Antenna Sidelobe Research Articles

Error sources and feasibility for microwave remote sensing of ocean surface salinity

A set of geophysical error sources for the microwave remote sensing of ocean surface salinity have been examined. The error sources include the sea surface temperature, sea surface roughness, atmospheric gases, ionospheric Faraday rotation, and solar and Galactic emission sources. It is shown that the brightness temperature effects of a few kelvin can be expected for most of these error sources. The key correction requirements for accurate salinity measurements are the knowledge accuracy of 0.5/spl deg/C for the sea surface temperature (SST), 10 mbar for the surface air pressure, 2/spl deg/C for the surface air temperature, 0.20 accuracy for the Faraday rotation, and surface roughness equivalent to 0.3 m s/sup -1/ for the surface wind speed. We suggest the use of several data products for corrections, including the AMSR-type instruments for SST and liquid cloud water, the AMSU-type product for air temperature, the scatterometer products or numerical weather analysis for the air pressure, coincidental radar observations with 0.2 dB precision for surface roughness, and on-board polarimetric radiometer channel for Faraday rotation. The most significant sky radiation is from the Sun. A careful design of the antenna is necessary to minimize the leakage of solar radiation or reflection into the antenna sidelobes. The narrow-band radiation from Galactic hydrogen clouds with a bandwidth of less than 1 MHz is also significant, but can be corrected with a radio sky survey or minimized with a notched (band-rejection) filter centered at 1.421 GHz. The other planetary and Galactic radio sources can also be flagged with a small data loss. We have performed a sampling analysis for a polar-orbiting satellite with 900 km swath width to determine the number of satellite observations over a given surface grid cell during an extended period. Under the assumption that the observations from different satellite passes are independent, it is suggested that an accuracy of 0.1 psu (practical salinity unit) is achievable for global monthly 10 latitude by 10 longitude gridded products.

Surface clutter due to antenna sidelobes for spaceborne atmospheric radar

A spaceborne radar for atmospheric observation must be able to detect atmospheric backscatter in the presence of clutter from the surface, due to antenna sidelobes. Such clutter can come from the same pulse as that observing the atmosphere if the radar antenna is pointed off-nadir. However pulses both prior and subsequent to the pulse observing the atmosphere can also contribute to clutter, and surface clutter can be a problem even for nadir-looking radars. Here, the problem is analyzed by deriving a method for computing clutter which includes effects of all contributing transmit pulses, Doppler shifting, finite receiver bandwidth, and curved Earth's surface. The results are applied to analysis of existing radars and design of future radar systems.

Ground clutter cancellation in incoherent radars: solutions for EISCAT Svalbard radar

Abstract. Incoherent scatter radars measure ionosphere parameters using modified Thomson scatter from free electrons in the target (see e.g. Hagfors, 1997). The integrated cross section of the ionospheric scatterers is extremely small and the measurements can easily be disturbed by signals returned by unwanted targets. Ground clutter signals, entering via the antenna side lobes, can render measurements at the nearest target ranges totally impossible. The EISCAT Svalbard Radar (ESR), which started measurements in 1996, suffers from severe ground clutter and the ionosphere cannot be measured in any simple manner at ranges less than about 120–150 km, depending on the modulation employed. If the target and clutter signals have different, and clearly identifiable, properties then, in principle, there are always ways to eliminate the clutter. In incoherent scatter measurements, differences in the coherence times of the wanted and unwanted signals can be used for clutter cancellation. The clutter cancellation must be applied to all modulations, usually alternating codes in modern experiments, used for shorter ranges. Excellent results have been obtained at the ESR using a simple pulse-to-pulse clutter subtraction method, but there are also other possibilities.Key words: Radio science (ionospheric physics; signal processing; instruments and techniques)

Ground clutter cancellation in incoherent radars: solutions for EISCAT Svalbard radar

<strong class="journal-contentHeaderColor">Abstract.</strong> Incoherent scatter radars measure ionosphere parameters using modified Thomson scatter from free electrons in the target (see e.g. Hagfors, 1997). The integrated cross section of the ionospheric scatterers is extremely small and the measurements can easily be disturbed by signals returned by unwanted targets. Ground clutter signals, entering via the antenna side lobes, can render measurements at the nearest target ranges totally impossible. The EISCAT Svalbard Radar (ESR), which started measurements in 1996, suffers from severe ground clutter and the ionosphere cannot be measured in any simple manner at ranges less than about 120–150 km, depending on the modulation employed. If the target and clutter signals have different, and clearly identifiable, properties then, in principle, there are always ways to eliminate the clutter. In incoherent scatter measurements, differences in the coherence times of the wanted and unwanted signals can be used for clutter cancellation. The clutter cancellation must be applied to all modulations, usually alternating codes in modern experiments, used for shorter ranges. Excellent results have been obtained at the ESR using a simple pulse-to-pulse clutter subtraction method, but there are also other possibilities.<br><br><b>Key words:</b> Radio science (ionospheric physics; signal processing; instruments and techniques)

