Relative Path Delays Research Articles

Facet-based simulator for bistatic scattering of the maritime scene with electrically large ships: Slope Summation Facet Model

This article developed a bistatic facetized sea scattering model, in which the sea surface is envisaged as a profile that is locally approximated by planar facets. The radar return from each local facet is associated to a so-called semi-deterministic scheme, which is established by combining the geometric optics limit of the Kirchhoff approximation with the Bragg components of the Bass–Fuks' two-scale model. In order to evaluate the complex reflective function of the facetized sea surface, a Slope Summation Facet Model is presented by simply associating the facet phase with its relative path delay. Significant computational efficiency and good agreement with experimental data are observed, which make the proposed facet model well suited for application to fast estimation on sea scattering, as well as further simulations on synthetic aperture radar imagery and the composite pattern of ship–sea scattering.

FACET-BASED TREATMENT ON MICROWAVE BISTATIC SCATTERING OF THREE-DIMENSIONAL SEA SURFACE WITH ELECTRICALLY LARGE SHIP

A feasible simulator, of which formulation and mechanism should be simple and time saving, is developed in this paper to overcome the di-culties of prediction on the EM scattering from three- dimensional (3-D) electrically very large ship-sea models. The work in this paper is twofold. First, the sea surfaces are supposed to be a combination of many locally-tilted slightly rough facets with two-scale proflles. The radar return from each local facet is associated to a semi- deterministic scheme which is established by combining the geometric optics limit of Kirchhofi Approximation (KA-GO) with the Bragg components of Bass-Fuks' two-scale model (BFTSM). Furthermore, we associate the complex re∞ective function of the respective facet by a so-called Phase-modifled Facet Model (PMFM), in which the facet's phase is treated approximately as a combination of inherent part that follows a homogeneous random distribution and coherent part associated with the relative path-delay. Second, in companion with the semi-deterministic treatment of the sea scattering model, a hybrid approximate algorithm is proposed to deal with the composite scattering of electrically large ship-sea model, which is entirely evolved through facets (for the sea surface) and wedges (for the ship target). The method of equivalent currents (MEC) and a hybrid frame which combines the four path model (FPM) with the quasi-image method (QIM) are employed to calculate the scattering characteristics of the ship-like target and ship-sea interactions, respectively. The entire simulator is of comparatively signiflcant computational e-ciency, and suitable for providing a preliminary prediction on the instantaneous complex re∞ective functions and normalized radar cross sections (NRCS) mean levels for electrically very large ship-sea model.

Entangled biphoton virtual-state spectroscopy of the A 2Σ +–X 2Π system of OH

This Letter describes the first application of entanglement-induced virtual-state spectroscopy to a molecular system. Non-classical, non-monotonic behavior in the two-photon absorption cross-section of the OH A–X system, induced by entangled photon pairs is theoretically demonstrated. A Fourier transform analysis of the biphoton cross-section permits access to the energy eigenvalues of intermediate rovibronic states with a fixed excitation energy. Our analysis reveals that the implementation of virtual-state spectroscopy of the OH A–X system requires tuning of the relative path delay τ e over a picosecond range with femtosecond resolution for a certain entanglement time T e.

Performance of adaptive linear interference suppression in the presence of dynamic fading

Adaptive linear interference suppression for direct-sequence (DS) code-division multiple access (CDMA) is studied in the presence of time- and frequency-selective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decision-directed and blind adaptive algorithms, which do not require a training sequence, are considered. Modifications to least squares adaptive algorithms are presented which are compatible with differential coding and detection. For frequency-selective fading, adaptive algorithms are presented based upon different assumptions concerning knowledge of the desired user's channel. Specifically, the cases considered are as follows: (1) perfect knowledge of the desired channel; (2) knowledge of only the relative path delays; and (3) knowledge of only one delay corresponding to the strongest path. Computer simulation results are presented which compare the performance of these algorithms with the analogous RAKE receivers. These results show that for case (3), even slow fading can cause a significant degradation in performance. Effective use of channel parameters in the adaptive algorithm reduces the sensitivity to fade rate, although moderate to fast fading can significantly compromise the associated performance gain relative to the RAKE receiver.

TOPEX microwave radiometer performance evaluation, 1992-1998

The stability and accuracy of the TOPEX Microwave Radiometer (TMR) measurement of the atmospheric path delay due to water vapor is assessed over the interval from launch (August 1992) through June 1998. Detailed global comparisons are made with path delays derived from the special sensor microwave imager (SSM/I) instruments and a network of 15 island radiosondes. The results provide consistent evidence that the TMR path delay measurements included an instrument-related downward drift of 1.0-1.5 mm/yr between October 1992 and December of 1996. The four-year drift correlates with an upward drift seen in the coldest TMR 18-GHz brightness temperature time series and is further supported by independent comparisons of TMR with ERS-1 and 2, GPS, and the Harvest Platform water vapor radiometer measurements. From January 1997 through June 1998, no significant relative path delay drift between TMR and SSM/I is seen in the comparison data, although anomalies do appear in early 1998. In terms of accuracy, both the SSM/I and radiosonde comparisons indicate no significant (>2%) scale error in the TMR path delay. An overall bias of <10 mm mag be present, but the comparisons are not consistent in this determination.

Multiphoton absorption cross section and virtual-state spectroscopy for the entangledn-photon state

Using a microscopic theory, we determine the entangled $n$-photon state generated both by the nonlinear process of spontaneous $n$-photon parametric down-conversion and in a cascade of $n\ensuremath{-}1$ crystals with two-photon parametric down-conversions. Expressions for the entangled $n$-photon absorption cross sections are also obtained. The absorption cross sections exhibit a linear dependence on the photon flux density, and depend on the entanglement times characterizing photons. The effect of relative path delay in the beams is discussed. It forms the basis for extracting the quadrupole moments between states involved in the interaction from the absorption cross section measured as a function of path delay. Effects competitive with entangled three-photon absorption are discussed.

Fringe formation and coherence of a soft-x-ray laser beam illuminating a Mach–Zehnder interferometer

We investigated the fringe visibility produced by a Mach-Zehnder interferometer illuminated by a collisionally pumped yttrium x-ray laser operating at 15.5 nm. Fringe visibility varied as a function both of relative path delay and of relative spatial overlap of the beams. This visibility information was extracted quantitatively from several interferograms and analyzed to produce a characterization of the temporal coherence, yielding a gain-narrowed linewidth of 1.3 pm for the 15.5-nm laser transition and spatial coherence consistent with an effective source size of approximately 220 microm +/- 50% at the x-ray laser output.

Microwave angle-of-arrival measurements under anomalous tropospheric propagation conditions

The operation is described of a diagnostic system designed to measure accurately the angle-of-arrival of tropospheric microwave signals at X-band frequencies in situations typical of terrestrial communications links; relative amplitudes, angles-of-arrival and relative path delays under multipath conditions are also accessible using the system. The results presented here indicate that, under single-path conditions, significant variations in angle-of-arrival in the vertical plane occur regularly and, further, that in many instances these variations exhibit a wave-like character.