The last decade has seen an increase in the awareness of the infrared signature of naval ships. New ship designs show that infrared signature reduction measures are being incorporated, such as exhaust gas cooling systems, relocation of the exhausts and surface cooling systems. Hull and superstructure are cooled with dedicated spray systems, in addition to special paint systems that are being developed for optimum stealth. This paper presents a method to develop requirements for the emissivity of a ship's coating that reduces the contrast of the ship against its background in the wavelength band or bands of threat sensors. As this contrast strongly depends on the atmospheric environment, these requirements must follow from a detailed analysis of the infrared signature of the ship in its expected areas of operation. Weather statistics for a large number of areas have been collected to produce a series of 'standard environments'. These environments have been used to demonstrate the method of specifying coating emissivity requirements. Results are presented to show that the optimised coatings reduce the temperature contrast. The use of the standard environments yields a complete, yet concise, description of the signature of the ship over its areas of operation. The signature results illustrate the strong dependence of the infrared signature on the atmospheric environment and can be used to identify those conditions where signature reduction is most effective in reducing the ship's susceptibility to detection by IR sensors.
The increased interest during the last decade in the infrared signature of (new) ships results in a clear need of validated infrared signature prediction codes. This paper presents the results of comparing an in-house developed signature prediction code with measurements made in the 3 5 μm band in both clear-sky and overcast conditions. During the measurements, sensors measured the short-wave and long-wave irradiation from sun and sky, which forms a significant part of the heat flux exchange between ship and environment, but is linked weakly to the standard meteorological data measured routinely (e.g., air temperature, relative humidity, wind speed, pressure, cloud cover). The aim of the signature model validation is checking the heat flux balance algorithm in the model and the representation of the target. Any uncertainties in the prediction of the radiative properties of the environment (which are usually computed with a code like MODTRAN) must be minimised. It is shown that for the validation of signature prediction models the standard meteorological data are insufficient for the computation of sky radiance and solar irradiation with atmospheric radiation models (MODTRAN). Comparisons between model predictions and data are shown for predictions computed with and without global irradiation data. The results underline the necessity of measuring the irradiation (from sun, sky, sea or land environment) on the target during a signature measurement trial. Only then does the trial produce the data needed as a reference for the computation of the infrared signature of the ship in conditions other than those during the trial.
Current missile-warning sensors on aircraft mostly operate in the ultraviolet wavelength band. Aimed primarily at detecting short-range, shoulder-fired surface-to-air missiles, the detection range of the sensors is of the same order as the threat range, which is 3-5 km. However, this range is only attained against older missiles, with bright exhaust flames. Modern missile developments include the use of new propellants, which generate low-intensity plumes. These threats are detected at much shorter ranges by current ultraviolet warning sensors, resulting in short reaction times. Infrared sensors are able to detect targets at a much longer range. In contrast with the ultraviolet band, in which a target is observed against an almost zero background, infrared sensors must extract targets from a complex background. This leads to a much higher false-alarm rate, which has thus far prevented the deployment of infrared sensors in a missile warning system. One way of reducing false-alarms levels is to make use of the spectral difference between missile plumes and the background. By carefully choosing two wavelength bands, the contrast between missile plume and background can be maximised. This paper presents a method to search for the best possible combination of two bands in the mid-wave infrared, that leads to the longest detection ranges and that works for a wide range of missile propellants. Detection ranges predicted in the infrared will be compared with those obtained in the ultraviolet, to demonstrate the increased range and, therefore, the increased reaction time for the aircraft.
The application of long-range infrared observation systems is challenging, especially with the currently available high spatial resolution infrared camera systems with resolutions comparable with their visual counterparts. As a result of these developments, the obtained infrared images are no longer limited by the quality of system but by atmospheric effects instead. For instance, atmospheric transmission losses and path radiance reduce the contrast of objects in the background and optical turbulence limits the spatial resolution in the images. Furthermore, severe image distortion can occur due to atmospheric refraction, which limits the detection and identification of objects at larger range. EOSTAR is a computer program under development to estimate these atmospheric effects using standard meteorological parameters and the properties of the sensor. Tools are provided to design targets and to calculate their infrared signature as a function of range, aspect angle, and weather condition. Possible applications of EOSTAR include mission planning, sensor evaluation and selection, and education. The user interface of EOSTAR is fully mouse-controlled, and the code runs on a standard Windows-based PC. Many features of EOSTAR execute almost instantaneous, which results in a user friendly code. Its modular setup allows its configuration to specific user needs and provides a flexible output structure.