Design of SLB systems in the presence of correlated ground clutter

Impulsive interference might increase the number of false alarms in a surveillance radar. The sidelobe-blanking (SLB) device, used in connection with the radar system, reduces the number of false alarms when the interference impinges on the antenna sidelobes. The paper describes how to design an SLB system operating in correlated Gaussian clutter with known Doppler spectrum plus white Gaussian thermal noise and impulsive interference. Closed-form expressions of the probability of false alarm, the probability of blanking a target received in the main lobe, and the probability of blanking a coherent repeater interference (CRI) are also derived for two statistical models of target/CRI, namely: Swerling-0 (known amplitude and random phase), and Swerling-I (random amplitude and random phase).

Evaluation of high frequency radar wave measurement

The spatial coverage, temporal availability and spectral and parameter accuracy of wave measurements using radars operating at the upper end of the high frequency (HF) radio band are discussed. The two radars used are the Ocean Surface Current Radar (OSCR) developed in the UK and the Wellen Radar (WERA) developed in Germany. The measurements show that useful accuracy is obtainable with very good potential for operational coastal monitoring. Direction biases over most of the frequency range are less than 15°. The relative bias in significant waveheight estimation in high sea-states is within ±5%. Although there is some deterioration in spectral performance in high sea-states, this is primarily confined to amplitudes at the higher ocean wave frequencies where a modelling approach could be adopted. Gaps in spatial and temporal coverage and some reduction in accuracy in mean direction estimation are due to surface current variability or antenna sidelobes.

Superconducting chip receivers for imaging application

Experimental details of a unique superconducting imaging array receiver are discussed. Each pixel contains an internally pumped receiver chip mounted on the back of the elliptical microwave lens. Each chip comprises a quasi-optical SIS mixer integrated with a superconducting flux-flow oscillator (FFO) both fabricated from the same Nb/AlO/sub x//Nb trilayer on a silicon substrate. Properties of the integrated lens antenna were studied using an externally pumped reference SIS mixer which showed antenna sidelobes below /spl sim/17 dB and a receiver double side band noise temperature, T/sub RX/(DSB), below 100 K within the frequency range 460-500 GHz that is close to the quantum noise. For the imaging array T/sub RX/(DSB)=150 K has been measured at 500 GHz using the internal flux-flow oscillator as a local oscillator (LO). A balanced SIS mixer was tested showing T/sub RX/(DSB)<100 K within the range of 480-510 GHz using the internal LO. A computer system was developed to control simultaneously the dc bias of the SIS mixer and the frequency and power provided by FFO. The system also performs automatic optimization of the receiver noise temperature.

Measuring ionospheric densities, temperatures, and drift velocities simultaneously at Jicamarca

Incoherent scatter autocorrelation function measurements are difficult to make in the F region at Jicamarca because of very strong clutter contamination by coherent echoes from unstable plasma waves in the E‐region electrojet that are aligned with the magnetic field. We have developed a more effective way to deal with this clutter that improves the quality of the temperature (and composition when light ions are present) data. Other coherent echoes (much weaker than electrojet echoes but stronger than incoherent scatter) are also received through the antenna sidelobes from field‐aligned irregularities in the 140–170 km altitude range during daytime. These latter echoes have a very narrow bandwidth, and so it is easy to measure their Doppler shift and obtain the vertical plasma drift velocity, which is proportional to the zonal electric field.

Impact of optical baffle on antenna pattern

One of the major concerns of antenna design for spacecraft applications is the effect of surrounding structures which can reflect and diffract the antenna's radiated energy and cause degradation in the antenna directivity, beam shape, and sidelobe levels. A case in point is the NASA Earth Observing System (EOS) spacecraft which has a large number of instruments on board. Among these is the Multiple Imaging Spectral Radiometer (MISR). The downlink antenna system (DAS) for the spacecraft is a very broad beam antenna and as such irradiates much of the spacecraft leading to the possibility of distortion of its pattern. The MISR, on the other hand, must be protected from optical glint resulting from the structures onboard the spacecraft and to this end the instrument has been fitted with a baffle. The optimal position for this baffle (from the point of view of the MISR instrument) places it in the near-field of the downlink antenna. The baffle material must be chosen so as to minimize the effect on the antenna pattern while adequately protecting the instrument. In an effort to select an optimum material, several optically black materials were considered, all of which were RF transmissive in varying degrees. The RF reflection coefficient of samples of these materials was found to be less than 0.03 as measured in a waveguide. The results were then used in an analytical assessment of the antenna pattern perturbation. Details of the modeling are described.

