discusses the differences between these models for a large number of realistic atmospheric conditions. In particular we present the differences between the calculated roughness heights, scaling constants, and vertical profiles with respect to various meteorological parameters.
During the first MAPTIP workshop in Oslo, Norway (May 94) after the trial (Oct. 93), a number of working groups were created. These groups reported some of their joint findings at the second workshop in Quebec City, Canada (May 95). This presentation summarizes the results obtained by the 'refractive effects in the visible and the IR group.' The experimental setups of the three different groups (DEV, FGAN-FfO & CELAR) and the experimental procedures used during the trial are presented along with a summary of the available datasets. Due to the large number of datasets only a select number were chosen for further in-depth analysis. These case studies were chosen using essentially two criteria: (1) interesting meteorological condition and (2) measurements were obtained by more then one group. The meteorological conditions for these case studies are presented along with comparisons of the measurements with the theoretical results obtained using both the PIRAM (CELAR) and L(W)WKD (DREV) models. Differences between the model predictions and some of the problems encountered are also discussed.
Using a modified form of the anomalous diffraction approximation we have been able to derive in closed form an analytic expression for the phase function of Mie scatterers integrated over an inverse power law (Junge) size distribution. The analysis explains the apparent singularity seen experimentally at the forward scattering angle. Simple relationships are also derived that relate the inverse power law as a function of scattering angle in the near forward direction to the power law of the size distribution. The parameters of the formula are the relative index of refraction and the inverse power of the size distribution. A comparison is given between the analytic formula and exact integration of the Mie scattering for spheres. This new phase function is used in the analysis of forward angle transmissometer- nephelometer data collected by DREV in the Arctic, Atlantic, and Pacific.
Two years ago we designed, built, and tested a ROV mounted range-gated imaging system. Given that the target covers at least one pixel at the maximum range of interest the model predicts that for the same laser power and under the condition where the field of illumination is matched to the field of view there is no performance penalty in increasing the field of view. In order to test this result we have built and deployed a second generation underwater imaging system whose field of view and field of illumination are matched and continuously variable from 60 mr to 600 mr in water. The laser source was also upgraded in power by a factor of 10 to a water cooled, 2-kHz, 400 mw doubled Nd:YLF laser. The light is collected by a 7-cm diameter zoom lens. The detector is a gated image intensifier with a 7-ns gate and a gain which is continuously variable from 500 to 1,000,000. An on-board image processor has been added to the system. It allows us to frame integrate in real-time and thus further improve system performance.
A careful analysis of a scattering and absorption database of the waters off the coasts of Canada shows that a laser-assisted camera system will have a significantly improved viewing performance over conventional systems. The laser underwater camera image enhancer system is a range-gated laser system that can be mounted on a remotely operated vehicle. The system uses a 2-kHz diode-pumped frequency-doubled Nd:YAG laser as an illumination source. The light is collected by a 10-cm-diam zoom lens. The detector is a gated image intensifier with a 7-ns gate and a gain that is continuously variable from 500 to 1,000,000. The system has been tested in a water tank facility at Defence Research Establishment Valcartier and has been mounted on the HYSUB 5000 remotely operated vehicle for sea trials. In the strongly scattering waters typical of harbor approaches, this system has a range of from three to five times that of a conventional camera with floodlights.
The crucial parameters required for the design of underwater optical systems are optical absorption and scattering as a function of location, depth, and wavelength in the ocean. DREV has developed, built, and deployed an underwater probe (NEARSCAT), whose sole purpose is to gather information about the underwater light field in the waters of interest to Canada. The instrument is unique in that it can scan all wavebands in the visible spectrum from 400 nm to 700 nm. It can also continuously sample up to 6 arbitrarily chosen wavelength bands simultaneously with a resolution of 10 nm. The instrument can separate the absorption and scattering components of seawater. This instrument was deployed at 16 locations along the East Coast of Canada, ranging from the north of Baffin Island to Cabot Strait. It was also deployed at 21 stations on the West Coast of Canada. The water column was sampled to a maximum depth of 300 m. The data was found to be extremely consistent and of high quality. We found that the waters were much less absorbing than was previously believed. A strong scattering layer was found to exist near the surface, and extending to a depth of 40 meters. This layer does not strongly absorb. The lack of absorption, the strong layering of scattering, and the predominance of narrow-angle forward scattering has important consequences for optical underwater systems as it implies that signal recovery techniques will be effective in the underwater environment and allow much better performance than was previously thought to be possible.
A careful analysis of the scattering and absorption data base of the waters off the coasts of Canada has persuaded us that a laser assisted camera system will have a significantly improved viewing performance over conventional systems. With this purpose in mind, we designed and built the laser underwater camera image enhancer system (LUCIE). The system uses a 2 kHz diode-pumped frequency-doubled Nd:YAG laser as an illumination source. The light is collected by a 10 cm diameter zoom lens. The detector is a gated image intensifier with a 7 ns gate and a gain which is continuously variable from 500 to 1,000,000. The gate delay is adjusted to the focal distance of the lens system. This ensures that only the scattering occurring near the target is seen by the camera system. In the strongly scattering waters typical of harbor approaches this system has a range of from 4 to 6 times that of a conventional camera with floodlights. The system has been tested in a water tank facility at DREV and has been mounted on the HYSUB 5000 remotely operated vehicle (ROV) for sea trials. The images from the system are sent to the surface via a high performance analog link with a bandwidth of 8 MHz. The images are processed to remove the effect of marine snow. This processing and the high repetition rate of the laser, which ensures a lack of speckle, both contribute significantly to the clarity of the images. The NEARSCAT transmissometer- nephelometer system is operated simultaneously with the LUCIE system and this allows us to have the fundamental data necessary for evaluating the performance of the imaging system and validating transmission, scattering, and imaging models.
The exponentially varying atmospheric density near the water surface can bend the radiation path and potentially affect optical detection and tracking by varying the maximum inter-vision range (MIVR) by causing a positioning error and producing mirages. Using a marine boundary-layer model in conjunction with ray-tracing, quantitative analysis of these effects as a function of meteorological conditions can be achieved and predictions on the nature and magnitude of the induced phenomena can be made. This simple form of analysis produces effects of significant magnitude depending on the conditions. However, the literature reports very few instances of these effects and the few data published on the subject lack the necessary information to relate the phenomena to the prevailing weather conditions. An experiment was conducted over the Ottawa River in the fall of 1991 to verify the occurrence, persistence, and magnitude of these refraction-induced phenomena and initiate the validation of our modelling approach. Both shortened and extended MIVRs as well as mirages were observed and the measurements made were in good agreement with the model predictions. Sample images taken under sub- and super-refraction conditions are presented.