The reduction of the laser hazard distance range using atmospheric attenuation has been tested with series of lidar
measurements accomplished at the Vidsel Test Range, Vidsel, Sweden. The objective was to find situations with low
level of aerosol backscatter during this campaign, with the implications of low extinction coefficient, since the lowest
atmospheric attenuation gives the highest ocular hazards.
The work included building a ground based backscatter lidar, performing a series of measurements and analyzing the
results. The measurements were performed during the period June to November, 2014.
The results of lidar measurements showed at several occasions’ very low atmospheric attenuation as a function of height
to an altitude of at least 10 km. The lowest limit of aerosol backscatter coefficient possible to measure with this
instrument is less than 0.3•10-7 m-1 sr-1. Assuming an aerosol lidar ratio between 30 – 100 sr this leads to an aerosol
extinction coefficient of about 0.9 - 3•10-6 m-1.
Using a designator laser as an example with wavelength 1064 nm, power 0.180 W, pulse length 15 ns, PRF 11.5 Hz,
exposure time of 10 sec and beam divergence of 0.08 mrad, it will have a NOHD of 48 km. With the measured aerosol
attenuation and by assuming a molecule extinction coefficient to be 5•10-6 m-1 (calculated using MODTRAN (Ontar
Corp.) assuming no aerosol) the laser hazard distance will be reduced with 51 - 58 %, depending on the lidar ratio
assumption.
The conclusion from the work is; reducing of the laser hazard distance using atmospheric attenuation within the NOHD
calculations is possible but should be combined with measurements of the attenuation.
KEYWORDS: Visibility, LIDAR, Backscatter, Mass attenuation coefficient, Cameras, Aerosols, Visibility through fog, Sensors, Modulation transfer functions, Signal attenuation
Laser rangefinders are used in various electro-optical (EO) fire control systems. They often operate at eye-safe wavelengths around 1.55 μm, which extends their utility. The paper investigates the use of a modified eye-safe laser rangefinder at 1.55 μm to obtain information on atmospheric attenuation and couple that information to the performance of active and passive EO sensors with an emphasis of lower visibility conditions. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along variable and slant atmospheric paths, especially in darkness. The experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths are presented. The backscatter curve is used to evaluate the extinction. These extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at two ranges from the sensor. TV and IR images of test targets along a 1.8 km path close to sea surface in the Baltic Sea were collected in parallel with the lidar. A weather station and a scintillometer collected weather and turbulence parameters. Results correlating the lidar attenuation with the imaging performance will be given.
KEYWORDS: Visibility, Backscatter, LIDAR, Sensors, Signal attenuation, Laser range finders, Aerosols, Visibility through fog, Atmospheric sensing, Mass attenuation coefficient
Laser Rangefinders are well established components in various electro-optical fire control systems. Recent range finders are often operating at eye safe wavelengths around 1.5 μm which extend their utility. One such extension is the use of the sensor for atmospheric sensing based on the measured backscatter signal. The present paper investigates the use of an eye-safe laser rangefinder at 1.5 μm to obtain information on atmospheric attenuation at various paths in the atmosphere. This knowledge can in turn be used in combination with atmospheric and target/background models to estimate the performance of other EO sensors like TV and thermal imagers beside the laser range finder itself. Such information can be of great value both for estimating own sensor capabilities at a given moment as well as estimating the threat capability. One obvious example is ship defense where it is difficult to obtain visibility along a variable atmosphere especially in darkness. The paper will describe the experimental equipment and the results from measurements of atmospheric backscatter along various atmospheric paths. The backscatter curve is used to evaluate the extinction. This extinction values are compared with those deduced from a point visibility meter and from echo measurements against two similar nets positioned at 2 ranges from the sensor. The results indicated a good correspondence between these results. Finally the results are illustrated in a system perspective by estimating the performance for thermal IR and other EO sensors.
