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191.
A unique method of determining a satellite's antenna direction error, which includes the effects of sensor-errors, reflector thermal distortion, etc., was developed. The principle of the method is to apply a nonlinear least squares method to variations in signal strength received simultaneously at various ground stations to obtain the movement of a premeasured antenna gain pattern. Applying this method to the Japanese medium-scale broadcast satellite for experimental purposes, the system's capability and usefulness were confirmed. 相似文献
192.
Ning Hsing Lu Eisenstein B.A. 《IEEE transactions on aerospace and electronic systems》1984,(6):830-834
An adaptive threshold detector to test for the presence of a weak signal in additive non-Gaussian noise of unknown level is discussed. The detector consists of a locally optimum detector, a noise level estimator, and a decision device. The detection threshold is made adaptive according to the information provided by the noise level estimator in order to keep a fixed false-alarm probability. Asymptotic performance characteristics are obtained indicating relationships among the basic system parameters such as the reference noise sample size and the underlying noise statistics. It is shown that, as the reference noise sample size is made sufficiently large, the adaptive threshold detector attains the performance of a corresponding locally optimum detector for detecting the weak signal were the noise level known. 相似文献
193.
Rijsdijk F.B. van der Spek G.A. 《IEEE transactions on aerospace and electronic systems》1978,(2):226-236
For a planar-array antenna with a monopulse feed horn, this study describes a simple algorithm for the determination of the direction of target echoes. Antenna pattern measurements of the array indicate that the direction sines of a received wavefront can be independently obtained with one simple relation between a normalized difference channel output and a direction sine. This paper determines the accuracy of the algorithm. 相似文献
194.
S. J. Bauer L. H. Brace D. M. Hunten D. S. Intriligator W. C. Knudsen A. F. Nagy C. T. Russell F. L. Scarf J. H. Wolfe 《Space Science Reviews》1977,20(4):413-430
The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail. 相似文献
195.
Multipath-adaptive GPS/INS receiver 总被引:2,自引:0,他引:2
Erickson J.W. Maybeck P.S. Raquet J.F. 《IEEE transactions on aerospace and electronic systems》2005,41(2):645-657
Multipath interference is one of the contributing sources of errors in precise global positioning system (GPS) position determination. This paper identifies key parameters of a multipath signal, focusing on estimating them accurately in order to mitigate multipath effects. Multiple model adaptive estimation (MMAE) techniques are applied to an inertial navigation system (INS)-coupled GPS receiver, based on a federated (distributed) Kalman filter design, to estimate the desired multipath parameters. The system configuration is one in which a GPS receiver and an INS are integrated together at the level of the in-phase and quadrature phase (I and Q) signals, rather than at the level of pseudo-range signals or navigation solutions. The system model of the MMAE is presented and the elemental Kalman filter design is examined. Different parameter search spaces are examined for accurate multipath parameter identification. The resulting GPS/INS receiver designs are validated through computer simulation of a user receiving signals from GPS satellites with multipath signal interference present The designed adaptive receiver provides pseudo-range estimates that are corrected for the effects of multipath interference, resulting in an integrated system that performs well with or without multipath interference present. 相似文献
196.
Space Science Reviews - The two isotopes of nitrogen, 14N and 15N, have relative abundances extremely variable among solar system reservoirs such as planets and their atmospheres, primitive and... 相似文献
197.
Shnidman D.A. Toumodge S.S. 《IEEE transactions on aerospace and electronic systems》2002,38(3):1023-1037
In radar systems, sidelobe blanking (SLB) is used to mitigate impulsive interference that enters the radar through sidelobes of the main antenna. SLB employs an auxiliary antenna channel with the output being compared with that of the main antenna channel and a decision is then made as to whether or not to blank the main channel output. SLB performance determination involves the evaluation of several probability functions. Based on the classical Maisel SLB architecture, this work extends previous performance results, in which detection was limited to the case of a single radar pulse with either Marcum or Swerling I target fluctuation. Probability expressions have been generalized to include both an arbitrary number of integrated pulses and target fluctuation models based on the gamma distribution. The Swerling fluctuation models are all special cases of the gamma distribution. Results are derived in terms of two generalized probability functions, one for detection and the other for blanking. With these generalized probability functions, the SLB design and performance results can be determined. Examples are presented and discussed. 相似文献
198.
Semenov Vladimir S. Kubyshkin Igor V. Kiendl Michael T. Biernat Helfried K. Rijnbeek Richard P. Besser Bruno P. 《Space Science Reviews》1997,82(3-4):451-461
In this article we address several criticisms of Petschek-type reconnection models which have recently been raised by Heikkila. We discuss features of the time-dependent Petschek-type models in the context of the solar wind-magnetosphere interaction, and point out that such models can incorporate and reproduce observed features at the magnetopause, such as plasma jets and erosion of the current sheet. We argue that some of Heikkila's criticisms can be attributed to weaknesses in the analysis due to incomplete experimental information, rather than to flaws in the concept of reconnection per se; in this category we include the question of which instability leads to the localised breakup of the magnetopause current sheet. Other criticisms are based on an adherence to steady-state models, and cannot be sustained within the extended time-dependent theory. We discuss, for example, how the time-dependent model can provide a consistent picture of how energy from the incoming solar wind is transferred and converted as it enters the magnetosphere. 相似文献
199.
A relative navigation system for formation flight 总被引:1,自引:0,他引:1
A relative navigation system based on both the Inertial Navigation System (INS) and the Global Positioning System (GPS) is developed to support situational awareness during formation flight. The architecture of the system requires an INS/GPS integration across two aircraft via a data link. A fault-tolerant federated filter is used to estimate the relative INS errors based on relative GPS measurements and a range measurement obtained from the data link. The filter is constructed based on a reduced-order model of the relative INS error process. A method for analyzing the filter performance is presented. A case involving two helicopters in formation flight is studied under three different night trajectories to account for the effect of vehicle motion on the INS state transition matrix. The results of the covariance analysis are compared with actual night results over an instrumented test range. 相似文献
200.
In the case of a single sinusoid or multiple well-separated sinusoids, a coarse estimator consisting of a windowed Fourier transform followed by a fine estimator which is an interpolator is a good approximation to an optimal frequency acquisition and measurement algorithm. The design tradeoffs are described. It is shown that for the fine-frequency estimator a good method is to fit a Gaussian function to the fast-Fourier-transform (FFT) peak and its two neighbors. This method achieves a frequency standard deviation and a bias in the order of only a few percent of a bin. In the case of short-time stationarity, for a moderate number of averages and for an adaptive threshold detector, only between 0.5 and 1 dB is lost when averaging is traded off for FFT length, in contrast to the asymptotic result of 1.5 dB. The COSPAS-SARSAT satellite system for emergency detection and localization is used to illustrate the concepts. The algorithm is analyzed theoretically, and good agreement is found with test results 相似文献