Advanced missile seeker technologies and missile propulsion systems with reduced electrooptical signatures constitute a serious threat to military platforms. Seekers using multiple spectral bands may be hard to jam using current countermeasure systems. Imaging seekers may have adaptive tracking algorithms to suppress the effect of current countermeasure strategies. Furthermore, countermeasures can only be used if a threat has been declared. Low-signature propellants in combination with the missile seen in head-on angle, where a bigger portion of the missile plume is shielded by the missile body, will make it extremely hard to detect the missile with current warning sensor systems. We report on the objectives and the status of the EDA CAT B project DEBELA (Detect Before Launch), which tries to address this threat, and which looked into potential technologies for future self-protection systems. The project focusses on within visual range threats and electrooptical sensors only. Candidate-technologies have been identified and tested in a field experiment on the premises of the Bundeswehr Technical Center (WTD 52) in Oberjettenberg, Germany.
In the framework of two research groups of the Science and Technology Organization (STO) Fraunhofer IOSB and partners conducted a field trial in a humid coastal environment in Florida (USA). The research groups investigate the sensitivity of EO/IR-TDAs (Electro-Optical/Infrared Tactical Decision Aids) to environmental factors and physics-based EO/IR scene simulation tools for decision support systems. Main objective of the trial was the collection of a dataset, that can be used to study/improve the quality of the current EO/IR scene simulation toolsets and to evaluate the impact of different parameters on the reliability of EO/IR-TDA predictions. An overview of the trial and results of a first analysis are given.
In the framework of a NATO research group Fraunhofer IOSB and partners conducted a field trial in an arid shrub land environment in southern New Mexico (USA). The group investigates environmental limitations of fielded EO-TDAs (Electro-Optical Tactical Decision Aids). Main objective of the trial was to study the impact of the atmosphere on imaging sensor performance with a focus on the effects of atmospheric extinction and near surface turbulence. An overview of the trial will be given, as well as an overview on EO-TDA development. Results of efforts to forecast the refractive index structure parameter using numerical weather prediction (NWP) models will be described, as well as the results of a perception study on the influence of turbulence on target acquisition ranges using MWIR imagery.
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 𝐶2𝑛. It is responsible for beam wander, blurring. and scintillation. “Ground truth” measurements of optical turbulence were carried out using a Boundary Layer Scintillometer (BLS2000). The measurements were taken along an optical path of 3.64 km and a height of 1.5 m. Additionally, height dependency of 𝐶2𝑛 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.
The atmospheric influence on wave propagation was investigated during the First European South African Transmission ExpeRiment (FESTER) 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.
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 Cn2. The strength of the integrated optical turbulence Cn2 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 Cn2 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.
Airborne platform effects on lasers and warning sensors (ALWS) has been a European collaborative research project to investigate the effects of platform-related turbulence on optical countermeasure systems, especially missile approach warning systems (MAWS) and directed infrared countermeasures (DIRCM). Field trials have been carried out to study the turbulence effects around a hovering helicopter and behind a turboprop aircraft with engines running on the ground. In addition different methods for modelling the effects have been investigated.
In the helicopter trials significant beam wander, scintillations and beam broadening were experienced by narrow divergence laser beams when passing through the down-wash of the hot engine exhaust gases. The measured effects considerably exceed the effects of atmospheric turbulence. Extraction of turbulence parameters for modelling of DIRCM-relevant scenarios show that in most cases the reduction of jamming power and distortion of jamming waveform can be expected to be small. The reduction of effects of turbulence is mainly related to the larger beam divergence and shorter Rayleigh length of DIRCM lasers compared to the experimental probe beams. Measurements using the turboprop platform confirm that tolerable effects on laser beam properties are found when the laser beam passes through the exhaust 15 m behind the outlet where the exhaust gases are starting to cool down.
Modelling efforts have shown that time-resolved computational fluid dynamics (CFD) calculations can be used to study properties of beam propagation in engine exhaust-related turbulence. Because of computational cost and the problem of validating the CFD results the use for system performance simulations is however difficult.
The hot exhaust gases emitted from aircraft engines create extreme optical turbulence in a local region. The effects on countermeasure system performance depend both on the system parameters and on the threat characteristics. With present-day DIRCM systems, the effects of even severe turbulence are often tolerable.
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 over a 1.8 km over-water link over False Bay.
The link ran from the Institute of Maritime Technology (IMT) at 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 with Cn2 with height is not always
found. These results are analyzed in terms of the meteorological scenario, and a comparison is made with a fourth optical
link providing optical turbulence data over a 8.69 km path from IMT to St. James, roughly perpendicular to the three 1.8
km paths.
