A Quasi-Optic Imaging Resonator for Plasma Diagnostics

A new resonant quasi-optic interferometer has been developed which has a unique capability to deduce spatial information about perturbing plasma media. The fundamental property of the resonator is that it is multiply degenerate. Its geometry consists of four spherical mirrors arranged in a confocal ring configuration such that the transverse modes are simultaneously resonant. Energy is coupled into and out of the resonator by thin dielectric film beam splitters. When perturbed, it is found that the resonant frequency of the system depends on the point at which the field is measured and this dependence is directly related to the geometry of the perturbation. The results of a theoretical analysis of the perturbation of the resonator are presented and preliminary experimental measurements of a 4-mm system are discussed.     

Mismatched filtering of odd-periodic binary sequences

Binary sequences with perfect periodic autocorrelation functions, as required in communications, radar, and measuring, are not known for any lengths >4. As a possible remedy, mismatched filtering can be used to entirely suppress any sidelobes of the periodic autocorrelation function at the expense of a reduced signal-to-noise ratio (SNR). In this work, the mismatched filtering method is extended to the odd-periodic autocorrelation function whose technical implementation is no more complex than that of periodic sequences. A new class of odd-periodic binary sequences is constructed that exist for many more lengths and exhibit significantly lower mismatched filtering losses than any known periodic sequences     

On Sonar Signal Analysis

Systems which utilize acoustic energy to explore an undersea environment are called sonars. This introductory and tutorial paper presents a discussion of active sonar signal analysis concentrating upon sinusoidal, linear FM, and pseudorandom echolocation pulses. Many previously published results concerning sonar signals and their haracteristics are integrated, collected, and presented in a unified form such as to portray typical signal design considerations.     

Product Correlator Performance for Gaussian Random Scenes

Contained herin is a derivation of two figures of merit for evaluating the performance of a product correlator and an associated discriminator curve. The general formulas are valid for both area and one-dimensional scenes. An explicit evaluation of the formulas is presented when the correlation functions are also Gaussian. In this case, the accuracy and probability of false match are found to take very simple forms.     

E region electric fields at the dip equator and anomalous conductivity effects

Zonal and vertical electric fields were estimated at E region heights in the Brazilian sector. Zonal electric fields are obtained from the vertical electric fields based on their relation through the Hall-to-Pedersen ionospheric conductivities ratio. The technique for obtaining the vertical electric field is based on its proportionality to the Doppler velocities of type 2 irregularities as detected by coherent radars. The 50 MHz backscatter coherent (RESCO) radar was used to estimate the Doppler velocities of the type 2 irregularities embedded in the equatorial electrojet. A magnetic field-line integrated conductivity model was developed to provide the conductivities. It considers a multi-species ionosphere and a multi-species neutral atmosphere, and uses the IRI 2007, the MISIS 2000 and the IGRF 10 models as input parameters for ionosphere, neutral atmosphere and Earth’s magnetic field, respectively. The ion-neutral collision frequencies of all the species are combined through the momentum transfer collision frequency equation, and different percentages of electron-neutral collisions were artificially included for studying the implication of such increase in the zonal electric field, which resulted ranging from 0.13 to 0.49 mV/m between the 8 and 18 h (LT), under quiet magnetic conditions.     

