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Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR)...
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Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR). Data from December 5 to 9, 1991, show the mean DGS velocities to be within 50 m s of the ISR velocities, that is, within the uncertainty levels of each instrument. The analysis highlights the dominance of the electric field in controlling the plasma motion. The measured velocities are generally height independent, as would be expected for an E field mapped along the magnetic field lines from high altitudes to ionospheric heights. In addition, the comparative analysis is used to study an ionospheric event where a large section of ionization was removed from the daytime cusp region by a strong anti-Sunward drift when the interplanetary magnetic field (IMF) Bz component changed orientation.
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Anomalous signal fluctuations due to ionospheric scintillation at 1.5 GHz appear occasionally on earth-space paths in maritime satellite communication. With regard to the characteristics of ionospheric scintillations at gigahertz ...
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Anomalous signal fluctuations due to ionospheric scintillation at 1.5 GHz appear occasionally on earth-space paths in maritime satellite communication. With regard to the characteristics of ionospheric scintillations at gigahertz frequencies, few data are available in the middle latitude region. This paper describes the results of 1.5-GHz scintillation measurements at Yamaguchi, Japan. Measurements were carried out by receiving a 1.5-GHz beacon signal from the MARISAT satellite over the Indian Ocean with an elevation angle of 17.3°. In 13 months of measurements, very severe scintillation with peak-to-peak fluctuations exceeding 30 dB was observed in the equinoctial month, and a number of spike-type scintillations were also observed, particularly at night Scintillation characteristics, such as diurnal and seasonal variations, amplitude distribution, spectrum, and depolarization are discussed.
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The Atmosphere Explorer (AE) photoelectron spectrometer experiment consists of electrostatic deflection-type electron spectrometers designed to measure the low-energy electron flux expected to be encountered along the AE orbit. Th...
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The Atmosphere Explorer (AE) photoelectron spectrometer experiment consists of electrostatic deflection-type electron spectrometers designed to measure the low-energy electron flux expected to be encountered along the AE orbit. The system has been designed to provide general information on the intensity, angular distribution, energy spectrum, and net flows along field lines as well as sufficient energy resolution to allow the fine details of the electron-energy spectrum to be studied. The instrumentation consists of two separate electron-spectrometer detector heads connected to a common electronics system for data processing and telemetry interfacing. The experiment will measure the electron-energy spectrum from 2 to 500 ev (and possibly to less than 0.1 ev). The energy resolution (ΔE/E) of the electron spectrometers will be approximately 2.5\% full width half maximum. The geometrical factor of the analyzers will be approximately 10 cm ster, independent of electron energy. Placement on the spacecraft will be arranged so as to give as little interference as possible from shadowing of electron trajectories by the spacecraft and contamination of the observed spectra by locally generated photoelectrons.
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This paper describes a new electrode structure that has been developed to measure the water content of artificial soil. Similar to a conventional setup, the water content is also extracted from the capacitive component of the admi...
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This paper describes a new electrode structure that has been developed to measure the water content of artificial soil. Similar to a conventional setup, the water content is also extracted from the capacitive component of the admittance. Results show that in the existence of high water conductivity $(sigma geq 1.5 hbox{S/m})$, the conventional rod-shaped pairs of electrodes are no longer suited for accurate water-content measurements. This is due to the occurrence of physical effects, including skin effect, proximity effect, and parasitic inductances. To reduce these physical effects, a new electrode structure has been designed, built, and tested. It is shown that, with the desired signal frequency of 20 MHz, the physical problems have significantly been reduced and that, even with water conductivity of up to $sigma = 2 hbox{S/m}$, reliable results can be obtained.
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The project Electronic Realization of the Kilogram has rebuilt the National Institute of Standards and Technology (NIST) watt balance, which has been fully operational for nine months. Short-term noise is at an all-time low with o...
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The project Electronic Realization of the Kilogram has rebuilt the National Institute of Standards and Technology (NIST) watt balance, which has been fully operational for nine months. Short-term noise is at an all-time low with overnight runs exhibiting standard deviations of parts in 10~(8). Recent improvements are discussed that have led to this enhanced performance, as are several significant error tests. Present watt results (i.e., Planck's constant) are now within 1 μW/W of CODATA values, but error checking to report a more precise value has not been completed.
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The direct measurement of the angle rate of moving objects using a radar with a spatially diverse electric field pattern, a measurement analogous to the measurement of the range rate of a moving objects, represents, along with the...
