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41.
42.
The expression of the probability density function PClick(T) of the duration r of the clicks is determined as a function of 1) the expected number of clicks per second, Felick 2) the expected number of times per second that the noise component x(t) in phase with the carrier has instantaneous amplitude larger than that of the carrier itself 3) the probability density function peross(T) of the duration of the time intervals in which x(t) < -A 4) the probability that the values of the noise component in quadrature with the carrier will be of opposite sign at the extremes of time intervals of duration v. Using for Pcross(T) an approximate formula that holds fairly well in the range of the values of the signal-to-noise power ratio ? usually encountered, PIick(T) has been calculated for several values of a and for two shapes of the noise spectrum, Gaussian and rectangular. Finally, an expression is given for the mean value of T.  相似文献   
43.
Optical data taken in January 1987 from La Silla, when compared with the discovery plate of 1975.2 show no proper motion for PSR 0833-45, while a very significant one was expected if the pulsar originated in the center of the Vela SNR, which has so far been associated with it. If such an association is to be retained, either an extreme asymmetry of the SNR is required, or both objects are much older than has so far been thought. Both alternatives have their difficulties, thus possibly casting doubts on the reality of this classic SNR/pulsar association.Based on observations collected at the European Southern Observatory, La Silla, Chile.  相似文献   
44.
This paper analyzes the operation of a phase-locked loop (PLL) preceded by a bandpass limiter (BPL) in the presence of impulsive noise. It is shown that the effect of the limiter consists essentially in a change of the statistics of the pulse strengths of the noise, so that the behavior of a BPL + PLL can be deduced from that of a simple PLL by suitable adjustment of the noise model. It is found that the limiter greatly enhances the PLL performance by reducing both the phase-error variance in the loop and the probability of cycle slippage. Finally, the design of the filter of the BPL is discussed, resulting in the conclusion that the best results are obtained by using a singletuned RLC circuit.  相似文献   
45.
The phenomenon of cycle slipping in a first-order phase-locked loop is reconsidered and a simple expression for the expected number ? of cycles slipped per second is given in the form of a rapidly converging series. In this way the computation of ? becomes much easier than with previously known formulas. A formula allowing the computation of the expected number of cycles gained or lost per second is also given. These results are obtained by extending a technique used by Viterbi. The outcome of experimental verifications of the theory is also reported.  相似文献   
46.
    
One of the most attractive scientific issues in the use of GNSS (Global Navigation Satellite System) signals, from a meteorological point of view, is the retrieval of high resolution tropospheric water vapour maps. The real-time (or quasi real-time) knowledge of such distributions could be very useful for several applications, from operative meteorology to atmospheric modelling. Moreover, the exploitation of wet refractivity field reconstruction techniques can be used for atmospheric delay compensation purposes and, as a very promising activity, it could be applied for example to calibrate SAR or Interferometric-SAR (In-SAR) observations for land remote sensing. This is in fact one of the objectives of the European Space Agency project METAWAVE (Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water vapour Effects), in which several techniques are investigated and results were compared to identify a strategy to remove the contribution of water vapour induced propagation delays in In-SAR products. Within this project, the tomographic reconstruction of three dimensional wet refractivity fields from tropospheric delays observed by a local GNSS network (9 dual frequency GPS receivers) deployed over Como area (Italy), during 12–18 October, 2008, was performed. Despite limitations due to the network design, internal consistency tests prove the efficiency of the adopted tomographic approach: the rms of the difference between reconstructed and GNSS observed Zenith Wet Delays (ZWD) are in the order of 4 mm. A good agreement is also observed between our ZWDs and corresponding delays obtained by vertically integrating independent wet refractivity fields, taken by co-located meteorological analysis. Finally, during the observing period, reconstructed vertical wet refractivity profiles evolution reveals water vapour variations induced by simple cloud covering. Even if our main goal was to demonstrate the effectiveness in adopting tomographic reconstruction procedures for the evaluation of propagation delays inside water vapour fields, the actual water vapour vertical variability and its evolution with time is well reproduced, demonstrating also the effectiveness of the inferred 3D wet refractivity fields.  相似文献   
47.
The GRASP mission — Gamma Ray Astronomy with Spectroscopy and Positioning — is currently under assessment by ESA as a future space astronomy mission. The GRASP telescope will be the first high-resolution spectral imager to operate in the gamma-ray region of the spectrum. This, coupled with its high sensitivity, will enable GRASP to address many basic questions related to the physics of celestial objects thus offering a major step forward in high-energy astrophysics.  相似文献   
48.
An experimental method capable of directly measuring the net radiant heat flux on complex geometry surfaces by non invasive methods has been devised, designed, realized and tested. The method is based on a computerized thermographic system capable of directly outputting the radiative heat fluxes absorbed by the surface to be examined. The calibration tests have shown the feasibility and the accuracy of the proposed method. Preliminary tests are presented which simulate the radiation flux from a jet exhaust.  相似文献   
49.
The current knowledge on celestial high-energy gamma-ray sources is reviewed on the occasion of the end of the COS-B satellite. The breakthroughs of such a mission are outlined together with its limitations. Future experimental possibilities are presented in the context of planned missions.  相似文献   
50.
    
In this article the author assesses trends in the major parameters governing space insurance over the past decade. Following the series of launch failures in the mid-1980s premium rates rose sharply, and they have generally remained high — too high, clients sometimes feel. Yet the insurers' financial balance remains consistently negative. The industry leaders can now perhaps bring stability to premium rates, and the climate could be further improved if policies more clearly defined responsibilities and liabilities. However, clients must beware of destroying the space industry market by taking advantage of its momentary weaknesses.  相似文献   
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