Narrow-beam Antennas Research Articles

S-Band FMCW Radar Measurements of Ocean Surface Dynamics

Abstract A ground-based frequency-modulated continuous wave microwave radar has been developed and applied to the measurement of ocean surface velocities. Full Doppler spectra are available, and both range and lime variations of the surface drifts can be recorded with resolutions down to 2.5 m and 0.3 s, respectively, over range extents of up to 1000 m. The ability of the radar to provide information over a wide range of sea conditions is demonstrated. These range from the identification of 5-cm scattering waves and their damping by a surface slick, to the propagation characteristics of long (∼200 m) ocean waves in deep and shallow water. The longwave propagation is in accord with linear wave theory. The relationship of the backscattered power at low grazing angles to the wave features is also demonstrated. The consequences of measuring radial velocity components at low grazing angles with a narrow beam antenna are considered. In this situation, wave height spectra and the principal wave direction are wel...

Studies of the Substructure of Severe Convective Storms Using a Mobile 3-mm-Wavelength Doppler Radar

An experiment whose objective was to determine the wind and reflectivity substructure of severe convective storms is detailed. A 3-mm-wavelength (95 GHz) pulsed Doppler radar was installed in a van and operated in the Southern Plains of the United States during May and early June of 1993 and 1994. Using a narrow-beam antenna with computer-controlled scanning and positioning the van several kilometers from targets in severe thunderstorms, the authors were able to achieve 30-m spatial resolution and also obtain video documentation. A dual-polarization pulse-pair technique was used to realize a maximum unambiguous velocity of ±80 m s−1. Analyses of data collected in a mesocyclone near the intersection of two squall lines, in a low-precipitation storm, and in a hook echo in a supercell are discussed. A strategy to achieve 10-m spatial resolution and obtain analyses of the internal structure of tornadoes is proposed.

Modulation techniques for high-speed wireless indoor systems using narrowbeam antennas

We consider high-speed wireless indoor links in which narrowbeam antennas are used to limit multipath delay spread. For this kind of link, the performances of simple 2- and 4-level modulation schemes are analyzed. The amplitudes and phases of all multipath rays are assumed to be static (no time variation), as expected for fixed links in an indoor environment. We consider coherent PSK with data-directed and pilot tone carrier recovery, differential PSK, and coherent and noncoherent forms of 2-level ASK. Our objective is to find techniques for which factors other than multipath limit the attainable bitrate. Thus for each scheme, analysis is used to identify regions of the multipath parameter set over which data eye closure occurs. Results are given for the 2- and 3-ray multipath channels which can arise in an indoor environment with narrowbeam antennas. Specifically, we assume two parallel walls approximately perpendicular to the direct ray and a narrowbeam antenna at one or both ends of the link. The 2-level schemes are found to be quite robust with respect to 2-ray multipath, and can mostly tolerate a second ray amplitude equal to that of the direct ray. The 4-level schemes are sensitive to 2-ray multipath, showing eye closure at many phases with a second ray power about 7.5 dB below the first (direct) ray. A similar sensitivity is found for 2-level schemes with 3-ray multipath. Finally, 4-level schemes with 3-ray multipath are significantly more sensitive, showing eye closure with a second ray power about 11 db below the direct ray. Thus, considerations of simplicity and robustness favor 2-level PSK schemes (coherent with pilot tone, or differential) for narrowbeam high speed transmission indoors. >

Analytical results for capacity improvements in CDMA

Examines the performance enhancements that can be achieved by employing spatial filtering in code division multiple access (CDMA) cellular radio systems. The goal is to estimate what improvements are possible using narrow-beam adaptive antenna techniques, assuming that adaptive algorithms and the associated hardware to implement these systems can be realized. Simulations and analytical results are presented which demonstrate that steerable direction antennas at the base station can dramatically improve the reverse channel performance of multicell mobile radio systems, and new analytical techniques for characterizing mobile radio systems which employ frequency reuse are described using the wedge-cell geometry of Rappaport and Milstein (1992). The authors also discuss the effects of using directional antennas at the portable unit. Throughout the paper the authors use phased arrays and steerable, fixed pattern antennas to approximate the performance of adaptive antennas in multipath-free environments. >

