Lars Trygve Heen

ShipIR model validation using spectral measurement results from the NATO SIMVEX trial

The ship signature model ShipIR/NTCS has been selected as a NATO standard. In 2001 Norway participated in the SIMVEX field trial arranged by NATO in Canada for validation of this model. The measurements were performed on a research vessel under different meteorological conditions, when the ship was sun illuminated and shaded, and also at night. This paper presents spectral results from our high resolution FTIR spectroradiometer, Bomem DA5. Using in-house software that enables correction of non-ideal properties of the spectroradiometer, we obtained improved absolute precision of calibrated spectra. The FTIR results are most interesting for sources with signatures deviating significantly from blackbody functions, like the ship plume, sun illuminated surfaces and sea and sky backgrounds. Ship surface and sea and sky background results have been compared with ShipIR/NTCS predictions. Results from plume measurements have been compared with simulated spectra, using the FASCODE atmospheric model, and we have estimated the plume temperature and the concentration of the most important IR contributing molecules.

Measurements of IR propagation in the marine boundary layer in warm and humid atmospheric conditions

A multinational field trial (SAPPHIRE) was performed at the Chesapeake Bay, USA, during June 2006 to study infrared ship signature and atmospheric propagation effects close to the sea surface in a warm and humid environment. In this paper infrared camera recordings of both land and ship mounted sources are analyzed. The cameras were positioned about 4 m above mean sea level. Several meteorology stations - mounted on land, on a pier and on a buoy - were used to characterize the propagation environment, while sensor heights were logged continuously. Both sub- and superrefractive conditions were studied. Measurements are compared to results from earlier field trials performed in Norway during typical North-Atlantic atmospheric conditions (cool air with little water content), and differences between medium wave and long wave infrared are emphasized. The ship mounted source - a calibrated blackbody source - was used to study contrast intensity and intensity fluctuations as a function of distance. The distance to the apparent horizon is also determined. In addition, normalized variance of intensity for land based sources has been calculated for a number of cases and these values can easily be converted to refractive index structure constant C2n-values. Measurement results are compared to results from the IR Boundary Layer Effects Model (IRBLEM).

Detection metrics and ship [D]RI

Well-known detection metrics based on Johnson criteria or Target Task Performance (TTP) models were developed for land-based targets [1,2]. In this paper we investigate how (whether) we can apply these metrics to especially recognition and identification of ships at sea. Large sea targets distinguish themselves from land-based targets by their large aspect ratio, when seen broad side, and their relatively large and hot plume. We shall only address the second of these two issues here. First, however, we shall investigate how the simple Johnson approach to recognition and identification stacks up against a TTP approach. The Johnson approach has clear and simple criteria to measure the target task performance. To apply the TTP model N50 (V50) values need to be found through observer trials. We avoid these trials here but estimate the criteria based on a comparison of the models. From analysis of LWIR and MWIR recordings of a multipurpose ship running outbound and inbound tracks, we find little difference between the two metrics. As mentioned, we study the effect of the plume on task performance ranges, by considering two different estimates for the target contrast: the average contrast and the root of the squares of this contrast and the standard deviation of the contrast. We argue that the plume skews the recognition and identification ranges to much too optimistic values when the standard deviation is included. In other words, although the plume helps to detect the target, it does not help the recognition or identification task. It seems a more careful definition of the temperature contrast needs to be applied when these models are used.

Obtaining spectral information from infrared scenarios using hyper-spectral cameras and cameras with spinning filter wheel

In the past decades the Norwegian Defence Research Establishment (FFI) has recorded and characterized infrared scenarios for several application purposes, such as infrared target and background modeling and simulation, model validation, atmospheric propagation, and image segmentation and target detection for civilian and defence purposes. During the last year FFI has acquired several new systems for characterization of infrared radiation properties. In total, five new infrared cameras from IRCAM GmbH, Germany, have been acquired. These cameras cover both the longwavelength and extended medium-wavelength infrared spectral bands. The cameras are equipped with fast rotating filter wheels which can be used to study spectral properties and polarization effects within these wavelength bands. This option allows the sensors to operate in user-defined spectral bands. FFI has also acquired two HyperCam sensors from Telops Inc, Canada, covering the long-wavelength and extended medium-wavelength spectral bands, respectively. The combination of imaging detectors and Fourier Transform spectroscopy allows simultaneous spectral and spatial characterization of infrared scenarios. These sensors may optionally be operated as high-speed infrared cameras. A description of the new sensors and their capabilities are presented together with some examples of results acquired by the different sensors. In this paper we present a detailed comparison of images taken in different spectral bands, and also compare images taken with the two types of sensors. These examples demonstrate the principles of how the new spectral information can be used to separate certain targets from the background based on the spectral information.

Measurements of IR propagation in the marine boundary layer during the September 2011 SQUIRREL trial

A multinational field trial (SQUIRREL) was performed at the Eckernförder Bucht, in the Baltic Sea, during September 2011 to study infrared ship signature and atmospheric propagation effects close to the sea surface in a cool environment. In this paper mid-wave infrared camera recordings of ship-mounted sources are analyzed. The camera was positioned about 6 m above mean sea level. Several meteorology stations - mounted on land, on a pier and on a buoy - were used to characterize the propagation environment, while sensor heights were logged continuously. Both sub- and superrefractive conditions were studied. Measurements are compared to results from an earlier field trial performed at Chesapeake Bay, in 2006, during warm and humid atmospheric conditions. The ship-mounted sources - two calibrated blackbody sources at 200 °C and 100 °C - were used to study contrast intensity and intensity fluctuations as a function of distance. The distance to the apparent horizon is also determined. Measurement results are compared to results from the IR Boundary Layer Effects Model (IRBLEM), and good agreement is found.

