Aperture Length Research Articles

The Tarija-Pulkovo Astrographic Catalogue Of The Southern Sky: First Results And Perspectives

A project of a new photographic catalogue of southern stars was proposed in 1977 by Polozhentsev and Potter (1978). The observations for this catalogue were begun in 1982 with an astrograph (field 5°× 5°) placed at the Bolivian-Soviet observatory near Tarija, Bolivia. The astrograph has an aperture and focal length of 23 and 230 cm, respectively. A total of 6420 plates must be photographed to realize the fourfold coverage of the Southern Hemisphere with a 2° overlap in both coordinates and of these 6405 plates have been taken as of January 1, 1987. V.V. Avramchuk, S.P. Pulyaev, Kh.I. Potter (USSR) and R.F. Zalle, V. Yamaguchi, and E. Espinoza took part in the observations.

Detection of buried plant

A wideband microwave holographic method for imaging buried objects with high azimuth and range resolution is described. The method incorporates a synthetic aperture approach in pulse radar systems, originally designed for the pulse-echo imaging, to improve azimuth resolution. The resolving capability of the method is discussed in terms of controlling parameters such as the synthetic aperture length, soil conductivity and dielectric constant, and antenna beamwidth. Since the propagation velocity of electromagnetic wave in soil, which varies from soil to soil, is an essential parameter for reconstructing object images by the method, a method of minimum squared error estimation of the propagation velocity from a pulse-echo image is proposed. Results of underground object image reconstruction from real pulse-echo data is reported to demonstrate high estimation accuracy of the propagation velocity and fine resolution of the reconstructed image.

SAR resolution in the presence of phase errors

The author analyzes the effects of phase errors on synthetic aperture radar (SAR). The theory is applied to the following question: how does the achievable resolution vary with the carrier frequency when optimum quadratic focus and/or optimum processing interval (synthetic aperture length) are used? Numerous related results are given, so that much of the material is tutorial. For phase errors corresponding to uncompensated motion, the best achievable RMS resolution with any phase error spectrum satisfies the derived equation. For motion-induced phase errors it is seen that resolution improves with increasing carrier frequency when the first term in the expression applies (e.g. for phase errors concentrated at low frequencies) and resolution is independent of carrier frequency when R delta /v/v is the smaller term (e.g. with broad band or high frequency phase errors).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Source localization in almost‐stratified waveguides

In our previous papers [E. C. Shang, C. S. Clay, and Y. Y. Wang, J. Acoust. Soc. Am. 78, 172 (1985); J. Acoust. Soc. Am. 77, 1413 (1985)], source localization techniques based on mode filtering have been developed for stratified waveguides. In this paper, some problems of source localization in almost‐stratified waveguides are discussed. Specifically, the Prony method is proposed for source bearing in range‐dependent waveguides. It is found that the Prony method only requires a “local almost‐stratified” condition, which means that within the data sampling aperture length the field can be treated as adiabatic modes. For most of the practical interesting cases, this condition can be satisfied quite easy.

Passive synthetic arrays

Passive‐synthetic apertures [W. Carey and N. Yen, J. Acoust. Soc. Am. Suppl. 1 75, S62 (1984)] were formed with experimental towed hydrophone data in a sound channel that supported RR and RSR transmission. These apertures were formed with lengths up to 95λ with coherent temporal processing gains approaching 0.75 of theoretical. These results and those of previous investigators [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752–753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] indicate that synthetic apertures can be formed by the coherent summation of the phase‐corrected summation of either hydrophone or subaperture beams over successive time samples when the synthetic aperture length is less than the spatial coherence length and the processing time is less than the temporal coherence length. The evaluation of synthetic apertures requires comparisons with conventional and other high‐resolution techniques. Comparisons between conventional array processing and high‐resolution techniques [maximum entropy (ME) and maximum likelihood (ML) methods] are performed by use of the analytical expressions developed by A. T. Parsons (AUWE, TN 700/83) for the determination of the array, integration, and the net processing gains. Analytical comparisons between conventional and synthetic aperture arrays formed with either the same number of hydrophones or with the same effective length but a different number of hydrophones, show that, when the spatial processing gain exceeds the loss in integration gain, then the use of synthetic apertures is advantageous. [Work performed while at NORDA.]

Microscope ocular fundus photography.

The ocular of any microscope may be used for the purpose of photography through the device if the camera utilizes the given optical conditions like an observer eye concerning aperture and focal length.