Topographic mapping using a multibeam radar altimeter

An airborne radar altimeter operating at 36 GHz is uniquely capable of measuring the topography of water, land, and ice surfaces. The Multimode Airborne Radar Altimeter (MARA) was designed to combine a narrow transmitted pulsewidth, a high transmitted power level, and a narrow antenna beam to produce a high-precision ranging capability at the nadir of the aircraft platform and at four fixed off-nadir angles out to 12/spl deg/ in the multibeam mode described in this paper, or a single beam scanning between /spl plusmn/22/spl deg/ in the scanning radar altimeter mode. Data collected over water and land surfaces are presented to demonstrate the potential of MARA for topographic mapping with accuracies and precisions of value in dynamic oceanography, geodesy, geology, hydrology, biogeochemistry, biogeography, and glaciology. >

Measurement of ocean wave spectra using narrow-beam HE radar

A data interpretation algorithm is developed to extract ocean wave information from HF radar backscatter observed by a narrow-beam antenna system. The basis of this measurement is the inversion of the integral equation representing the second-order radar cross section of the ocean surface. This equation is numerically inverted by approximating it as a matrix equation and pseudoinverting the kernel matrix using a singular value decomposition. As a test of this algorithm, comparisons are made between wave spectrum estimates obtained from a WAVEC buoy and a pair of 25.4-MHz ground wave radars, using data collected during the 1986 Canadian Atlantic Storms Program (CASP). Overall, the results of this experiment have been positive and have demonstrated both the basic feasibility of the inversion algorithm and the wave sensing capability of HF radar. For example, significant wave height estimates deduced by two radars differed from the buoy, in an absolute value sense, by only 0.12 m on average. When using only one radar, the mean difference of this important parameter from the buoy was a reasonable 0.33 m. >

On the spectral analysis and interpretation of incoherent scatter plasma line echoes

This paper describes a model of the physical interactions involved in incoherent scatter that result in specific plasma line spectral shapes as a function of range and frequency. This model is quite successful in duplicating the naturally occurring plasma line spectra observed at high latitudes. Moreover, the inherent sensitivity of the data collection method suggests a much more general applicability. For example, we will demonstrate a sensitivity of the method to plasma density irregularities in the medium that rivals or exceeds that possible with radio wave scintillation techniques. Unlike height‐integrated scintillation measurements, however, this new method can range resolve the fluctuations in plasma density. We are able to determine the altitude of the F layer peak to an accuracy of ∼ ± 1 km even with a probing pulse of 72 km in length. Our density fluctuation measurements indicate a residual level of ∼ 1% fluctuations after a linear‐temporal trend is removed. This indicates spatial variations in plasma density of this order within our relatively narrow (0.5°) antenna beam. These spatial variations are enhanced during periods when the F layer peak cutoff frequency is decreasing. Such behavior is consistent with increased plasma structure on the trailing edge of F region plasma blobs. Our technique involves not only model fitting but also a data collection method that minimizes range smearing. This data collection mode has already been successfully implemented at the Sondrestrom, Greenland, incoherent scatter radar facility.

MLS: a total system approach

A typical approach and landing operation is described. The microwave landing system (MLS) is then examined, its design characteristics and how it works are shown, and how the MLS design fulfils the user's operational requirements by protecting the guidance signal from reflected signal interference is highlighted. MLS angle system accuracy is discussed in great detail, and its reliability, integrity, and coverage volume are briefly considered. MLS availability at any runway to all aircraft types and their landing scenarios, which is accomplished using narrow scanning beam antennas, is examined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Eastward propagation of PC4‐5 range CNA pulsations in the morning sector observed with scanning narrow beam riometer at L=6.1

A new type of cosmic noise absorption (CNA) pulsation is observed with a scanning beam riometer with a spatial resolution of 10 km, at Syowa station, Antarctica (L=6.1). This type of CNA pulsation has a short azimuthal wavelength (80–600 km), corresponding to a large azimuthal m‐number (20–180). Most CNA pulsations with a period of 1 to 10 min propagate eastward at a speed of 200 m/s to 3 km/s in the morning sector (5–10 MLT). The eastward propagation direction is opposite to that of CNA pulsations with a small m‐number as reported by Olson et al. (1980). The pulsation period tends to increase, while the propagation speed decreases with increasing local time. These results suggest that the source of this new type of CNA pulsation first appears in the nightside magnetosphere and moves toward noon. It is pointed out that the finding of this new CNA pulsation is due to the high spatial resolution (10 km) and narrow antenna beam (13°) of the scanning riometer system.