Localizing a shooter in a complex scenario is a difficult task. Acoustic sensors can be used to detect blast waves. Radar technology permits detection of the projectile. A third method is to detect the muzzle flash using electro-optical devices. Detection of muzzle flash events is possible with focal plane arrays, line and single element detectors. In this paper, we will show that the detection of a muzzle flash works well in the shortwave infrared spectral range. Important for the acceptance of an operational warning system in daily use is a very low false alarm rate. Using data from a detector with a high sampling rate the temporal signature of a potential muzzle flash event can be analyzed and the false alarm rate can be reduced. Another important issue is the realization of an omnidirectional view required on an operational level. It will be shown that a combination of single element detectors and simple optics in an appropriate configuration is a capable solution.
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.
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.
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 principal goals was recording of static and dynamic thermal infrared signatures under different environmental conditions for both validations of existing thermal equilibrium signature prediction codes, but also to aid development of dynamic thermal signature models. A small scientific work boat (called Sea Lab) was used as the principal target and sensor platform. Painted metal plates of different thicknesses were also used as infrared targets on-board Sea Lab to study static/dynamic thermal signatures and were also fitted with pyrgeometers, pyrometers and iButton temperature sensors/loggers. First results focused on the variable of thermal signatures as function of environmental conditions and the accuracy of calculated source temperatures (from measured radiometric temperatures) compared to the physical temperature measurements of the plates.
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.
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.
The performance of electro-optical platform protection systems can be degraded significantly by the propagation
environment around the platform. This includes aero-optical effects and zones of severe turbulence generated by engine
exhausts. For helicopters rotor tip vortices and engine exhaust gases that are pressed down by the rotor airflow form the
so called downwash phenomena. The downwash is a source for perturbations. A wide range of spatial and temporal
fluctuations in the refractive index of air can occur. The perturbations from the turbulent flow cause detrimental effects
on energy delivery, angle of arrival fluctuations, jam-code transmission, tracking accuracy and imaging performance in
general. Therefore the effects may especially have a severe impact on the performance of laser-based protection systems
like directed infrared countermeasures (DIRCM). The chain from passive missile detection and warning to obtaining an
optical break-lock by the use of an active laser system will be influenced.
To anticipate the installed performance of an electro-optical defensive aids suite (DAS) for helicopter platforms it is
necessary to develop models for the prediction of the perturbations. Modelled results have to be validated against
experimental findings. However, the data available in open literature on the effects of rotor downwash from helicopters
on optical propagation is very limited. To collect necessary data and to obtain a first impression about the magnitude of
occurring effects the European defence agency group (EDA) on “airborne platform effects on lasers and warning sensors
(ALWS)” decided to design and perform a field trial on the premises of the Italian Air Force Flight Test Center in Pratica
di Mare, Italy. ALWS is a technical arrangement under the Europa MoU among France, Germany, Italy, Sweden and the
United Kingdom.
The hot exhaust gases from engines on helicopters are pushed down by the rotor in a turbulent flow. When the optical
path of a laser beam or optical sensor passes through this region severe aberrations of the optical field may result. These
perturbations will lead to beam wander and beam distortions that can limit the performance of optical countermeasure
systems.
To quantify these effects the Italian Air Force Flight Test Centre hosted a trial for the “Airborne platform effects on
lasers and warning sensors” (ALWS) EDA-project. Laser beams were propagated from the airport control tower to a
target screen in a slant path with the helicopter hovering over this path. Collimated laser beams at 1.55-, 2- and 4.6-μm
wavelength were imaged with high speed cameras. Large increases in beam wander and beam divergence were found,
with beam wander up to 200 μrad root-mean-square and increases in beam divergence up to 1 mrad.
To allow scaling to other laser beam parameters and geometries formulas for propagation in atmospheric turbulence were
used even though the turbulence may not follow Kolmogorov statistics. By assuming that the plume is short compared to
the total propagation distance the integrated structure parameter through the plume could be calculated. Values in the
range 10-10 to 10-8 m1/3 were found when the laser beams passed through the exhaust gases below the helicopter tail. The
integrated structure parameter values calculated from beam wander were consistently lower than those calculated from
long term spot size, indicating that the method is not perfect but provides information about order of magnitudes.
The measured results show that the engine exhaust for worst case beam directions will dominate over atmospheric
turbulence even for kilometer path lengths from a helicopter at low altitude. How severe the effect is on system
performance will depend on beam and target parameters.
Atmospheric effects limit the performance of any electro-optical (EO) system. Tasks such as laser communications or horizontal-path imaging for long-range surveillance are highly affected by environmental effects. In the majority of cases, effects like turbulence impose a fundamental limitation to the capability of EO systems. In this paper, we give an overview of the limiting factors and we will show possibilities for restoration of images degraded by atmosphere.
The exhaust from engines introduces zones of extreme turbulence levels in local environments around aircraft. This may
disturb the performance of aircraft mounted optical and laser systems. The turbulence distortion will be especially
devastating for optical missile warning and laser based DIRCM systems used to protect manoeuvring aircraft against
missile attacks, situations where the optical propagation path may come close to the engine exhaust. To study the extent
of the turbulence zones caused by the engine exhaust and the strength of the effects on optical propagation through these
zones a joint trial between Germany, the Netherlands, Sweden and the United Kingdom was performed using a medium
sized military turboprop transport aircraft tethered to the ground at an airfield. This follows on earlier trials performed on
a down-scaled jet-engine test rig.