A proposal for searching for electromagnetic wave sources by using a synthetic aperture technique

AbstractUnwanted electromagnetic waves radiated from equipment making use of electromagnetic energy create numerous societal problems. The prevention of these problems has been studied extensively. In this paper, a method for estimating the location of the unwanted electromagnetic waves is studied. Since it is difficult to make the antenna aperture large especailly at low frequencies, a search for electromagnetic wave source via a synthetic aperture technique is proposed. In the estimated electromagnetic source distribution by this method, images similar to the antenna sidelobes appear. It is demonstrated by computer simulation that the images can be suppressed by use of the synthetic aperture method with subarrays or use of Mill's cross method. Discrepancies between the true source location and the estimated location have been found in regard to the synthetic aperture length and the geometrical relationship with respect to the aperture center. It has been found that the antenna configuration with the synthetic aperture length of 10 λ can estimate an electromagnetic wave source within 10 λ square with accuracy of less than 1/4 λ square. Therefore this method is sufficiently practical if the location of the source is restricted somewhat.

Imaging Subglacial Topography by a Synthetic Aperture Radar Technique

A synthetic aperture radar (SAR) technique has been used to image part of the grounding-line region of Bach Ice Shelf in the Antarctic Peninsula. The radar was sledge-mounted and operated in a pulsed mode with a carrier frequency of 120 MHz. The coherently detected output was recorded photographically as in-phase and quadrature components. Because the system was essentially stationary for each measurement, there was no doppler information about the reflecting points as in the more commonly used airborne and satellite-based SARs. Instead, the phase history was used directly to identify point targets by a correlation method.Three sounding runs were carried out over the grounding line to give views of the area from separate directions. An aperture length of 104 m was necessary to achieve 8 m resolution in the along-track direction for an ice thickness of 290 m. The mapped swath was 88 m wide. Corrections to the data were made to allow for density variations and absorption in the ice. The back-scatter coefficient showed greater variations in echo strength over grounded ice compared with floating ice and texture analysis of the radar image revealed a statistically significant difference between these two regimes.

Imaging Subglacial Topography by a Synthetic Aperture Radar Technique

A synthetic aperture radar (SAR) technique has been used to image part of the grounding-line region of Bach Ice Shelf in the Antarctic Peninsula. The radar was sledge-mounted and operated in a pulsed mode with a carrier frequency of 120 MHz. The coherently detected output was recorded photographically as in-phase and quadrature components. Because the system was essentially stationary for each measurement, there was no doppler information about the reflecting points as in the more commonly used airborne and satellite-based SARs. Instead, the phase history was used directly to identify point targets by a correlation method. Three sounding runs were carried out over the grounding line to give views of the area from separate directions. An aperture length of 104 m was necessary to achieve 8 m resolution in the along-track direction for an ice thickness of 290 m. The mapped swath was 88 m wide. Corrections to the data were made to allow for density variations and absorption in the ice. The back-scatter coefficient showed greater variations in echo strength over grounded ice compared with floating ice and texture analysis of the radar image revealed a statistically significant difference between these two regimes.

Kinetics of condensation in supersonic expansion (Ar)

The temperature and density in an overexpanded jet (Ar) were mapped by the method of characteristics. The kinetics of condensation in the supersaturated gas along the central streamline was analyzed on the basis of four models, subject to previously developed thermodynamic relations which must apply during the homogeneous condensation of any supersaturated vapor. Here we report on detailed dynamic analyses of the rates of cluster growth. Each association: An−1+A1⇄An was represented either by a single stabilization step, or by a sequence of two energy removing collisions, for which kinetic parameters based on Lennard-Jones (LJ) potentials were estimated. Coupled differential rate equations (up to 192) were solved, both for cases when the heat of condensation was neglected, and when it was partially or fully distributed throughout the fluid. The distribution of cluster sizes declines exponentially, according to N(n)=A exp(−an)+B exp(−bn), in agreement with generally observed distributions for monatomic species. For source pressures of 3 and 10 atm the condensation process becomes kinetically controlled within a distance downstream somewhat less than twice the aperture diameter. Our calculations indicated that after about three aperture lengths ‘‘freezing in’’ takes place if the heat of condensation is neglected. When the heat of condensation is distributed (partitioned in one of several plausible ways) freezing in is delayed beyond four aperture diameters. A modest fraction of the nascent clusters retain some of the heat of condensation and remain at a higher temperature than the ambient gas.

Laser beam-quality/aperture-shape scaling relation

Many high-energy lasers (HELs) have noncircular output apertures. Some are rectangular in shape with or without a central or noncentral (up to 30%) obscuration. However, most high-energy laser propagation codes (especially those developed for systems analysis) model the aperture as either an unobscured circle or as a circle with fixed (e.g., 10%) obscuration. We present a beam-quality/aperture-shape scaling relation which can be useful when applying these codes to realistic designs for HELs. Our analysis also yields a generalized formula for angular size of the Airy disk and definitions of a characteristic aperture length and aperture quality.