A simple prediction method for L-band multipath fading in rough sea conditions

Quantitative analysis of the coherent and incoherent power of sea-reflected waves at 1.5 GHz was performed using multipath-fading data obtained by field experiments. The experiments were carried out for shore-to-shore, satellite-to-shore, and satellite-to-ship paths, and antennas with gains from 13 dBi to 21 dBi were used for signal reception. Results indicate that in rough sea conditions where the incoherent component is dominant, the power of the incoherent component can reach the maximum power of the coherent component (generated in calm sea) in most cases, except for a few cases such as when measurements using narrow-beam antennas or elevation angles below 7 degrees are made. On the results obtained, a simple prediction method for multipath fading caused by sea reflection is presented. >

Radar altimetry response from rough surfaces

Radar altimetry from SKYLAB, GEOS-3 and SEASAT has clearly shown the capability of the technique for measurements of wave-height, tidal action and the deviation from the perfect geoid. In order to further evaluate the performance over ice and land areas, a simulation model was developed, which estimates the pulse response from an arbitrary ground surface. Simulation results for different surface conditions e.g. sea, sea-ice, and land areas (including forests and lakes) display both possibilities and main limitations of the technique. For a lot of land surface applications, the ground resolution of existing altimetry systems seems too poor. Possible improvements in future by using more narrow-beam and scanning antennas are analyzed.

Doppler radio observations of 3‐meter irregularities in the polar cap E region

The first VHF Doppler radar system to measure echoes from ionospheric plasma instabilities in the polar cap was set up in 1982 in the Northwest Territories. High‐resolution spectra were achieved through the use of CW transmissions. A scattering volume over Sachs Harbour (geomagnetic latitude ∼77°) was defined by the intersection of the narrow antenna beams from the two widely spaced transmitter arrays (at Inuvik and Cambridge Bay) and the common receiver array (at Fort Franklin). The two bistatic radio links so formed had bisectors which differed in azimuth by 60°, and which were at mean magnetic aspect angles of ∼9° and 13° from perpendicularity with the earth's field. In this report, Doppler spectra are presented for a moderately disturbed period on July 7, when the scattering region was in the polar cap. In spite of this scattering location, the Doppler spectra show all the usual spectral types (I, II, and III) typical of the auroral zone. Although type III (ion cyclotron) echoes might have been anticipated at these large aspect angles (the k∥/k⊥ ratio could be as large as 0.25) in association with strong polar cap field‐aligned currents, the presence of the type I and II echoes is not explicable on the basis of present two‐stream and gradient drift instability theories. Whereas the spectral types in the polar cap and auroral zone show substantial similarities, the temporal morphology of the polar cap echoes is quite different from that of auroral zone echoes.

Observations of magnetic aspect effects in auroral radar backscatter

A series of experiments have been performed to investigate VHF auroral backscatter. The equipment is basically a bistatic CW radar using narrow beam antennas. One objective has been to investigate the dependence of backscatter power and spectral characteristics on aspect angle: the angle between the line of sight and the local magnetic field in the scattering region. The results indicate that the effects of aspect angle are much more complex than earlier measurements have indicated.

Narrow multibeam satellite ground station antenna employing a linear array with a geosynchronous arc coverage of 60°, part II: Antenna design

A dually polarized narrow ( ) beam antenna which provides a geosynchronous arc coverage of 60\deg is proposed and analyzed. To track the geosynchronous arc accurately, the properly oriented antenna produces a conically scanned beam by means of a linear array of feed horns with bias cut apertures illuminating a pair of parabolic cylinder reflectors in an imaging arrangement. This design, with reduced size array and singly carved reflectors, is relatively simple to construct. Calculations for a 0.35\deg beamwidth Ku band earth station antenna show a 0.05\deg pointing accuracy with scan loss due to aberrations less than 1 dB and little pattern degradation throughout the scan region. For multiple beam capability, a Rotman lens is proposed and analyzed as a means of exciting the feed array. It allows communication with multiple satellites spaced as close as 1\deg .