Comparison of distance dependence of ship signature and intensity of ship exhaust gas measured in both MWIR and LWIR transmission bands

A ships exhaust gas contains both hot gas molecules, which emit infrared radiation at specific wavelengths (line emitters), and soot particles which emit broad-banded, like a black body. Our modeling shows that the observed radiance from these emissions falls at different rates with distance. The attenuation of intensity is caused by absorption and scattering of the emitted radiation in the atmosphere. The hottest part of the exhaust plume is spatially confined to a relative small volume. Usually, a ships hull and its superstructure have a higher temperature than the sky or sea background. The temperature difference is generally not very large. However, the ship has a spatial extent that is much larger than the plumes. In this work we study how both the emitted radiation from the plume and the ships total signature decrease with increasing distance. This study is based on experimental data that was collected during a measurement campaign at the southwest coast of Norway. Shore-based digital IR cameras, both LWIR and MWIR, recorded image sequences of ships as they sailed away from close to shore (~ 1 km) in a zigzag pattern out to about 10 km. We used a statistical method to identify the gas cloud pixels and used their integrated radiance as a measure for the plume intensity. The ship signature is defined here as the integrated radiance over all the ships pixels in the imagery. From infrared spectroscopic data, collected using a Fourier Transform Infrared spectrometer aimed at the ships plume when the ship is close to shore, a model is obtained for the composition of the exhaust gas. This model was used to perform FASCODE simulations to study numerically the attenuation with distance of the plume radiance. Our work shows that this approach may be well suited to explain the observed signal decay rate with distance.

Measurements and modeling of the vertical radiance profile of sea and sky backgrounds using infrared sensors

In order to detect an object, the object has to be distinguished from its background. Often a contrast number is defined, the difference between the signals from the object and its background. Hence, detailed knowledge of both is required. Background measurements made during two measurement campaigns are compared with results from ShipIR modeling efforts. Specifically, background radiance profiles, extracted from infrared camera recordings and spectrometer recordings of the sea and sky, and spectral features are highlighted.

Measurements of the relative intensity of ship exhaust gas as a function of distance to infrared sensors

We present results from infrared imaging experiments, performed under hot and humid conditions at Chesapeake Bay, Maryland, USA in the summer of 2006. Specifically, the objective was to study the intensity of the exhaust gases from a ship at different distances. In particular there is an interest to quantify the intensity decrease of the plume with distance and correlate this with simulations of atmospheric transmission. For this purpose the ship ran a predetermined course making broad-side passes at predetermined distances from the shore-based IR camera as part of the course. The distances were 1.6, 2.4, 3.2, 4, 6, and 8 km. The cameras are sensitive in the 3 - 5 μm and 8 - 12 μm wavelength ranges. Digital recordings were made during the ship broad-side passes. It is challenging to identify gas cloud pixels against a background because the pixels are not necessarily clustered. We present a statistical method to identify the gas cloud pixels, calculate their average intensity, and determine the contrast between the gas pixels and the background pixels as a function of distance. The contrast versus distance data are then compared with simulations using standard atmospheric transmission software.

Measurements of the vertical radiance profile using infrared sensors

Measurements of the spectral radiance of the sky and the sea, taken near Halifax during the September 2001 SIMVEX trial, indicated that the use of user defined atmospheric profiles, i.e. high altitude atmospheric contributions, were necessary in order to obtain agreement between measurements and results from simulations using atmospheric radiance codes. This paper analyzes data obtained under hot and humid conditions during the SAPPHIRE trial at Chesapeake Bay, Maryland, USA, in the summer of 2006. Digital recordings of the sea and sky background were made using cameras sensitive in both the 3 - 5 μm and 8 - 12 μm wavelength range. The center of the field of view of the cameras was pitched from -5 to +15 degrees. In parallel with the imaging experiments, spectrometric data was collected at the same time. In addition, many different types of meteorological data were collected. Measurements of the vertical radiance profile near the horizon will be compared with simulation results from ShipIR using various meteorological input parameters.

Measurements of IR and visual propagation in the marine boundary layer

Two field trials have been performed on the west coast of Norway to study propagation effects (in particular refraction and turbulence effects) close to the sea surface. A complete meteorological station and a temperature profile buoy were used to characterize the propagation environment, while sensor height was logged continuously. Land and ship mounted sources were studied using infrared (midwave IR and longwave IR FPAs) and visual cameras at about 4 m above mean sea level (MSL). The land-based sources were mounted about 2-13 m above MSL, while the ship mounted sources were 10 m above sea level. Both sub- and superrefractive conditions were studied during the trials. The sensors were mounted on a programmable motion controller, which allowed extraction of absolute apparent pitch angles of the imaged sources. Apparent horizon distances have been determined for the ship sources, while mirror plane positions and apparent elevation (pitch) angles have been determined for the land sources. In addition, normalized variance of intensity (scintillation index) has been calculated for a number of cases. The scintillation index can easily be converted to refractive index structure parameters (Cn2), one of the key parameters characterizing optical turbulence. Measurement results are compared to results from the IR Boundary Layer Effects Model (IRBLEM *). *) IRBLEM is proprietory to the Department for National Defence of Canada as represented by DRDC-Valcartier