Effects of prey density, prey size and predator size on rates of feeding by an intertidal predatory gastropod Morula marginalba Blainville (Muricidae), on several species of prey

As a prerequisite for models of foraging behaviour of the whelk, Morula marginalba Blainville (Muricidae), the effects of variation in density of prey on the rate of feeding of the predator were examined in field conditions for three coexisting species of prey. Densities of prey used were those at which the prey, two limpets and a barnacle, occurred naturally in the rocky intertidal habitat. Large limpets, Cellana tramoserica (Sowerby) can resist attacks by predatory gastropods by raising the mantle over the outside of the shell. These experiments showed that no C. tramoserica were killed by Morula marginalba even at very great densities and with no alternative prey present. For the small limpet Patelloida latistrigata (Angas), one of the whelk's most highly preferred prey, juveniles were eaten 1.4 times as fast as adults. Fitting the random predator equation gave greater attack coefficients and shorter handling times for juvenile than adult limpets. Sizes of both predator and prey affected rates of eating barnacles, Tesseropora rosea (Krauss), but not in a simple way. Whelks of 15-mm aperture length ate adult barnacles 4.2 times faster than did 12-mm whelks, but there was no significant difference in the rates at which the two sizes of snail ate juvenile barnacles. Rates of feeding on T. rosea and Patelloida latistrigata increased significantly with prey density. These results form a basis for including the density of prey in models of spatial dispersion of the predatory gastropod Morula marginalba.

Microwave holographic imaging of underground objects

The resolving capability of underground imaging radar employing a multifrequency holographic approach is characterized in terms of controlling parameters such as the synthetic aperture length, soil conductivity and dielectric constant, and antenna beamwidth. The propagation velocity of electromagnetic wave in soil, which varies from soil to soil, is an essential parameter for reconstructing object images using the holographic approach. Hence, a method of estimating the propagation velocity and also the depth of an object from a pulse-echo image based on the minimum squared error is proposed. The result of underground object image reconstruction from real pulse-echo data is reported to demonstrate high estimation accuracy of the propagation velocity and fine resolution of the reconstructed image.

Variability of shell growth and morphology of the wall-plate junctions of the intertidal barnacle Tesseropora rosea (Cirripedia: Tetraclitidae)

Variability in shell growth and morphology of the wallplate junctions of Tesseropora rosea (Krauss) were studied in 1983 in New South Wales, Australia, at two sites with differing wave exposure. The wall-plate junctions, a combination of “dado” (geometrically interlocking) and “lap” bints, increase in complexity with age, but do not differ between sites. The number of parietal tubes is fixed at settlement and does not differ between sites, but differs among wall plates in a manner consistent with their evolutionary history (rostrum > laterals > carina). The number of secondary septa increases with age, but does not differ between sites. In epilithic T. rosea, growth of the alae contributes to orifice enlargement. Epizoic T. rosea (on limpets) have conspicuous radii and display diametric growth. At the more exposed site, the shell wall was sleeper, and the aperture length, height and weight of the shell increased at a faster rate than at the less exposed site, but the maximum height of the shell was less. These differences appear to be due to a combination of greater feeding rates and greater erosion rates at the exposed site. The greater maximum height attained at the more sheltered site may be due to greater individual longevity. Juveniles at the sheltered site had thinner septa in the laterals than those at the exposed site. Septa in adults were rhinner at the exposed site, but this is due to age differences between sites in the adult populations.

Properties of the beam noise distribution function for horizontal arrays in ship‐induced noise fields

An analysis is presented of the environmental and array system factors governing the fluctuations in the beam noise of a horizontal array in a ship‐induced noise field. A model is used that relates the cumulative distribution function to the directivity and side‐lobe characteristics of the system and the strength and anisotropy of the shipping distribution. The results are threefold. First, for a given environment, an increase in the array length results in a large increase in the prevalance of low noise periods at the expense of a modest increase of high noise periods. Second, for sufficiently large apertures, a “noise floor” develops in that noise values larger than the noise floor occur with high probability, whereas values less than the noise floor occur only rarely. Finally, the noise floor level is essentially independent of the noise anisotropy and of further increases in aperture length, and it increases with degradation in side‐lobe level.

