Wide-Band Airborne Radar Operating Considerations for Low-Altitude Surveillance in the Presence of Specular Multipath

Abstract-Reliable detection of low-altitude platforms while simultaneously maintaining a desired search rate can be extremely difficult due to the presence of multipath. Wide-band operation in combination with frequency diversity is a sensible approach to not only mitigate, but in some cases exploit multipath channel characteristics. While a great deal of knowledge exists for characterizing the frequency dependencies of complicated multipath channels, relatively little attention has been given to examining how this knowledge could be exploited with wide-band radar sensors. The utilization of multipath channel characteristics is considered for the scenario of an airborne wide-band radar sensor performing low-altitude surveillance in a maritime environment. A brief overview of applicable multipath phenomenology is presented leading to a description of the propagation conditions selected for the construction of a representative channel. A generalized wide-band model of the sensor engagement applicable to the resolved and unresolved domains of the interference regime is utilized in combination with the simulated channel. Wide-band short pulse and linear frequency modulation waveforms are employed to consider waveform modulation characteristics in combination with desirable sensor bandwidth and frequency diversity for nominal operation at -band. A brief discussion on implementation possibilities is also included

A Numerical Study Investigating Sensitivity of Radar Wave Propagation to the Marine Atmospheric Boundary Layer Environment

Radar is a remote sensor that is useful in scientific and military applications. The environment affects the accuracy of radar measurements as well as the predictability of a radar system’s performance. Because of the complexity of the dynamic processes occurring in the marine atmospheric boundary layer (MABL), which includes the lowermost troposphere and ocean surface, the impact of the environment on radar is intricate and difficult to assess. To better understand the relative importance of various aspects of the MABL environment on radar wave propagation, this study evaluates the sensitivity of radar wave propagation to the MABL environment using a global sensitivity analysis (SA) method, the extended Fourier amplitude sensitivity test (EFAST), and the Variable Terrain Radio Parabolic Equation (VTRPE) simulation, which calculates propagation power of radar waves in a wide variety of marine atmospheric conditions. A total of 16 environmental parameters are examined, 8 parameterizing the rough ocean surface, and 8 parameterizing the atmospheric vertical refractivity profiles. Radar frequencies of 3, 9, and 15 GHz are each simulated with horizontal (HH) and vertical (VV) polarization, resulting in sensitivity calculations for 6 different cases. The study is conducted for a domain of 1 km in altitude and 60 km in range using a low grazing angle generic air/sea surveillance radar. The relative importance of the different parameters varied much more with frequency than polarization. The EFAST method takes into account parameter interactions, which are found to be significant and can be essential to correctly interpret the significance of a parameter. Results show that the atmospheric mixed layer parameters are most important, particularly the height of the mixed layer. Overall, swell period is the most significant ocean surface parameter. However, sea directionality is also important at 3 GHz, and sea surface roughness and salinity are important at 9 and 15 GHz, respectively. Sensitivities to ocean surface parameters, except those related to directionality, become more prominent as radar frequency increases, and some sensitivity differences with respect to polarization occur regarding sea surface characteristics. Due to spatial variability of sensitivity throughout the domain, regional analysis is performed, using short (0-10 km), mid (10-30 km), and long (30-60 km) range, and low (0-200 m), mid (200-600 m), and high (600-1000 m) altitude divisions (9 regions). The most sensitive parameter in each low altitude region, from short to long range, is evaporation duct height and mixed layer height (mid and long range). The mixed layer height is the most sensitive parameter in all mid-altitude regions. At high altitude, the most sensitive parameter varies with frequency, except at short range where it is the mixed layer refractivity gradient (i.e., M-gradient). At mid-range, the most sensitive parameters are the inversion layer strength, mixed layer M-gradient, and mixed layer height for 3, 9, and 15 GHz respectively. At long range, the inversion strength is the most sensitive parameter at 3 GHz, while at 9 and 15 GHz it is the wind speed. These regional sensitivity results, along with those for the whole domain, can be used to determine which environmental parameters need to be specified with high accuracy when accounting for their effects on propagation for various radar systems and applications. This sensitivity information can also be used to help guide field measurements for simulation validation studies as it indicates what aspects of the environment need to be focused on for such experimental campaigns. Furthermore, these results provide guidance on prioritization of environmental characterization in numerical weather prediction (NWP) and inversion studies (e.g., refractivity from clutter (RFC) studies), which are the two most common numerical methods currently used to address environmental effects on propagation. Additionally, the methodology presented in this study can be used and applied to similar problems that seek to understand the sensitivity to environmental effects on other remote sensors, such as infrared (IR), optical, and acoustic sensors

Fuel injector: Air swirl characterization aerothermal modeling, phase 2, volume 1

A well integrated experimental/analytical investigation was conducted to provide benchmark quality relevant to a prefilming type airblast fuel nozzle and its interaction with the combustor dome air swirler. The experimental investigation included a systematic study of both single-phase flows that involved single and twin co-axial jets with and without swirl. A two-component Phase Doppler Particle Analyzer (PDPA) was used to document the interaction of single and co-axial air jets with glass beads that simulate nonevaporating spray and simultaneously avoid the complexities associated with fuel atomization processes and attendant issues about the specification of relevant boundary conditions. The interaction of jets with methanol spray produced by practical airblast nozzle was also documented in the spatial domain of practical interest. Model assessment activities included the use of three turbulence models (k-epsilon, algebraic second moment (ASM), and differential second moment (DSM)) for the carrier phase, deterministic or stochastic Lagrangian treatment of the dispersed phase, and advanced numerical schemes. Although qualitatively good comparison with data was obtained for most of the cases investigated, the model deficiencies in regard to modeled dissipation rate transport equation, single length scale, pressure-strain correlation, and other critical closure issues need to be resolved before one can achieve the degree of accuracy required to analytically design combustion systems