Laser beams were propagated along the axis of the aircraft at different distances relative to the engine exhaust and the
spatial beam profiles and intensity scintillations were recorded with cameras and photodiodes. A second laser beam path
was directed from underneath the loading ramp diagonally past one of the engines. The laser wavelengths used were 1.5
and 3.6 μm. In addition to spatial beam profile distortions temporal effects were investigated. Measurements were
performed at different propeller speeds and at different distances from exhaust nozzle to the laser path.
Significant increases in laser beam wander and long term beam radius were observed with the engine running.
Corresponding increases were also registered in the scintillation index and the temporal fluctuations of the instantaneous
power collected by the detector.
Airplane based laser systems for DIRCM, active imaging and communication are important applications attracting
considerable interest. The performance of these systems in directions where the laser beam points close to or through the
exhaust plume from the jet engines may be severely reduced. A trial to study these phenomena using a downscaled jet-engine
test rig was carried out. The results on propagation of laser beams along and across the plume from these trials are
presented. For laser beams propagation along the engine axis an OPO based source producing co-propagating laser
beams at 1.52 and 3.56 μm was used. The beams were projected on a screen and imaged with separate IR cameras to
study beam wander and spot degradation. Propagation across the plume was studied with a 532 nm laser projected on a
screen and imaged by a high speed camera. The engine thrust and the distance between the engine nozzle and the laser
beams were varied to study the effects of changing conditions. Scaling to full size engines and performance implications
for DIRCM is discussed.
The NATO Panel SET-088 TG-51 has the charter to investigate infrared research topics relating to Littoral Ship Self-Defence. The two main research areas for TG-51 are low-altitude maritime IR propagation phenomenology and ship
signature properties. Atmospheric scintillation and refraction prediction models were validated in several trials
conducted by different NATO groups. So far most trials were conducted in cold waters. In June 2006, TG 51 performed
the SAPPHIRE trial (Ship and Atmospheric Propagation PHenomenon InfraRed Experiment) to collect data in littoral
areas under conditions of warm sea temperatures. The location of the trial was the US Naval Research Laboratory's
Chesapeake Bay Detachment (CBD) field site on Chesapeake Bay. The objectives of the trial were to validate ship
signature models and scintillation/refraction models. In the SAPPHIRE trial, the purpose of FGAN-FOM was to
investigate the influence of changing weather conditions on the apparent elevation of a target. Therefore, we set up an
IR-camera at CBD overlooking Chesapeake Bay observing a set of lights installed on an Island in 16 km distance. In
this paper we discuss and analyse the measured elevations and compare them to the propagation model IRBLEM
(IR Boundary Layer Effects Model) by DRDC, Canada.
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 performance of Electro-Optical systems operating within the marine boundary layer is severely influenced by atmospheric conditions. Vertical temperature and humidity gradients lead to refraction and turbulence effects. Refraction causes focusing and defocusing of rays, detection range limitations, mirage formation and angular deviation. Associated with the VAMPIRA (Validation Measurement on Propagation in the IR and Radar) experiment, we made investigations on the apparent elevations of point targets. The location of the trial was at the coast line of the Baltic Sea, overlooking a bay. An IR camera system was fixed on a pier at an altitude of 6.5 m. The camera continuously recorded a series of lights 8.2 km away at the other side of the bay. With these recordings we obtained 24-hours-observations of refraction effects due to changing weather conditions. In addition to the VAMPIRA measurements a similar experiment was performed in tropical waters. First results of this trial are given here. Aim of this paper is to discuss and analyse the measured elevations and compare them to the propagation model IRBLEM (IR Boundary Layer Effects Model by DRDC, Canada).
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.
KEYWORDS: Black bodies, Atmospheric propagation, Ocean optics, Sensors, Long wavelength infrared, Data modeling, Atmospheric modeling, Signal to noise ratio, Refraction, Meteorology
The propagation of IR radiation through the marine boundary layer is very much dependent on vertical temperature gradients. Due to the Air-Sea Temperature Difference (ASTD) the distance to the visible horizon for an imaging system can be shorter (ASTD < 0) or larger (ASTD > 0) than the distance to the geometric horizon. To analyse these phenomena FGAN-FOM took measurements in the mid and long wave IR. Location of the experiment was the Baltic Sea. A ship, equipped with IR point sources, was tracked while it was sailing in and out up to, and beyond, the horizon. Weather conditions during the measurement period showed interesting variations in ASTD and atmospheric turbulence (see paper 4884-11). Especially strong sub-refractive effects have been observed with ASTDs up to -5.0 °C. This paper deals with the analysis of the detection range of point targets under different meteorological conditions. Experimental results are compared with the propagation model IRBLEM (IR Boundary Layer Effects Model) which was developed by DRDC-RDDC - Valcartier, Canada.
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