The formation of a synthetic aperture with towed hydrophones

Passive‐synthetic apertures were formed with experimental data from the coherent summation of subapertures formed from 1–4 hydrophone groups at successive time samples. The results show that the coherent signal gain for synthetic apertures with lengths up to 95 λ was a function of the number of successive time samples (Nts) described by α 10 log(Nts) with α∼0.7. The hydrophones were towed with a speed‐to‐wavelength ratio of 0.168 s−1 at a depth of 26 λ. A calibrated moored source was at a depth of 37 λ in 3200 m of water at distance‐to‐wavelength ratio of between 20 and 30 × 103 from the receiver. The stable environment was characterized by a sound‐speed profile with a negative gradient of − 0.130 s−1 to a sound channel axis of 150 m and a positive gradient of + 0.016 s−1 to a critical depth of 1825 m. The resulting propagation was RR and RSR with losses of between 90 and 100 dB. These results show that for these environmental‐acoustic conditions the synthetic apertures can be formed by the coherent summation of phase corrected subaperture beams over successive time samples as long as the synthetic aperture length is less than the single path coherence length and the synthetic aperture processing time is less than the signal's temporal coherence length. The results presented represent an extension of earlier work [R. Fitzgerald, J. Acoust. Soc. Am. 60, 752‐753 (1976); R. Williams, J. Acoust. Soc. Am. 60, 60–73 (1976)] by the demonstration that coherent gain can actually be achieved with resolution such that multipath vertical arrival angle differences can be resolved.

A geometric analysis of growth in gastropod shells, with particular reference to turbinate forms

Nucella lapillus and Littorina littorea afford examples of turbinately coiled shells with no space between the columella and the inner face of the whorls. Three constants are sufficient to determine the form of such shells: λ, the ratio between diameters of successive whorls; β, the semi-apical angle and ρ, the ratio of aperture length and breadth in the apertural plane passing through the axis. The variation of these three constants with size was examined and shown to be relatively small and insignificant in Nucella, but in Littorina it was significant.For Nucella and Littorina these shell characteristics were used to relate for the nth whorl, the length along the shell spiral (ln) to the shell height (Hn), the latter being the measure usually employed in growth studies. This relation was derived mathematically and confirmed empirically aswhere α is the angle of the logarithmic spiral given by α = tan−1 2π sin β/loge λ. The increased resolution of the micro-growth band increments, which are measured along the shell spiral was directly related to the ln:Hn ratios.Since the formula for the ln/Hn ratio (= shell conversion factor) includes λ, β and ρ, the three constants necessary to determine shell form in turbinately coiled shells of the type described, the shell conversion factor may be a better index of shell shape than indices based on a single constant. In species where the shell grows isometrically this ratio will remain constant, whereas changes in shell shape with size will influence this ratio. Such changes in shell shape during growth can be quantified in terms of the shell constants and when included in calculating the shell conversion factor, will show the relative magnitude of shell shape changes brought about by growth. The shell conversion factor may also be used to study the effects of ecological variations on the geometry of the shell.The conical shell of the limpet, Patella vulgata, is geometrically a much simpler system in which direct shell measurements were used to relate length along the direction of maximum growth to the antero-posterior diameter of aperture.

Array beam noise characterization

The formulation of the stochastic process for beam noise of a horizontal array operating at low frequency is analyzed based on the array's response to the distant ship-induced noise field. The general form of beam noise characteristic function obtained with nonhomogeneous compound Poisson process assumption for shipping distribution is related to three major operation environment parameters: the mean number of shipping density, the array aperture length and beam pattern, and random fluctuation and range dependence in the transmission loss function. This characteristic function can be used to derive the probability of observing low noise region in terms of frequency-azimuth-time representation of noise surface for some specific operation scenarios. Computer modeling for ship lane dominant noise field and uniform shipping distributed noise field are performed for investigating the sensitivity of the relevant parameters to the array's beam noise statistical property.

The occurrence of low beam noise level in ship induced noise field

The formulation of the stochastic process for beam noise of a horizontal array operating at low frequency is analyzed based on the array's response to the distant ship-induced noise field. The general form of beam noise characteristic function obtained with nonhomogeneous compound Poisson process assumption for shipping distribution is related to three major environmental parameters: the mean number of shipping density, the array aperture length and beam pattern, and random fluctuation and range dependence in the transmission loss function. This characteristic function can be used to derive the probability of observing low noise regions in terms of frequency-azimuth-time representation of noise surface for certain acoustic environments. Computer modeling for ship lane dominant noise field and uniform shipping distributed noise field were performed for investigating the relationship among the relevant parameters in the array's beamforming output. The results illustrate the sensitivity of beam noise statistical characteristics to the angular ship density.

Polarization-maintaining single-mode fibers

Polarization-maintaining single-mode fibers will find application in acoustooptic sensors and fiber gyroscopes. In this study both stress-induced birefringence and elliptical core polarization-maintaining single-mode fibers were developed and evaluated. The two fiber types were compared in terms of core dimension, numerical aperture, attenuation, and beat length, a measure of the polarization maintenance. For comparative beat length values the stress-induced birefringence fibers had lower attenuation and larger core diameter than the elliptical core fibers.