The exhaust gas plume is an important and sometimes dominating contributor to the infrared signature of ships. Suppression of the infrared ship signatures has been studied by TNO for the Royal Netherlands Navy over considerable time. This study deals with the suppression effects, which can be achieved using a spray of cold water in the inner parts of the exhaust system. The effects are compared with the effect of cooling with air. A typical frigate size diesel engine serves as an example for gas flow, composition and temperature of the plume. The infrared emission of the cooled an un-cooled exhaust gases is calculated. Both the spectral behaviour and the integrated values over typical bands are discussed. Apart from the signature also some advantages of water exhaust gas cooling for the ship design are discussed.
Missile warning systems operating in the ultraviolet part of the spectrum have become a common part of the suite of self-defence systems of modern aircraft. These systems have a low false alarm rate and a detection range of several kilometers against man-portable surface-to-air missiles. The performance of the missile warning systems depends on several factors, including weather and threat type. This paper uses a generaic missile warning sensor and a recently developed model to predict missile plume UV radiance, to demonstrate the variability in detection range for a number of typical threats, weather types, aircraft speeds and warning system lay-outs. The variation in sensor performance present in the results shows that an assessment of the level of platform self-protection prior to each mission should be performed.
As a result of the deployment of UV missile warning systems, recent years have seen an increasing interest in threat assessment in the UV band. Unfortunately, due to the different nature of the physical processes that are needed to describe a missile signature in the UV, available codes for the IR can not be applied. As a result, the development of a UV missile plume signature model was initiated. This paper presents a model for the prediction of UV missile plume signatures, that takes into account relevant physical mechanisms in a missile plume. The model is based on first principles, predicting the radiance from CO-O chemiluminescence and hot particles in the plume, which are the dominant sources of radiation in the UV wavelength band considered. Scattering of radiation on particles in the plume can be important for particle-rich propellants and is accounted for in the code. The multiple scattering algorithm has been set up to handle any number of directions in an axi-symmetric medium; the algorithm presents a novel way of solving the radiative transfer problem. Several examples are shown, to illustrate scattering processes in missile plumes. A number of validation tests are presented to show the model's performance. At this stage, comparisons with real data are under progress.
The advent of low-observable (stealth) ships, of which the new Air Defense and Command Frigate (LCF) is an example, must be followed by an increased interest in signature control during operations. Taking full operational advantage of the stealth character of low-observable ships requires on-board signature control. At all times, the ship's visibility for expected threats must be known. This paper presents an active infrared-signature control system, that will be used on board the LCF. The system is the first step towards a full signature management system, which takes into account all systems affecting the signature during operations. Such a system should not only consider electro- optical signatures, but all relevant signatures: radar, acoustic, magnetic, etc.
The first results are presented from a quantitative model describing the aerosol production in the surf zone. A comparison is made with aerosol produced in the surf zone as measured during EOPACE experiments in La Jolla and Monterey. The surf aerosol production was derived from aerosol concentration gradients measured downwind from the surf zone, after correction for the background size distribution that was measured upwind from the wave breaking zone. The aerosol production model was originally developed from measurements performed along the Baltic coast. The model predicts the aerosol production from the total energy dissipated in the wave breaking zone, calculated from the coastal bathymetry and deep-water surface wave field. In the present work, the parameterization of the aerosol production in the wave breaking zone is maintained, but the energy dissipation in the wave breaking zone is calculated using a different model that produces more realistic surf zone widths. Wave data were obtained from buoys off the Californian coast, while bathymetry data were supplied by the Scripps Institute of Oceanography. Observed and predicted aerosol production in the surf zone are in good agreement, for both sites. The predicted aerosol flux reproduces the day-to-day variations and even some of the observed variations on a time scale of several hours.
Aerosol concentrations over the surf were measured during the EOPACE (Electro-Optical Propagation Assessment in Coastal
Environment) Surf-i experiment in La Jolla, California. Particle size distributions were measured on the beach (at three
levels) and across the surf (one level). Concentrations of droplets smaller than i im in diameter are little affected by the surf,
while those with diameters in the 1-10 im range increase by up to two orders of magnitude. Clear vertical gradients were
observed, which vary with particle size. No relation could be established between the surf production and wind speed or wave
properties. Extinction coefficients at visible and infrared wavelengths calculated from the particle size distributions show that
these are enhanced by a factor of 30 to 100, depending on the wavelength. Using the measured concentrations as boundary
condition, calculations with a simple dispersion model show the gradual decrease in the concentration with fetch in off-shore
winds. In on-shore winds the surf-enhanced aerosol concentration is effective over only a short range, but nevertheless
significant transmission losses may occur. Obviously, these conclusions apply only to the surf encountered during this
specific experiment. The effects of the surf in other areas and other ambient conditions will be assessed from the analysis of
data collected at a different location and in different conditions.