2nd NASA CFD Validation Workshop

The purpose of the workshop was to review NASA's progress in CFD validation since the first workshop (held at Ames in 1987) and to affirm the future direction of the NASA CFD validation program. The first session consisted of overviews of CFD validation research at each of the three OAET research centers and at Marshall Space Flight Center. The second session consisted of in-depth technical presentations of the best examples of CFD validation work at each center (including Marshall). On the second day the workshop divided into three working groups to discuss CFD validation progress and needs in the subsonic, high-speed, and hypersonic speed ranges. The emphasis of the working groups was on propulsion

COBE's search for structure in the Big Bang

The launch of Cosmic Background Explorer (COBE) and the definition of Earth Observing System (EOS) are two of the major events at NASA-Goddard. The three experiments contained in COBE (Differential Microwave Radiometer (DMR), Far Infrared Absolute Spectrophotometer (FIRAS), and Diffuse Infrared Background Experiment (DIRBE)) are very important in measuring the big bang. DMR measures the isotropy of the cosmic background (direction of the radiation). FIRAS looks at the spectrum over the whole sky, searching for deviations, and DIRBE operates in the infrared part of the spectrum gathering evidence of the earliest galaxy formation. By special techniques, the radiation coming from the solar system will be distinguished from that of extragalactic origin. Unique graphics will be used to represent the temperature of the emitting material. A cosmic event will be modeled of such importance that it will affect cosmological theory for generations to come. EOS will monitor changes in the Earth's geophysics during a whole solar color cycle

Innovation: Key to the future

The NASA Marshall Space Flight Center Annual Report is presented. A description of research and development projects is included. Topics covered include: space science; space systems; transportation systems; astronomy and astrophysics; earth sciences; solar terrestrial physics; microgravity science; diagnostic and inspection system; information, electronic, and optical systems; materials and manufacturing; propulsion; and structures and dynamics

CHARACTERISTICS OF REFRACTIVITY AND SEA STATE IN THE MARINE ATMOSPHERIC SURFACE LAYER AND THEIR INFLUENCE ON X-BAND PROPAGATION

Predictions of environmental conditions within the marine atmospheric surface layer (MASL) are important to X-band radar system performance. Anomalous propagation occurs in conditions of non-standard atmospheric refractivity, driven by the virtually permanent presence of evaporation ducts (ED) in marine environments. Evaporation ducts are commonly characterized by the evaporation duct height (EDH), evaporation duct strength, and the gradients below the EDH, known as the evaporation duct curvature. Refractivity, and subsequent features, are estimated in the MASL primarily using four methods: in-situ measurements, numerical weather and surface layer modeling, boundary layer theory, and inversion methods. The existing refractivity estimation techniques often assume steady homogeneous conditions, and discrepancies between measured and simulated propagation predictions exist. These discrepancies could be attributed to the exclusion of turbulent fluctuations of the refractive index, exclusion of spatially heterogeneous refractive environments, and inaccurate characterization of the sea surface in propagation simulations. Due to the associated complexity and modeling challenges, unsteady inhomogeneous refractivity and rough sea surfaces are often omitted from simulations. This dissertation first investigates techniques for steady homogeneous refractivity and characterizes refractivity predictions using EDH and profile curvature, examining their effects on X-band propagation. Observed differences between techniques are explored with respect to prevailing meteorological conditions. Significant characteristics are then utilized in refractivity inversions for mean refractivity based-on point-to-point EM measurements. The inversions are compared to the other previously examined techniques. Differences between refractivity estimation methods are generally observed in relation to EDH, resulting in the largest variations in propagation, where most significant EDH discrepancies occur in stable conditions. Further, discrepancies among the refractivity estimation methods (in-situ, numerical models, theory, and inversion) when conditions are unstable and the mean EDH are similar, could be attributed to the neglect of spatial heterogeneity of EDH and turbulent fluctuations in the refractive index. To address this, a spectral-based turbulent refractive index fluctuation model (TRIF) is applied to emulate refractive index fluctuations. TRIF is verified against in-situ meteorological measurements and integrated with a heterogenous EDH model to estimate a comprehensive propagation environment. Lastly, a global sensitivity analysis is applied to evaluate the leading-order effects and non-linear interactions between the parameters of the comprehensive refractivity model and the sea surface in a parabolic wave equation propagation simulation under different atmospheric stability regimes (stable, neutral, and unstable). In neutral and stable regimes, mean evaporation duct characteristics (EDH and refractive gradients below the EDH) have the greatest impact on propagation, particularly beyond the geometric horizon. In unstable conditions, turbulence also plays a significant role. Regardless of atmospheric stability, forward scattering from the rough sea surface has a substantial effect on propagation predictions, especially within the lowest 10 m of the atmosphere

NASA Tech Briefs, March 1988

Topics include: New Product Ideas; NASA TU Services; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; and Life Sciences

Index to NASA Tech Briefs, 1975

This index contains abstracts and four indexes--subject, personal author, originating Center, and Tech Brief number--for 1975 Tech Briefs

NASA Tech Briefs, August 1993

Topics include: Computer Graphics; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences; Books and Reports