Showing papers on "Scintillation published in 1983"

Optical scintillation statistics for IR ground-to-space laser communication systems.

Abstract: Statistical estimates of selected scintillation parameters for an infrared laser ground-to-space communication system are presented for a point-receiving aperture. The quantities estimated here are the fraction of time that the signal power is both above and below a given value, the mean number of times per second the signal power crosses a given signal level, and the mean duration of both surges and fades for a given log-irradiance variance.

The scintillations for weak atmospheric turbulence using a partially coherent source

Abstract: The scintillation index for weak atmospheric turbulence is formulated for a partially coherent source (in space and time). We obtain results for two cases when the detector has a slow and a fast response time. Our solutions correctly reduce to the known scintillation index in the limit of coherent and incoherent sources. Transmitter aperture averaging and frequency averaging effects are shown as special cases of our results. The limitations of the source model used and the solution obtained for a fast detector are discussed.

Observations and modeling of multi-frequency VHF and GHz scintillations in the equatorial region

Abstract: Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi-frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two-component power law irregularity spectrum with a shallow large-scale regime and a steeper small-scale regime, and a spectral break at about 1-km scale size.

Evaporation derived from optical and radio-wave scintillation

Abstract: The determination of the evaporation from scintillation measurements at several wavelengths is discussed. The refractive-index structure parameter Cn2 derived from the observed amplitude scintillation on a 30-GHz radio link is compared to a spot measurement of this quantity at a much lower height than that of the radio link. After free convection height scaling, a difference of a factor of 3 is found. This factor is discussed. A calculation of the evaporation from the observed radio-wave scintillation yields good agreement with calculations based on the Priestley-Taylor formula.

Aperture averaging of scintillation for space-to-ground optical communication applications.

Abstract: : We derive as useful engineering formula for the aperture averaging factor of optical scintillation that exhibits the explicit parametric dependence on wavelength, collector diameter, and path weighted integrals of the index structure constant profile. Numerical results presented here indicate that care should be exercised in the application of available index structure constant profile models to optical communication system performance studies. (Author)

A Hexagonal Bar Positron Camera: Problems and Solutions

Abstract: As the ability of positron imaging devices to obtain many transverse sections simultaneously with improved spatial resolution has increased, so has the number of scintillation crystals and photomultipliers required. While the PETT III built around 1975 used only 48 crystals and photomultipliers, current state of the art systems under development use 280 or 512 crystals per ring and may have several thousand crystal/photomultiplier assemblies in a complete system. The cost of these complex systems precludes their use in routine diagnostic medicine, restricting their availability to a few research centers. A continuous position-sensitive detector provides a method of avoiding the costly one-resolution element-per-detector design. The authors have investigated position-sensitive one-dimensional scintillation detectors for positron imaging to assess the problems encountered in their use and potential solutions. Special emphasis was placed on avoiding solutions which increased the complexity of the device.

Modeling of spaced‐receiver scintillation measurements

Abstract: Spaced-receiver scintillation measurements are modeled applying scintillation theory together with model spectral representations of nonfrozen turbulent media. The effects of velocity distribution of scatterers, diffusion of irregularities and velocity gradient across the scattering layer on parameters derived from spaced-receiver scintillation experiments are studied. Both correlation and dispersion analyses are considered. The results from modeling are compared with observational data from the equatorial region. It will be demonstrated that self-consistent models can be constructed in interpreting the data and information about the drift velocity field in the ionosphere can be obtained from spaced-receiver experiments.

Spectral density of millimeter wave amplitude scintillations in an absorption region

Abstract: A detailed theory including the effect of water vapor fluctuations for the spectral density of the log amplitude scintillations of a radio wave propagating in an absorption medium is presented. The scintillation spectra obtained from links at 55.5 and 36.1 GHz on a common 4.1 km path are given together with the relevant meteorological data. Results show that the lower corner frequency predicted by Ott and Thompson, for the enhancement of the scintillations in an absorption region, is a good approximation.

Coordinated measurements of low-energy electron precipitation and scintillations/TEC in the auroral oval

Abstract: A case study of coordinated observations of low-energy ( 10 radians for fluctuation frequencies >00067 Hz) were found to be well correlated with intense fluxes (>109 particles (cm2 s sr)−1) of precipitated low-energy electrons Total electron content and magnetometer measurements indicate that the onset of the precipitation event was about 10 min prior to the DMSP pass Within this time scale, the ionization generated in the F region could reach the topside so that the thermal sensor on board the DMSP satellite was able to measure a factor of 2–3 density enhancement at 840 km The latitudinal width of these density structures is consistent with that of F region blobs observed at Chatanika The gradient scale length measured in the topside was only 30 km, which was probably responsible for the fast growth rate of the scintillation-producing irregularities The phase to amplitude scintillation ratio changed rather drastically compared to quiet magnetic times, however, implying that increased convection velocities during these magnetic disturbances were partially responsible for the enhanced phase scintillation