Development of a Low RCS Reflector Antenna

ABSTRACT RCS (Radar Cross Section) reduction by proper shaping is applied to the problem of reducing the RCS levels of a center fed parabolic reflector antenna system such that antenna gain, sidelobe level, and physical size of the reflector are preserved. A detailed numerical - physical investigation of the electromagnetic scattering phenomena associated with parabolic reflector antennas is presented. The electromagnetic scattering phenomenon are identified and categorized according to their contributions to the overall RCS level of the antenna system. The dominant scattering phenomena is used to formulate a low RCS design requirement for the design of low RCS reflector antennas. These antennas have radiation characteristics similar to center fed parabolic antennas, but have greatly reduced RCS levels. A new low RCS reflector antenna system is designed to replace an existing center fed parabolic reflector antenna system. The new antenna is designed by combining a high frequency, ray based, reflector design technique with the low RCS design requirements. The newly designed low RCS reflector antenna system is found to have transmitting characteristics similar to that of the original center fed reflector parabolic antenna but with RCS levels reduced by up to 25 dB.

Ground clutter model for airborne MPRF radars in look-down search mode

The performances of a medium pulse repetition frequency (MPRF) radar operating in air-to-air look-down mode are strongly influenced by the presence of ground clutter. The clutter echo received both on the antenna main beam and side lobes can increase the number of false alarms. To correctly account for the effects of ground clutter, one has to develop a model capable of reproducing clutter characteristics as a function of range and frequency. The authors propose a model to estimate range/Doppler distribution of ground clutter mean power. The proposed discrete model permits an easy development of simulation packages which quantify the effects of ground clutter in MPRF systems and which allow the evaluation of MPRF radars performances in most typical operating conditions. Three-dimensional plots of the range/Doppler distribution of mean ground clutter power, obtained by a FORTRAN code that implements the model, are presented. Moreover, the dependence of ground clutter power on important mission and radar parameters is discussed.

Localized and strongly unstable plasma regions in the auroral E region ionosphere and implications for radar experiments

The results in this paper were obtained with SAPPHIRE, a new auroral Doppler radar experiment designed to study meter‐scale E region irregularities. SAPPHIRE is a dual 50‐MHz continuous wave, phased array, multibeam, bistatic system which is capable of performing cross‐beam measurements from two widely different directions. There are two transmitters, each of which probes the auroral electrojet plasma over a large spatial target grid area of multiple intersections that determine 16 scattering regions or cells. Initial observations using untapered antenna arrays showed a class of scatter characterized by a narrow power spectrum peaking at the same Doppler shift in all, or several, observing cells simultaneously. These are strong echoes ranging in lifetime from a few tens of seconds to a few minutes and occurring preferentially in the midnight and morning magnetic time sectors. The analysis showed that this scatter is strongly anisotropic in azimuth and comes from localized regions of spatially coherent large‐amplitude plasma waves that produce mostly type III, but also type I and the rare type IV, radar auroras. By using many events and analyzing a large number of Doppler spectra, we found that type III echoes are the strongest observed, having on the average relative intensities at least 15 dB higher than the type I echoes. The observations relate to the “short discrete radar auroras” which are known to originate in spatially confined, dynamic plasma regions. The possibility exists that the large free energy for instability in these active regions is provided from intense electric fields and/or very sharp electron density gradients, both expected to occur at times near the edges of discrete auroral arcs. Finally, the present results confirm that, because of the large dynamic range of radio auroral echoes, strong scattering regions lead to the complete domination, at times, by backscatter through antenna sidelobes. For the localized regions of strong type III and type I echoes, this means that the conventional 3‐dB antenna beam width scale size of the scattering region is unrealistic. Obviously, this has important implications for the radar auroral experiments and the interpretation of observations.

Rainfall Estimation from ESMR-5 Measurements and Application to El Niño

An algorithm to estimate monthly 5° × 5° area-averaged rain rate over the oceans from January 1973 to December 1976 using single-channel microwave data from the Nimbus-5 satellite has been developed. This study extends the work of Shin et al. by including the full width of scan angles (from −50° to 50°) in order to reduce sampling error. The scan-angle dependence of the estimated rain rate due to variable antenna sidelobe effects, surface emissivity, and propagation pathlength is eliminated using a statistical method. A globally uniform beam-filling correction factor of 2.2 is applied in this study. Comparison with island station rainfall measurements over the Pacific shows a remarkably high correlation between two data in the equatorial dry zone and South Pacific convergence zone (SPCZ) but a low correlation in the extratropics and equatorial western Pacific. It is also proved that the retrieved rain rates are statistically significant. The rainfall deviations from non-El Niño years April 1973 to December 1975 reveal the temporal and spatial variations produced by the 1972–73 and 1976–77 El Niño episodes. We observe an increase of rainfall over the eastern and central equatorial Pacific Ocean and a decrease over the equatorial western Pacific Ocean and eastern Australia during these events. Consistent with previous work, the rainfall anomaly of the 1972–73 El Niño was much stronger than that of the 1976–77 El Niño.

