共查询到20条相似文献,搜索用时 31 毫秒
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NTSC的双混频时差测量系统试运转结果分析 总被引:1,自引:1,他引:1
中国科学院国家授时中心(NTSC)新进口的由德国Timetech公司制造的双混频时差测量系统(dual mixer time difference system,DMTD)已经通过了试运行。介绍了DMTD的工作原理和设备结构。NTSC时频基准实验室的主钟(MC)信号作为DMTD的频率参考信号,5个氢钟和18个铯钟的频率信号作为被测信号与MC信号进行相位比对。用频率分配放大器输出的多路MC信号也作为被测信号用以监测DMTD本身的精度和稳定度。给出了DMTD和时间间隔计数器TIC实际测量结果的比较及误差分析。测量结果表明DMTD特别适用于频率短期稳定度非常高的氢原子钟这样的频标之间的频率和时间比对。该设备将用于NTSC的守时工作,不久的将来也将用于铯喷泉与氢钟的频率比对。 相似文献
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Extracting the neutral hydrogen(HI) signal is a great challenge for cosmological 21 cm experiments; both the astrophysical foregrounds and receiver noise are typically several orders of magnitude greater than the 21 cm signal. However, the different properties of the 21 cm signal, foreground and noise can be exploited to separate these components. The foregrounds are generally smooth or correlated over the frequency space along a line of sight(Lo S), while both the 21 cm signal and noise vary stochastically along the same Lo S. The foreground can be removed by filtering out the smooth component in frequency space. The receiver noise is basically uncorrelated for observations at different times, hence for surveys it is also uncorrelated in different directions, while the 21 cm signal, which traces the large scale structure, is correlated up to certain scales. In this exercise, we apply Wiener filters in frequency and angular space to extract the 21 cm signals. We found that the method works well. Inaccurate knowledge about the beam could degrade the reconstruction, but the overall result is still good, showing that the method is fairly robust. 相似文献
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We develop a theory for radar signal scattering by anisotropic Langmuir turbulence in the solar corona due to a t+l ⇄ t process. Langmuir turbulence is assumed to be generated within a cone by a narrow type III burst electron beam. Using wave-kinetic
theory we obtain expressions for the frequency shift, scattering cross-section of the turbulence, coefficient of absorption
(due to scattering) and optical depth. On the basis of those expressions we give some estimates for an echo spectrum. We show
that the minimum radar echo frequency shift is determined by the minimal phase velocity of the Langmuir waves, the maximum
shift is determined by the electron beam velocity, but in any case it can not exceed −wt/2 (decay) and wt (coalescence), where
wt is the frequency of a radar signal. The angular characteristics of the scattered signal differ dramatically for the cases
of coalescence and decay. The signal is scattered into a narrow cone high above the specular reflection point (wp ≪ wt), but
in the vicinity of wp ∼ wt/2 the red-shifted echo is scattered isotropically, while the blue-shifted echo is scattered into
a even narrower cone. We show that absorption (due to scattering) increases with increasing radar frequency. The dependence
of the absorption on the local plasma frequency is strongly determined by the Langmuir turbulence spectrum. Our theory shows
that the role of the nonlinear scattering process t+l ⇄ t is essential and that such process can be used for radar studies of the spectral energy density of anisotropic Langmuir turbulence. 相似文献
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The effective observation of burst events in solar radio research has been impeded by various interference signals,especially interference signals with a wide frequency range and high intensity,as they can partially or completely obscure the observation of burst events.Image processing methods that directly remove the interference signal channels and subtract the average of the interference signal channel are not suitable for processing all types of interference signals.This paper proposes the use of a specific kind of recurrent neural networks,called long short-term memory networks,to predict the value of the radio frequency interference signals with high intensity of the burst event in the solar radio spectrum.The predicted interference can then be removed in accordance with the principle that signals can be linearly added.Therefore,predicted value is subtracted from the data containing the burst event signals and the RFI signals(The radio frequency interference signals to be processed in this article refer to the signal of the broadcast signal that can be received in the frequency range,the signal transmitted by the mobile phone,and the signal transmitted by the sea vessel,and the like) to remove the interference.Then,in order to reduce the error caused by the stepwise prediction in the network and further improve the prediction accuracy,this paper analyzes the characteristics of the value of the radio interference and applies the digital mapping method to convert the prediction problem into the classification problem in the time series.The experimental results show that the proposed method can effectively remove the radio interference in the solar spectrum and clearly show the burst events. 相似文献