Radio aurora magnetic and streaming aspect sensitivities on 6 simultaneous links at 50 MHz

During August 1981, a 50 MHz c.w. radar system was operated in central Canada to measure auroral scatter amplitudes and Doppler spectra from a scattering region centered near 66° magnetic invariant latitude (L ≈ 6.0). Narrow beams from 3 transmitters, differing in frequency by 1 KHz, were directed to cover a common volume of the ionosphere over a ground location at 56.3°N, 103.5°W. The scattered signals were received on narrow beam antennas at two receiving sites, and recorded in analog form on magnetic tape under the control of an AIM65 microcomputer. The analog tapes were digitized later and FFT-processed to obtain Doppler spectra and amplitudes. The 6 transmission paths were designed to provide several magnetic aspect angles varying by 1.5°-7° from perpendicularity with the earth's field B and two streaming aspect angles differing by ∼38°. The objective was to employ controlled geometric factors to study the functional dependency of signal amplitudes and Doppler shifts on magnetic and streaming aspect angles. Several hundred hours of excellent data were obtained in continuous operation during the month of August 1981. Preliminary results will be reported.

A Procedure for Estimating the Power Pattern of a Narrow-Beam Antenna

In EMC analysis it is often required to assess the mutual interference between stations without detailed knowledge of the antenna patterns involved. Typically, only the mainbeam gain is available from large-scale spectrum-management data systems. This paper formalizes a simple method for estimating the power pattern of narrow-beam antennas from this admittedly insufficient information. Estimated patterns are compared with measured antenna patterns.

Adaptive Main-Beam Nulling for Narrow-Beam Antenna Arrays

Narrow-beam, low-sidelobe antennas may be used to enhance communication in the presence of sidelobe interferers. Protection against main-beam interferers as well can be obtained through the use of an adaptive multibeam antenna. Such an antenna, suitable for time-multiplexed, multichannel signals is described here. The objective is to permit successful communication and signal direction-of-arrival tracking in the presence of a large number of sidelobe interferers and a small number of main-beam interferers.

Type 1 radar echoes from the equatorial electrojet with double‐peaked Doppler spectra

Normal type 1 radar echoes obtained from relatively large zenith angles have a power spectrum with a single narrow peak whose Doppler shift corresponds approximately to the acoustic velocity in the medium. On some occasions, however, this single maximum splits into two distinct peaks, separated in phase velocity on one occasion by 270 m/s. This bifurcation is most easily observed at large zenith angles during daytime when a narrow antenna beam is used. It has also been seen in a daytime experiment in which radars at Jicamarca and Huancayo simultaneously probed the same region from two different radar zenith angles. The bifurcation has been observed so far only to the west of Jicamarca, over the Pacific Ocean. This spectral splitting could be caused by vertical electron density gradients, such as those associated with ‘blanketing’ sporadic E layers. A sufficiently sharp (scale lengths of a few hundred meters or less) positive gradient on the underside of the layer and negative gradient on the topside would cause the type 1 velocity to be decreased and increased, respectively, during the day, by amounts as large as those observed.

A review of earth-based radar mapping of the moon

Lunar radar mappings carried out in the late 1960's and 1970's have provided several valuable insights into lunar surface processes. These radar mappings used the delay-Doppler technique developed by Gordon Pettengill and his colleagues. These radar mappings also needed the narrow antenna beams, now available with large radio telescopes such as those at the Arecibo and Haystack Observatories. Two-element radar interferometers have provided resolution of the delay-Doppler ambiguity at meter wavelengths and provided topographic information at centimeter wavelengths. These techniques have provided high-resolution lunar radar-maps at 3.8-cm, 70-cm, and 7.5-m wavelengths, a set of wavelengths which span the window available for Earth-based radar mapping of the Moon.

New formulas for tropospheric scatter path loss

The predicted performance of tropospheric scatter transmission is investigated in terms of spectrum slope, refractive index variance, and scale of turbulence. The common volume integral is evaluated to yield a useful expression for path loss in terms of these parameters. Formulas for the coupling loss in the asymptotic case of very wide and very narrow antenna beams are derived. Approximate formulas for intermediate size antenna beams are deduced and compared with more exact computer calculations. Our results suggest that previous path loss prediction methods do not place enough emphasis on turbulent scatter and that a model with spectrum slope 11/3 should be used. The resulting model provides an initial step toward the development of a simpler and more realistic alternative to the NBS model, and also refines a related approach by Gjessing.