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The direct measurement of the angle rate of moving objects using a radar with a spatially diverse electric field pattern, a measurement analogous to the measurement of the range rate of a moving objects, represents, along with the measurements of range, range rate, and angle, a fourth basic radar measurement. Recently introduced and experimentally demonstrated, the theoretical accuracy of the direct measurement of angle rate is derived in this paper, and it is compared to the measurements of range, range rate, and angle in the context of the optimal signal forms for the best measurement accuracy. Signal forms achieving optimal accuracy for each measurement are discussed; example implementations of high-accuracy measurements are compared to the optimal forms; and the limitations of simultaneous measurements of pairs of measurements are derived. Combining the angle rate measurement with the three other standard radar measurements may provide future radar systems the capability to simultaneously and instantaneously measure the position and 3-D trajectory of moving objects without compromising the accuracy of any individual measurement.
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In 1972 and again in 1989-1990 measurements of the incidental radio noise from two cities in central Illinois were made in the range 144-412 MHz by flying over in a light airplane equipped with downward looking antennas. The citie...
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In 1972 and again in 1989-1990 measurements of the incidental radio noise from two cities in central Illinois were made in the range 144-412 MHz by flying over in a light airplane equipped with downward looking antennas. The cities were well resolved by the antennas, so the data could be expressed as brightness temperatures. Care was taken to avoid contamination of the data by radiation from transmitters and the airplane's systems. Variation of brightness with frequency, time of day, and season of the year was determined. Comparison of the 1972 and 1989-1990 data shows no significant changes between epochs.
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Searching similarity between a pair of shapes or data is an important problem in data analysis and visualization. The problem of computing similarity measures using scalar topology has been studied extensively and proven useful in...
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Searching similarity between a pair of shapes or data is an important problem in data analysis and visualization. The problem of computing similarity measures using scalar topology has been studied extensively and proven useful in the shape and data matching. Even though multi-field or multivariate (consists of multiple scalar fields) topology reveals richer topological features, research on building tools for computing similarity measures using multi-field topology is still in its infancy. In the current article, we propose a novel similarity measure between two piecewise-linear multi-fields based on their multi-resolution Reeb spaces - a newly developed data-structure that captures the topology of a multi-field. Overall, our method consists of two steps: (i) building a multi-resolution Reeb space corresponding to each of the multi-fields and (ii) proposing a similarity measure between two multi-resolution Reeb spaces by computing a list of topologically consistent matching pairs (of nodes) and the similarity between them. We demonstrate the effectiveness of the proposed similarity measure in detecting topological features from real time-varying multi-field data in two application domains - one from computational physics and one from computational chemistry.
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A new strategy to reduce the far-field reconstruction error due to the finite size of the measurement region in planar and cylindrical near-field/far-field transformation techniques is presented. The approach exploits the (spatial...
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A new strategy to reduce the far-field reconstruction error due to the finite size of the measurement region in planar and cylindrical near-field/far-field transformation techniques is presented. The approach exploits the (spatial) band-limitation property of the radiated field in order to estimate the field outside the measurement area. A number of different estimation algorithms are investigated, with reference to both the accuracy of the estimation and the required computational effort. Numerical and experimental results show that with a small computational effort, it is possible to obtain a significant improvement of the reconstructed far-field pattern with respect to that obtained without near-field extrapolation.
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We report initial measurements of ionospheric propagation parameters, particularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an inclin...
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We report initial measurements of ionospheric propagation parameters, particularly the total electron content (TEC), using the recently launched FORTE satellite. FORTE, which orbits the Earth at an altitude of 800 km and an inclination of 70°, contains a set of wideband radio receivers whose output is digitally recorded. A specialized triggering circuit identifies transient, broadband radio events, which include radiation from lightning, transionospheric pulse pairs, and man-made sources. Event data are transmitted to the ground station for analysis. In this paper we examine signals transmitted from an electromagnetic pulse generator operated at Los Alamos. The transmitter produces nearly impulsive signals in the VHF range. The received signal is dispersed by the ionosphere, and the received signal can be analyzed to deduce the total electron content along the path. By comparing the slant TEC thus measured with results from a ray-tracing code, we can deduce the vertical TEC to 800 km. Data from eight passes are presented. These types of data (in larger quantities) are of interest to operators of radar altimeters, who need data to corroborate their corrections for the ionospheric TEC. The combination of FORTE TEC data to 800 km and TEC measurements to 20,000 km (the Global Positioning System orbital altitude) can provide useful information for assessing the validity of models of plasmaspheric electron density. Initial estimates of the plasmaspheric density, on two daytime passes, are about 6 TECU. The signal received by FORTE, which is linearly polarized at the transmitter, is split into two magnetoionic modes by the ionosphere. The receiving antenna is also linearly polarized and therefore receives both modes. By measuring the beat frequency between the two modes, we can deduce the product of the geomagnetic field and the cosine of the angle between the field and the propagation vector. The possibility of using the measured slant TEC and the beat frequency to geolocate impulsive signals is discussed.
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