A Passive Ranging Technique for Objects within the Marine Surface Layer

Infrared Search and Track (IRST) systems are important to the surface Navy for the detection of low-flying missile threats. Infrared signals propagating within the marine atmospheric surface layer are frequently distorted by strong vertical fluxes. One particular distortion that occurs commonly is the subrefractive mirage. During subrefractive mirage conditions, an imaging sensor or camera will record two distinct images of a single point source. A subrefractive mirage image can be exploited to provide both height and range information. A technique for passive ranging is described, and a case study using field test data is presented as an example of the concept. A Passive Ranging Technique for Objects within the Marine Surface Layer Stephen Doss-Hammel SSC San Diego INTRODUCTION Infrared Search and Track (IRST) systems are designed to operate within the marine atmospheric surface layer. This environment can be difficult for radar systems. A reliable passive infrared (IR) system has the potential to provide useful target detection data. However, the near sea surface environment can also distort images in the infrared. In particular, refraction effects have a strong effect on IR systems, and the occurrence of mirages is not uncommon. This report describes work to exploit one type of mirage, the inferior mirage, to determine range and height of the source creating the mirage image. REFRACTIVE EFFECTS AND RAY-TRACE TECHNIQUES The primary computational tool chosen for the analysis of refractive effects was a widget-based simulator called IRWarp that predicts refractive effects [1]. IRWarp uses meteorological conditions as input data for a ray-trace module [2]. The ray-trace data are used to generate detailed information about geometrical transformations induced by the propagation environment. The ray-tracing method used within IRWarp is from a model by Lehn [3]. The radius of curvature r of a ray is given by: r = nT 2  (1) α(λ)(Tρg + pdT/dz) where T = absolute temperature, ρ = density, p = pressure, g = gravitational acceleration, n = refractive index, and α(λ) = (77.6 + 0.584/λ) × 10 for wavelength = λ. It is also assumed that the ray slope does not exceed 10 milliradians. The formulation in Eq. (1) applies to visible and infrared wavelengths. Pressure A is relatively constant for the measurements made, and the prime determinant of the radius of curvature of near-horizontal rays was the vertical temperature gradient. The ray-trace algorithm first defines the vertical temperature profile as a set of discrete layers, each with a characteristic temperature gradient and refractivity gradient. A characteristic radius of curvature is then assigned to each layer using Eq. (1). The vertical temperature profile is based upon a surface-layer similarity theory developed by Monin and Obukhov. For the current study, an approach was followed based upon bulk methods for calculating turbulence Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.