Aperture Smoothing Research Articles

Multi-aperture fiber coherent combining system in urban horizontal atmospheric laser link

Multi-aperture receiver with optical combining architecture is an effective approach to overcome the turbulent atmosphere effect on the performance of the free-space optical (FSO) communications. In this paper, we built a 1 km urban horizontal link, and adopted beam combining system with four apertures at receiver. By analogy with aperture smoothing factor, we proposed the concept of fluctuation suppression ratio to evaluate the mitigation ability of the system on atmospheric fading. The experimental results confirmed that the beam combining system can perform best when the aperture interval is larger than the coherent length, and the performance decreases with the increase of the coherent length. Moreover, we found that the change of phase differences between the input beams is more than 40 πrad, which provides reference for future system design.

Generalised persymmetric parametric adaptive coherence estimator for multichannel adaptive signal detection

In this study, the authors deal with the problem of detecting a signal in partially homogeneous environments, where both the test data and the training data share the same covariance matrix up to an unknown scaling factor. A generalised persymmetric parametric adaptive coherence estimator (GPer-PACE) detector is proposed, where the disturbance is modelled as a multichannel autoregressive process. To mitigate the effect of limited training samples, the subspatial aperture smoothing is performed in the design of the authors’ GPer-PACE detector. Moreover, the persymmetric structure information is exploited to further reduce the sample requirements. The performance of the GPer-PACE is assessed by numerical examples. The results show that the GPer-PACE outperforms other traditional detectors in sample-deficient scenarios.

Aperture averaging beam-wander of multi-Schell quantization beam in a turbulent atmosphere

We study the effects of the mode number of the multi-Schell Gaussian model beams on aperture averaging wander of quantization Gaussian beams propagating through weak turbulence fluctuation channel. The results demonstrate that there is an optimal sub-Schell beam number Mopt, in here the beam wander is minimum. Because of the beam spreading of turbulence and vacuum diffraction, and aperture smoothing effect of the multi-beam jitter, the impact of zenith angle of channel and turbulent outer scale on the beam wander is tiny. The beam wander of the multi-Schell Gaussian model beams is increased as the increasing of the wavelength λ.

STATISTICAL PROPERTIES OF RANDOM SPARSE ARRAYS

Theoretical models that can be used to predict the range of mainlobe widths and the probability distribution of the peak sidelobe levels of two-dimensionally sparse arrays are presented here. The arrays are considered to comprise microphones that are randomly positioned on a segmented grid of a given size. First, approximate expressions for the mean and variance of the squared magnitude of the aperture smoothing function are formulated for the random arrays considered in the present study. By using the variance function, the mean value and the lower end of the range i.e., the first 1 per cent of the mainlobe width distribution, can be predicted with reasonable accuracy. To predict the probability distribution of the peak sidelobe levels, distributions of levels were modelled by using a Weibull distribution at each peak in the sidelobe region of the mean squared magnitude of the aperture smoothing function. The two parameters of the Weibull distribution were estimated from the means and variances of the levels at the corresponding locations. Next, the probability distribution of the peak sidelobe levels were identified by following a procedure in which the peak sideload level was determined as the maximum among a finite number of independent random sidelobe levels. It was found that the model obtained from that approach predicts the probability density function of the peak sidelobe level distribution reasonably well for the various combinations of the two different numbers of microphones and the various grid sizes tested in the present study. The application of these models to the design of random, sparse arrays having specified performance levels is discussed.

CCD drift-scan imaging lunar occultations: A feasible approach for sub-meter class telescopes

A CCD drift-scanning technique for lunar occultation (LO) observations is presented. While this ap- proach has been addressed before by Sturmann (1994) for the case of large telescopes, the technical validity has never been discussed for sub-meter class telescopes. In contrast to Sturmann's scheme, the proposed technique places the CCD in the image plane of the telescope. This does not represent a problem in the case of small telescopes, where the practical angular resolution attainable by LO is not limited by aperture smoothing. Photon- generated charge is read out at millisecond rates on a column by column basis, as the diraction pattern of the occulted star is being tracked. Two LO events (SAO 79031 and SAO 77911) were observed to demonstrate the feasibility of the method. Data analysis was carried out, yielding unresolved angular diameters for both objects. We show, however, that the technique could be useful for close binary detections with small telescopes. A dis- cussion of the limiting resolution and magnitude imposed by our instrumentation is carried out, showing that the drift-scanning technique could be extended to 1{2 m telescopes for stellar diameter determination purposes. Finally, we point out that the technical demands required by this technique can be easily met by most small professional observatories and advanced amateurs.

Measurement and modelling of amplitude and phase scintillations in an Earth–space path

This paper gives a complete summary of the theory and experimental results obtained by Portsmouth Polytechnic on the subject of amplitude and phase scintillations on a down-link path through the atmosphere during the Orbital Test Satellite campaign of research at 11.786 GHz and special and emphasis is given to engineering applications. The paper introduces first a brief summary of relevant theoretical studies, inclusive of antenna aperture smoothing, which facilitates the understanding of the scintillation process and the use of the experimental results in system design and planning. The results include chart recording observations, spectra of the scintillations and correlation with meteorological conditions together with results of cumulative distributions of scintillation fade, fade rate and intensity. The subject of propagation-induced phase noise is also dealt with and includes the theoretical estimates of the phase noise due to turbulence and precipitation as well as an outline of the experimental techniques used to measure the f.m. noise of the beacons.

Antenna aperture effects on measurements of propagation through turbulence

The first Born approximation is applied to propagation of a spherical wave through a layer of random irregularities to an aperture antenna. The effect of the receiving aperture on measurements of the variance of in-phase and quadrature components of the received signal, and the variances of their spatial derivatives is determined. In the weak scattering limit, these quantities are related to the scintillation index, mean-square phase fluctuations, mean-square angle-of-arrival fluctuations, and fading rates. Physical situations corresponding to the generic problem considered here include satellite communication through ionospheric fluctuations, solar wind observations, observations through irregularities caused by instabilities in barium release clouds, etc. Numerical results are presented which show the aperture effect for irregularities with power law spectra of the form <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S(K) \propto (1 + K^{2}L_{0}^{2})^{-N/2}</tex> for values of the spectral index <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> ranging from 3 to 5 and for many ratios of the three lengths, turbulent outer scale <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L_{0}</tex> , aperture radius, and Fresnel length. These results are presented in convenient form for estimating the aperture effect in many cases of interest. Aperture effects (aperture smoothing) generally dominate the behavior of the spatial derivatives, while a nearfield effect is shown which causes enhancement of the variance of the in-phase fluctuations over the value predicted ignoring the receiving aperture. It is suggested that this effect may be useful to explain some aspects of observed gigahertz scintillation.

A survey of clear-air propagation effects relevant to optical communications

The theory and observations of the optical propagation effects of the clear turbulent atmosphere are reviewed, with particular attention to those characteristics most important to the designer of an optical communication system. Among the phenomena considered are the variance, probability distribution, spatial covariance, aperture smoothing, and temporal power spectrum of intensity fluctuations, and similar quantities for phase fluctuations and angle of arrival.