Debris swarms seen by SMEI

The large 3° × 60° fields-of-view of the Solar Mass Ejection Imager (SMEI) instruments are oriented on the stabilized Coriolis satellite to image most of the sky each Sun-synchronous orbit. Besides observing coronal mass ejections, the SMEI mission objective, SMEI also has detected a plethora of Earth-orbiting satellites (resident space objects or RSOs) brighter than ∼8th magnitude at a rate of about 1 per minute. Occasionally, SMEI sees an RSO swarm: a sudden onset of a large number of RSOs, many more than the nominal rate, upto dozens detected in a 4-s frame. These swarms usually last for a few minutes. A sample of six such RSO ensembles is analyzed in this paper in which the distance and the direction of the velocity vector for individual objects are estimated. We present the observational evidence indicating that the swarms must be near-field objects traveling in orbits near that of Coriolis, and that the relatively speeds between the objects and Coriolis are low. Further, analyses indicate that the RSOs are quite close (<20 m) and are generally moving radially away from the satellite. The predicted encounter geometries for Coriolis passing through or near a small debris cloud is, generally, quite inconsistent with the observations. The most likely explanation consistent with the observations is that SMEI is seeing debris being ejected from the Coriolis spacecraft itself. An analysis of distance and brightness for a subset of the RSOs indicates that the median diameter of the debris particles is ∼80 μm.     

The Magnetospheric Multiscale Magnetometers

The success of the Magnetospheric Multiscale mission depends on the accurate measurement of the magnetic field on all four spacecraft. To ensure this success, two independently designed and built fluxgate magnetometers were developed, avoiding single-point failures. The magnetometers were dubbed the digital fluxgate (DFG), which uses an ASIC implementation and was supplied by the Space Research Institute of the Austrian Academy of Sciences and the analogue magnetometer (AFG) with a more traditional circuit board design supplied by the University of California, Los Angeles. A stringent magnetic cleanliness program was executed under the supervision of the Johns Hopkins University’s Applied Physics Laboratory. To achieve mission objectives, the calibration determined on the ground will be refined in space to ensure all eight magnetometers are precisely inter-calibrated. Near real-time data plays a key role in the transmission of high-resolution observations stored on board so rapid processing of the low-resolution data is required. This article describes these instruments, the magnetic cleanliness program, and the instrument pre-launch calibrations, the planned in-flight calibration program, and the information flow that provides the data on the rapid time scale needed for mission success.     

Output Signal-to-Noise Ratios for Amplitude-Modulated Noise-Like Lasers

A general expression of the output SNR of a photodetector is derived for a noise-like laser amplitude-modulated by a stationary Gaussian random modulating signal in the presence of a background light. The electric field Vx(t) of the noise-like laser is assumed to be a stationary narrowband Gaussian random process with zero mean. Two types of modulating signal are considered, the baseband and bandpass modulating signals. More specifically, the effects of the center frequency of the modulating signal, the modulating degree, the bandwidth ration of the noise-like laser to the modulating signal, the effective average quantum rate, and input CNR on output SNR are studied. The detection characteristics of the noise-like laser are also made clear by comparison with the case of a coherent laser.     

Magnetic field of the earth and planets

Knowledge of geomagnetism and its effects was recently much augmented during the IGY; when added to what is known of stellar fields, interplanetary fields, and meteoric magnetism, and combined with magnetic theory, this knowledge not only suggests that much may be learned of our neighbors in space by making an early determination of their magnetic fields, but also provides a foundation for many inferences and rough estimates on the magnetic fields of other solar-system bodies. Present estimates range from 660 cgs for Jupiter to 0.004 for the smaller satellites and the planetoids.     

Taxonomy of Human Wayfinding Tasks: A Knowledge-Based Approach

Abstract

Although the term “Wayfinding” has been defined by several authors, it subsumes a whole set of tasks that involve different cognitive processes, drawing on different cognitive components. Research on wayfinding has been conducted with different paradigms using a variety of wayfinding tasks. This makes it difficult to compare the results and implications of many studies. A systematic classification is needed in order to determine and investigate the cognitive processes and structural components of how humans solve wayfinding problems. Current classifications of wayfinding distinguish tasks on a rather coarse level or do not take the navigator's knowledge, a key factor in wayfinding, into account. We present an extended taxonomy of wayfinding that distinguishes tasks by external constraints as well as by the level of spatial knowledge that is available to the navigator. The taxonomy will help to decrease ambiguity of wayfinding tasks and it will facilitate understanding of the differentiated demands a navigator faces when solving wayfinding problems.