Composite Bound on Arrival Time Estimation Errors

Time of arrival (TOA) estimation of narrowband signals is a problem of considerable practical interest in radar and sonar applications. A new technique is presented to analyze the mean square error (MSE) performance of TOA estimation schemes, based on recently developed lower bound. We obtain a complete characterization of the MSE as a function of the signal and noise parameters. The results are given in a simple closed-form analytical expression.     

JOURNAL OF AEROSPACE POWER Published quarterly by The Chinese Society of Aeronautics and Astronautics

The Journal of Aerospace Power is a leading publication of its kind in China devoted to the advancement of the science and technology of aerospace power.It provides a common medium through which academic and industrial researchers and designers can communicate results, review progress and identify requirements for further work in this increasingly significant field.      

The autocorrelation of m-sequences over nonprime finitefields

Nonbinary m-sequences (maximal length sequences) for spread-spectrum communication systems that have a two-level autocorrelation are presented. The autocorrelation function of an m -sequence over the Galois field of q elements GF(q), where q=p^k, for p a prime and k an integer greater than 1, is developed and shown to be bilevel when the elements of GF(q) are expressed as elements of a vector space over the pth roots of unity     

COS-B views on the diffuse galactic gamma-ray emission and some point sources

The correlation between diffuse galactic gamma rays and gas tracers is studied using the final COS-B database and H i and CO surveys covering the entire galactic plane. A good quantitative fit to the gamma rays is obtained, with a small galacto-centric gradient in the gamma-ray emissivity per hydrogen atom. The average ratio of H₂ column density to integrated CO temperature is determined, the best estimate being (2.3 ± 0.3) × 10² molecules cm^–2 (K km s^–1)^–1. Strictly taken, this value is an upper limit. The corresponding mass of molecular hydrogen in the inner galaxy, derived using both 1st and 4th quadrants, is 1.0 × 10⁹ M .The softer gamma-ray spectrum towards the inner galaxy found in previous work can be attributed to a steeper emissivity gradient at low energies and/or to a softer gamma-ray spectrum of the emission distributed like molecular gas. A steeper emissivity gradient at low energies could be related to cosmic-ray spectral variations in the Galaxy, to different distributions of cosmic-ray electrons and nuclei, or to a contribution from discrete sources. A softer spectrum for the emission associated with molecular clouds may be physically related to the clouds themselves (i.e., cosmic-ray spectral variations) or to an associated discrete source distribution.New results on the temporal and spectral characteristics of the high-energy (50 MeV to 5 GeV) gammaray emission from the Vela pulsar are presented. The whole pulsed flux is found to exhibit long-term variability. Five discrete emission regions within the pulsar lightcurve have been identified, with the spectral characteristics and long-term behaviour being different. These characteristics differ significantly from those reported earlier for the Crab pulsar. However, geometrical pulsar models have been proposed (e.g., Morini, 1983; Smith, 1986) which could explain many of these features.     

A design procedure for the phase-controlled parallel-loadedresonant inverter

High-frequency-link power conversion and distribution based on a resonant inverter has been recently proposed. The design of several topologies is reviewed and a simple approximate design procedure is developed for the phase-controlled parallel-loaded resonant inverter. This design procedure seeks to ensure the expected benefits of resonant conversion and is verified by data from a laboratory 2.5 kVA, 20-kHz converter. A simple phasor analysis is introduced as a useful approximation for design purposes. Experimental results show that under transient conditions such as load short-circuit, a reversal of the expected commutation sequence is possible. This should be accounted for in the design of the power circuit, or prevented by the design of the controller     

Frequency estimation techniques for high dynamic trajectories

A comparison is presented of four different estimation techniques applied to the problem of continuously estimating the rapidly varying parameters of a sinusoidal signal, observed in the presence of additive noise. Frequency estimates are emphasized, although phase and/or frequency rate are also estimated by some of the algorithms. These parameters are related to the velocity, position, and acceleration of the maneuvering receiver or transmitter. Estimated performance at low carrier-to-noise ratios and high dynamics is investigated for the purpose of determining the useful operating range of an approximate maximum likelihood estimator, an extended Kalman filter, a cross-product automatic frequency loop and a phase-locked loop. Numerical simulations are used to evaluate performance while tracking a common trajectory exhibiting high dynamics     

前台阶流场的传热实验研究

本文尝试用瞬态实验方法, 在卢德威格风洞实验台上, 对小台阶高度 (H=4.0mm)的前台阶二维流场进行传热实验研究, 测定了当雷诺数为 11800至32200时的流场内热流通量和努谢尔特数的变化规律以及努谢尔特数随雷诺数变化的准则关系式。实验结果是令人满意的。      

Optimal speckle reduction in polarimetric SAR imagery

Speckle is a major cause of degradation in synthetic aperture radar (SAR) imagery. With the availability of fully polarimetric SAR data, it is possible to use the three complex elements (HH, HV, VV) of the polarimetric scattering matrix to reduce speckle. The optimal method for combining the elements of the scattering matrix to minimize image speckle is derived, and the solution is shown to be a polarimetric whitening filter (PWF). A simulation of spatially correlated, K-distributed, fully polarimetric clutter is then used to compare the PWF with other, suboptimal speckle-reduction methods. Target detection performance of the PWF, span, and single-channel |HH|² detectors is compared with that of the optimal polarimetric detector (OPD). A novel, constant-false-alarm-rate (CFAR) detector (the adaptive PWF) is as a simple alternative to the OPD for detecting targets in clutter. This algorithm estimates the polarization covariance of the clutter, uses the covariance to construct the minimum-speckle image, and then tests for the presence of a target. An exact theoretical analysis of the adaptive PWF is presented; the algorithm is shown to have detection performance comparable with that of the OPD