Gigahertz scintillations associated with equatorial patches

Abstract: In a program designed to compare measurements of scintillation activity in the equatorial anomaly region, observations were made of satellite beacon signals transmitting at frequencies ranging from 137 MHz to 7 GHz. Recordings were made at Ascension Island in January–February 1981, during a month of very high solar flux and a high occurrence of scintillations. Saturation was noted in the VHF-UHF range with levels of 8 dB peak to peak at 4 GHz and 3 dB peak to peak at 7 GHz. Statistics of occurrence of various levels for 1.5 and 4 GHz are given in the paper. The hypotheses of vertical or horizontal irregularity sheets or rods within the patches of irregularities were examined with data from the Global Positioning System satellites. Vertical sheets were eliminated as a possibility. A comparison of scintillations with 6300-A airglow images, which map regions of depleted electron density over the entire sky, showed that in the anomaly region, maximum scintillation activity occurs within the patch and not at the walls of the patch.

The Impact of Strong Scintillation on Space Based Radar Design II: Noncoherent Detection

Abstract: Electromagnetic signals which propagate through strongly disturbed regions of the ionosphere can experience angular scattering, causing appreciable amplitude and phase scintillation and angle-of-arrival fluctuations. The performance of a space based radar (SBR) subject to degradation due to signal propagation through a highly disturbed ionospheric channel is considered here. Pertinent characteristics of the disturbed channel and the received radar signal are described. The effects of the propagation path are investigated and the differences between monostatic and bistatic operation are presented. Results are presented which show the effect of severe scintillation on the coherent target detection performance of an SBR. It is shown that coherent detection performance can be seriously degraded in a scintillation environment if scintillation effects are not considered in the radar design.

A Positron Tomograph Employing a One Dimension BGO Scintillation Camera

Abstract: A detector employing a pseudo-continuous ring of BGO and analog type position sensing has been constructed. It is fabricated using 360 2 × 3 × 0.4cm BGO detector elements, a light guide 1 × 1.6 × 51cm ID and 90 2cm diameter PM tubes. The position of a scintillation event is identified using logic based on a maximum likelihood estimator. A stationary high spatial resolution high detection efficiency tomograph is achieved using this approach.

Ocean flow measurements using acoustic scintillation

Abstract: A wave propagating in a medium having random fluctuations in refractive index will suffer phase and amplitude perturbations. In the receiving plane, a random interference pattern will appear and this so‐called scintillation pattern will vary in time for two reasons: (1) the decay of the refractive‐index fluctuations producing the amplitude perturbation (eddy decay) and (2) advection of the eddies by the flow. In the case where eddy lifetimes are long compared with the scintillation period, we can derive estimates of flow from a statistical analysis of the scintillation pattern. In this paper, we discuss the propagation theory and report measurements of oceanic flows by analysis of the acoustic scintillation pattern produced by the density fluctuations in the ocean. By mounting a 214‐kHz source and two receivers on opposite sides of a barge such that the axis of propagation is perpendicular to the direction of travel, we induce a known flow rate equal to the barge velocity. We compute the slope of the time‐lagged covariance function of the logarithm of the amplitude at the two detectors. The slope is proportional to the path‐averaged flow transverse to the propagation path. Simultaneous measurements with a current meter provide sea truth. We have shown that such a technique will measure flow velocity with reasonable accuracy. An interesting result of the measurements is the demonstration that sound speed fluctuations in the spatial window 15–30 cm satisfy the Kolmogorov spectral slope for an inertial subrange, at the shallow depth (2.1 m) of the observation.

Scintillation Detectors Using Large Area Silicon Avalanche Photodiodes

Abstract: Low noise, large area silicon avalanche photodiodes have now been made to replace photomultiplier tubes in certain scintillator detector applications in which overall size and ruggedness are important. Previous devices have been limited by poor optical sensitivity and very small active size. These sensors when coupled to a 1-inch NaI(T1) crystal have produced pulse height spectra with resolution for Cs 137 gamma radiation as low as 9.5% FWHM. This achievement is the result of improvements in both silicon material quality and device design. Based on these results the future prospects of such detectors are very encouraging.

Scintillation detector array employing zig-zag plates

Abstract: A scintillation detector array for use in computerized tomography comprises a housing having a wall section substantially transparent to x-ray or gamma-ray radiation and which has, disposed within, a plurality of adjacent, triangular prism shaped chambers. The chambers have alternate, oppositely disposed bases and contain a scintillation medium. A photodetector is mounted on the base of each of the chambers. The detector array converts x-ray intensity levels of impinging x-ray radiation to related electrical intensity levels.

