5,669 research outputs found
Coherent integration
Coherent integration which is a digital filtering process and was applied to MST radar observations is discussed. It is simple to implement with either hardware or software and is appropriate for the very narrow band signals usually received by MST radars. By filtering the signal before performing spectral processing, the computations required for FFT or similar analysis are greatly reduced. Coherent integration does not increase the signal-to-noise ratio per unit bandwidth in the signal band. It filters out much of the wideband noise, which could also be done by full FFT processing of the raw signal
Pulse compression using binary phase codes
In most MST applications pulsed radars are peak power limited and have excess average power capacity. Short pulses are required for good range resolution, but the problem of range ambiguity (signals received simultaneously from more than one altitude) sets a minimum limit on the interpulse period (IPP). Pulse compression is a technique which allows more of the transmitter average power capacity to be used without sacrificing range resolution. As the name implies, a pulse of power P and duration T is in a certain sense converted into one of power nP and duration T/n. In the frequency domain, compression involves manipulating the phases of the different frequency components of the pulse. One way to compress a pulse is via phase coding, especially binary phase coding, a technique which is particularly amenable to digital processing techniques. This method, which is used extensively in radar probing of the atmosphere and ionosphere is discussed. Barker codes, complementary and quasi-complementary code sets, and cyclic codes are addressed
School of Electrical Engineering.
The possibility of making the antenna of an MST radar too large is pointed out. It is not that the signal ceases to become stronger beyond some critical antenna size; the received scattered signal actually becomes weaker as the antenna size is increased whenever the target is in the near field, Fresnel region of the antenna. The Arecibo antenna is a case in point. It is supposed that MST work would benefit to use a feed which illuminated only a portion of the dish
Radar interferometer measurements
A few attempts have been made to use MST radar interferometers in scatter probing of the ionosphere. A brief description of radar interferometer construction is given. These interferometer observations do not appear to have been successful
Overview of on-line data processing for MST radars (keynote paper), part 7
The most important aspects of the processing of MST radar data are discussed. The important points of on-line data processing for MST radar are reviewed. The goals of the on-line and now almost exclusively digital processing, procedures are to achieve good altitude resolution and coverage, good frequency (Doppler shift) resolution, and good time resolution, while avoiding, the problems of range and frequency ambiguity (aliasing), ground clutter, and interference. Achieving optimum results requires pulse compression and some coherent integration. The first allows full utilization of the average power capability of the transmitter and the second reduces the computing requirements
The van allen radiation belt
Van Allen radiation belt and energetic protons and electrons trapped in geomagnetic fiel
Capabilities and limitations of the Jicamarca radar as an MST radar
The Jicamarca radar (Long. 76.52W, Lat. 11.56S), located at 20 km from Lima at approximately 500 meters over sea level, is surrounded by mountains which provide a good shield from man-made interference. The radio horizon goes from a few hundred meters, across the dry valley where it is located, to 15 km, along the valley in the direction of the continental divide. This limits the clutter to 15 km, except for one high peak at 21 km. It is the most equatorial of all existing MST radars. Its proximity to the Andes, makes its location unique for the study of lee waves and orographic-induced turbulence. Vertical as well as horizontal projections of MST velocities are obtained by simultaneously pointing with different sections of the antenna into three or four different directions. The transmitters, receivers, and systems for data acquisition, processing, and control are included
Ionospheric E-region Irregularities Produced by Non-linear Coupling of Unstable Plasma Waves
Ionospheric E region irregularities produced by nonlinear coupling of unstable plasma wave
The Perception of Globally Coherent Motion
How do human observers perceive a coherent pattern of motion from a disparate set of local motion measures? Our research has examined how ambiguous motion signals along straight contours are spatially integrated to obtain a globally coherent perception of motion. Observers viewed displays containing a large number of apertures, with each aperture containing one or more contours whose orientations and velocities could be independently specified. The total pattern of the contour trajectories across the individual apertures was manipulated to produce globally coherent motions, such as rotations, expansions, or translations. For displays containing only straight contours extending to the circumferences of the apertures, observers' reports of global motion direction were biased whenever the sampling of contour orientations was asymmetric relative to the direction of motion. Performance was improved by the presence of identifiable features, such as line ends or crossings, whose trajectories could be tracked over time. The reports of our observers were consistent with a pooling process involving a vector average of measures of the component of velocity normal to contour orientation, rather than with the predictions of the intersection-of-constraints analysis in velocity space.Air Force Office of Scientific Research (90-0175, 89-0016); National Science Foundation, Office of Naval Research, Air Force Office of Scientific Research (BNS-8908426
The equatorial E-region and its plasma instabilities: a tutorial
In this short tutorial we first briefly review the basic physics of the E-region of the equatorial ionosphere, with emphasis on the strong electrojet current system that drives plasma instabilities and generates strong plasma waves that are easily detected by radars and rocket probes. We then discuss the instabilities themselves, both the theory and some examples of the observational data. These instabilities have now been studied for about half a century (!), beginning with the IGY, particularly at the Jicamarca Radio Observatory in Peru. The linear fluid theory of the important processes is now well understood, but there are still questions about some kinetic effects, not to mention the considerable amount of work to be done before we have a full quantitative understanding of the limiting nonlinear processes that determine the details of what we actually observe. As our observational techniques, especially the radar techniques, improve, we find some answers, but also more and more questions. One difficulty with studying natural phenomena, such as these instabilities, is that we cannot perform active cause-and-effect experiments; we are limited to the inputs and responses that nature provides. The one hope here is the steadily growing capability of numerical plasma simulations. If we can accurately simulate the relevant plasma physics, we can control the inputs and measure the responses in great detail. Unfortunately, the problem is inherently three-dimensional, and we still need somewhat more computer power than is currently available, although we have come a long way
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