A growing problem for the Police and Security Forces has been to prevent potentially hostile individuals to pass a checkpoint, without using lethatl violence. Therefore the question has been if there is a laser or any other strong light source that could be used as a warning and dazzling device, without lethal or long term effects. To investigate the possibilities a field trial has been performed at a motor-racing track. A green CW laser with an irradiance on the eye of maximum 0.5 MPE, as defined by ICNIRP [1] and the ANZI standard [2], was used as a dazzle source. Ten drivers have been driving with dipped headlights through a course of three lines with orange cones. In every line there has been only one gate wide enough to pass without hitting the cones. The time through the course, the choice of gates and the number of cones hit have been measured. For every second trial drive through the track, the driver was exposed to the laser dazzler. The background illuminances ranged from a thousand lux in daylight to about ten millilux in darkness. The protective effect of the sun-visor of the car was investigated. The drives visual system was carefully examined before and after experimental driving and a few weeks after the experimental driving to verify that no pathological effects, that could potentially be induced by the laser exposure, pre-existed or occurred after the laser exposure. An analysis of variance for a within subjects design has been used for evaluation. It was found that green laser light can have an obvious warning effect in daylight. Dazzling does reduce the drivers ability to make judgments and manouver the car in twilight and darkness. A sun-visor can reduce the glare and give the driver an improved control, but that perception can be unjustified. No damage to the visual system was observed.
Interferometric hyperspectral imagers using infrared focal plane array (FPA) sensors have received increasing interest
within the field of security and defence. Setups are commonly based upon either the Sagnac or the Michelson
configuration, where the former is usually preferred due to its mechanical robustness. However, the Michelson
configuration shows advantages in larger FOV due to better vignetting performance and improved signal-to-noise ratio
and cost reduction due to relaxation of beamsplitter specifications. Recently, a laboratory prototype of a more robust and
easy-to-align corner-cube Michelson hyperspectral imager has been demonstrated. The prototype is based upon an
uncooled bolometric FPA in the LWIR (8-14 μm) spectral band and in this paper the noise properties of this
hyperspectral imager are discussed.
We have performed a field trial to evaluate technologies for stand-off detection of biological aerosols, both in daytime
and at night. Several lidar (light detection and ranging) systems were tested in parallel. We present the results from three
different lidar systems; one system for detection and localization of aerosol clouds using elastic backscattering at
1.57 μm, and two systems for detection and classification of aerosol using spectral detection of ultraviolet laser-induced
fluorescence (UV LIF) excited at 355 nm. The UV lidar systems were utilizing different technologies for the spectral
detection, a photomultiplier tube (PMT) array and an intensified charge-coupled device (ICCD), respectively. During the
first week of the field trial, the lidar systems were measuring towards a semi-closed chamber at a distance of 230 m. The
chamber was built from two docked standard 20-feet containers with air curtains in the short sides to contain the aerosol
inside the chamber. Aerosol was generated inside the semi-closed chamber and monitored by reference equipments, e.g.
slit sampler and particle counters. Signatures from several biological warfare agent simulants and interferents were
measured at different aerosol concentrations. During the second week the aerosol was released in the air and the
reference equipments were located in the centre of the test site. The lidar systems were measuring towards the test site
centre at distances of either 230 m or approximately 1 km. In this paper we are presenting results and some preliminary
signal processing for discrimination between different types of simulants and interference aerosols.