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《New Astronomy》2004,9(2):83-101
The polarization of the Cosmic Microwave Background (CMB) is a powerful observational tool at hand for modern cosmology. It allows to break the degeneracy of fundamental cosmological parameters one cannot obtain using only anisotropy data and provides new insight into conditions existing in the very early Universe. Many experiments are now in progress whose aim is detecting anisotropy and polarization of the CMB. Measurements of the CMB polarization are however hampered by the presence of polarized foregrounds, above all the synchrotron emission of our Galaxy, whose importance increases as frequency decreases and dominates the polarized diffuse radiation at frequencies below ≃50 GHz. In the past the separation of CMB and synchrotron was made combining observations of the same area of sky at different frequencies. In this paper, we show that the statistical properties of the polarized components of the synchrotron and dust foregrounds are different from the statistical properties of the polarized component of the CMB, therefore one can build a statistical estimator which allows to extract the polarized component of the CMB from single frequency data also when the polarized CMB signal is just a fraction of the total polarized signal. Our estimator improves the signal/noise ratio for the polarized component of the CMB and reduces from ≃50 to ≃20 GHz, the frequency above which the polarized component of the CMB can be extracted from single frequency maps of the diffuse radiation. 相似文献
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Enrique Villa Juan L. Cano Beatriz Aja J. Vicente Terán Luisa de la Fuente Ángel Mediavilla Eduardo Artal 《Experimental Astronomy》2018,45(1):127-146
This paper describes the analysis, design and characterization of a polarimetric receiver developed for covering the 35 to 47 GHz frequency band in the new instrument aimed at completing the ground-based Q-U-I Joint Tenerife Experiment. This experiment is designed to measure polarization in the Cosmic Microwave Background. The described high frequency instrument is a HEMT-based array composed of 29 pixels. A thorough analysis of the behaviour of the proposed receiver, based on electronic phase switching, is presented for a noise-like linearly polarized input signal, obtaining simultaneously I, Q and U Stokes parameters of the input signal. Wideband subsystems are designed, assembled and characterized for the polarimeter. Their performances are described showing appropriate results within the 35-to-47 GHz frequency band. Functionality tests are performed at room and cryogenic temperatures with adequate results for both temperature conditions, which validate the receiver concept and performance. 相似文献
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Simulation technique for whistler mode signal propagating through inhomogeneous plasma using WKB approximation has been developed (Singh, K., Singh, R.P., Ferencz, O.E., 2004. Simulation of whistler mode propagation for low latitude stations. Earth Planet Space 56, 979-987). In the present paper, we have used it for the analysis of recorded signals at low latitudes and estimated the nose frequency, which is not observed on the dynamic spectra. At low latitudes nose frequency is ∼100 kHz or more and therefore it is absent in the dynamic spectra due to attenuation of the signal at higher frequencies. The importance of nose frequency is in determining the exact path of propagation, which is required in probing of ambient medium. It is shown that the method permits to study the nose frequency variation, it can be used to deduce physical parameters as the global electric field. A case study permits to get a reasonable value of the electric field, which up to now could not be done at very low latitude. 相似文献
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BPM短波授时台新的时频监控系统 总被引:1,自引:1,他引:0
陕西天文台BPM短波授时台搬迁后 ,其时频控制与监测工作从原BPM发播钟房转为由监控室监控钟房承担。介绍了新的控制系统原理与方法 ,并对发播的BPM时号精度作了分析。 相似文献
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Chris Koen 《Monthly notices of the Royal Astronomical Society》2009,392(1):190-204
The problem considered is that of testing for small changes over time in the properties of a sinusoidal signal contaminated by noise. Two test statistics are proposed. The first is a frequency domain statistic based on the spectrum of the data, while the second is a time domain statistic due to Nyblom. Power studies are used to show that the Nyblom statistic is generally the better. The form of the Nyblom statistic appropriate for phase-change tests also performs well in testing for frequency changes. 相似文献