Coordinated study of subkilometer and 3‐m irregularities in the F region generated by high‐power HF heating at Arecibo

Abstract: High-power high-frequency transmitters near Arecibo were used to generate artificial ionospheric irregularities in the F region. Radio star scintillation observations at 430 MHz were performed at Arecibo Ionospheric Observatory with the 305-m antenna, and radar backscatter measurements at 50 MHz were simultaneously made from Guadeloupe Island to probe the subkilometer and 3-m irregularities in the heated volume. Scintillation studies indicate a low-frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low-frequency temporal structure translates to spatial dimensions of 1–2 km. The frequency of the modulation envelope is found to be controlled by the heater power and is related to the dominant irregularity wavelength generated by the self-focusing instability. Scintillation spectra imply a steep power law index of −5 in the scale length range of about 300 m to 150 m and a shallow index of −2 at less than 150 m. The steep power law index may arise from an aperture averaging effect of the large 305-m antenna at Arecibo. Simultaneous measurements of 430-MHz scintillations and 50-MHz radar backscatter from field-aligned striations were performed to show that subkilometer irregularities can be generated by both O and X mode heating whereas the 3-m irregularities are excited only by the O mode heating, as is predicted by the theories of self-focusing and parametric instability. The width of the 50-MHz echo Doppler spectra is observed to be very narrow, only ∼2–3 Hz, and independent of the background plasma drift, implying that the frequency bandwidth of the scattered signal is probably controlled by the instability process.

The measurement of large-scale density fluctuations in toroidal plasmas from the 'phase scintillation' of a probing electromagnetic wave

Abstract: The plasma electron density fluctuation spectrum can be obtained from the measurement of the phase fluctuation spectrum of a probing electromagnetic wave measured some distance from the plasma. If the mean square fluctuation of the probing beam is much less than unity and the observer-plasma distance is well within the Fresnel distance of the irregularities, a direct relationship exists between the density and phase spectra. As an example of this procedure, an experiment is described which uses a probing beam wavelength of 2 mm to measure the density irregularities. It is then proposed that a suitable wavelength for the present large toroidal plasma devices is in the 10 mu m region.

The Oriented Scintillation Spectrometer Experiment for the Gamma-Ray Observatory

Abstract: : The Oriented Scintillation Spectrometer Experiment (OSSE) for the Gamma Ray observatory is described. OSSE uses four identical NaI(Tl)-CsI(Na) phoswich detectors to provide gamma-ray line and continuum detection capability in the 0.05-10 MeV energy range. Additioal gamma-ray and neutron detection capability is achieved above 10 MeV. Each detector has a CsI annular shield and a tungsten alloy collimator which define a 5 x 11 deg (FWHM) field-of-view. The detectors have independent, single-axis orientation systems which permit offset pointing to provide source-background subtraction. The sensitivity for line gamma rays the 0.05-10 MeV region will be .00002 - .00003 photons/cc-s for a .000001-second observation period. The several modes of data acquisition and the emphases for the eplanned observational program are discussed.

Interplanetary scintillation observations of the solar wind close to the Sun and out of the ecliptic

Abstract: A brief review is given of recent developments in the observation of the solar wind by the method of interplanetary scintillation. The emphasis is on observations of the velocity structure, the electron density and the effect of propagating disturbances in the interplanetary medium as detected principally by intensity and phase scintillation and by spectral broadening.

Radar target doppler frequency scintillation simulator and method

Abstract: Generating five Gaussian random variable, statistically identical but independent, signals, squaring four of the signals and combining two or four of the signals to provide the target power return, or radar cross section, of Swerling case one or three targets, respectively. Utilizing the fifth Gaussian random variable and dividing it by the square root of the target power return signal, and multiplying the result by the bandwidth of the fifth Gaussian random variable and a scale factor to provide an output signal representative of the target Doppler frequency scintillation.

Scintillation camera with pulse height analyzer

Abstract: A radiation imaging apparatus provided with a pulse height analyzer for checking whether the pulse height of the energy signal produced in response to each scintillation event falls within an energy window set for the particular (x,y) coordinate position of the scintillation. The pulse height analyzer has a memory for storing a plurality of energy windows each for a particular one of the coordinate positions in the scintillator and each having a particular central level and a particular width such that the nonuniformity of the sensitivity of the detector due to the position-dependency of the energy signal and/or other causes can be corrected. A preliminary pulse height analyzer may be provided to increase the counting rate of energy signals useful for image production.

A Quantitative Comparison of the Refractive Index Structure Parameter Determined from Refractivity Measurements And Amplitude Scintillation Measurements at 36 GHz

Abstract: Results are presented of the determination of the atmospheric refractive index structure parameter C2n, deduced from amplitude scintillation measurements made on a 36-GHz radio link and also from refractivity measurements using a turbulence probe. These measurements were made in a town environment (central London) and covered a range of atmospheric conditions. The two methods of measurement give good agreement, particularly in convective conditions, and provide further experimental evidence of the applicability of Tatar-ski's wave propagation theory in a turbulent medium to millimeter wavelengths.