Radio wave propagation over sea paths is influenced by the local meteorological condition at the atmospheric layer near
the surface, especially during ducts. Duct condition can be determined by measurements of local meteorological
parameters, by weather forecast models or by using inverse methods. In order to evaluate the feasibility of using inverse
methods to retrieve the refractivity profiles a measurement of RF signals and meteorological parameters were carried
out at a test site in the Baltic. During the measurements, signal power from two broadcast antennas, one at Visby and
one at Vastervik, were received at Musko, an island south of Stockholm. The measurements were performed during the
summer 2005 and the data was used to test the software package for inversion methods, SAGA (Seismo Acoustic
inversion using Genetic Algorithms, by Peter Gerstoft UCSD, US). Refractivity profiles retrieved by SAGA were
compared with the refractivity profiles calculated from measured parameters, during parts of the experiment, from
rocket sounding, radio sounding, local meteorological measurements using bulk model calculations, and also obtained
by the Swedish operational weather forecast model HIRLAM. Surface based duct height are predicted in relative many
situations even though the number of frequencies or antennas height has to be increased to diminish the ambiguous of
the refractive index profile.
KEYWORDS: Electro optical modeling, Sensors, Data modeling, Monte Carlo methods, Atmospheric modeling, RGB color model, Reflectivity, Solid modeling, Computer aided design, Temperature metrology
Computer programs for prediction of optical signatures of targets and backgrounds are valuable tools for signature assessment and signature management. Simulations make it possible to study optical signatures from targets and backgrounds under conditions where measured signatures are missing or incomplete. Several applications may be identified: Increase understanding, Design and assessment of low signature concepts, Assessment of tactics, Design and assessment of sensor systems, Duel simulations of EW, and Signature awareness. FOI (the Swedish Defence Research Agency) study several methods and modeling programs for detailed physically based prediction of the optical signature of targets in backgrounds. The most important commercial optical signature prediction programs available at FOI are CAMEO-SIM, RadThermIR, and McCavity. The main tasks of the work have been: Assembly of a database of input data, Gain experience of different computer programs, In-house development of complementary algorithms and programs, and Validation and assessment of the simulation results. This paper summarizes the activities and the results obtained. Some application examples will be given as well as results from validations. The test object chosen is the MTLB which is a tracked armored vehicle. It has been used previously at FOI for research purposes and therefore measurement data is available.
A lidar system for determination of vertical profiles of the aerosol extinction coefficient, operating with 355 nm and 532 nm laser wavelengths, is presented. Lidar measurements will be performed within the European collaboration named EARLINET-ASOS and data will be used as a part of the assessment of a method for prediction of the observation range of optical systems in the atmosphere, based on meteorological weather forecast models. The lidar system has been extended with a detector channel for elastic scattering at 532 nm and Raman scattering from N2 at 387 and 607 nm to enable determination of the aerosol extinction coefficient. Extinction coefficients and backscatter coefficients will later be compared to corresponding values received from meteorological predictions of atmospheric aerosols. The aerosols prediction will be calculated using weather forecast model (HIRLAM) together with a forward dispersion model in the chemical transport model MATCH. Atmospheric scattering calculations (Mie-calculations) will give extinction and backscatter coefficients so comparison can be with lidar measurements. The design of the lidar system and the first measurements will be presented.
A method for indirect determination of the refractive index profile in the atmospheric layer just above the sea surface with LIDAR is described. One important application is prediction of performance and particularly the maximum observation range of optical sensors. A lidar is placed near the sea surface and the lobe is scanned in a small vertical sector near the horizon. Returns from particles are recorded. The range to the point, where the laser beam reaches the sea surface, is determined from the lidar returns and stored together with the corresponding elevation angles. This information is compared to results from an optical propagation model to determine the vertical profile of the refractive index, which fits to the lidar results. Test measurements have been performed and results will be presented.
With the purpose of establishing a model for the IR extinction of atmospheric aerosols in relation to weather parameters, we have performed measurements with a transmissometer and meteorological sensors. A CO2-lidar is used to measure aerosol extinction at 106 microns in slant paths in order to investigate the altitude dependence of the extinction. The beam from a pulsed TEA-laser is emitted into the atmosphere and backscattered radiation is detected. The signal is digitized and used as input in a computer program to solve the lidar equation. The extinction coefficient vs range or altitude is obtained. The principle and characteristic properties of the two instruments are described and examples of obtained results are presented.
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