Experience from one year of operating a boundary-layer profiler in the center of a large city

Abstract. Since May 1992 a small, 915-MHz profiler has been operated continuously in downtown Montreal. It is a five-beam system employing a microstrip array antenna, located atop a 14-story office building that houses several academic departments of McGill University. The data are used for research on precipitation physics and the clear-air reflectivity in addition to wind profiling. We are especially interested in situations in which the reflectivities of the clear air and the precipitation are comparable. This permits the study of interactions between the precipitation and the clear air, a new area of research made possible by wind profilers. On clear days in the summer, 30-min consensus winds can often be measured to an altitude of 3 km, but ground clutter in the antenna sidelobes interferes with measurements below 600 m. Rain when present often permits wind profiling down to 100 m and up to 6 km or higher. On cold winter days there are some periods when the reflectivity is too weak at all levels to permit wind estimation. Falling snow, however, provides readily detectable echoes and serves as a good tracer of the wind and so allows profiling over its full altitude extent. The best conditions for observing interactions between precipitation and the clear air are when light rain falls through a reflective layer associated with a frontal surface or inversion. Unexpectedly, flocks of migrating birds sometimes completely dominate the signal at night in the spring and fall seasons.

Antenna sidelobes in the presence of flat reflectors

Reflecting surfaces near the antenna may produce surprisingly large sidelobes even if the designed free-space sidelobes in the directions of these surfaces are very small. Calculations of such sidelobes are presented for an example of a linear aperture with a parabolic weighting function for the received signal amplitude.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Pattern synthesis of the offset reflector antenna system with less complicated phased array feed

The performance degradation of an offset reflector antenna with off-axis scanning feed and distorted reflector surface can be improved by using a phased array feed. Generally, both analog attenuators and phase shifters are used in phased array feed. Yet, it seems to make the feed system very complicated. In this paper, a phase-only gradient search (POGS) algorithm is developed to optimize the performance of offset reflector antenna systems using phased array feed that is equipped with phase shifters only. This technique not only can greatly simplify the complexity of phased array feed, but also can provide the reflector antenna with better antenna gain and sidelobe level control capability. Simulation and experimental results are presented to illustrate the excellent performance of offset reflector, with limited beam scanning capability, can be obtained by using this less complicated phased array feed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

First observation of the upper stratospheric vertical wind velocities using the Jicamarca VHF radar

The Jicamarca VHF radar (50 MHz) has detected atmospheric echoes in the so‐called ”gap region” from 30 to 60 km heights. These echoes are, for the first time, discriminated from clutter echoes, using both co‐polarized (co‐pol) and cross‐polarized (x‐pol) arrays to monitor the clutter component which may enter the antenna sidelobes. The atmospheric scatterings in this region are shown to be composed of refractivity layers with thickness as of 1–2 km as in other middle atmospheric height ranges, suggesting that these scatterings are caused by thin turbulent layers. The height profile of inferred vertical wind velocities indicates a wavy structure. The wave amplitude, as a whole, increases with height, suggesting the activity of upward‐propagating gravity waves.

The electrical characteristics of a conical horn-reflector antenna employing a corrugated horn

The electrical properties of a conical horn-reflector antenna of 2-3 degrees beamwidth have been investigated over the frequency range of 12-18 GHz. It is shown that the employment of a corrugated horn reduces the characteristic spillover side lobe of the conical horn-reflector antenna by 15 dB and yields excellent main beam circularity over the full Ku band. This results from the taper of the field amplitude toward the edges of the horn aperture in both the E-plane and the H-plane illumination. The performance of the antenna makes it suitable for use in closely packed interferometers for radio astronomy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Corrections for the effects of a radome on antenna surface measurements made by microwave holography

Measurements of the surface deviations of a parabolic antenna by microwave holography have been corrected for the effects of an enclosing radome. The largest correction, which is due to diffraction from the metal space frame, is made by using a model computed from the radome structure. This model accounts for the changing diffraction as the antenna is scanned during the interferometric mapping of the complex beam pattern. Correction for reflections from the radome panels is made by simultaneously measuring the beam pattern at multiple frequencies to provide delay discrimination to reject antenna sidelobes generated by multiple reflections which arrive with delays different from that of the main beam.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Evaluation of surface clutter for the design of the TRMM spaceborne radar

Surface clutter interference through antenna sidelobes on rainfall measurements by spaceborne rain radar is quantitatively examined in order to clarify design criteria for the antenna. The received intensities of both rain echo and sea clutter are evaluated numerically, assuming appropriate precipitation and sea surface scattering models and a realistic antenna pattern for a phased array antenna, which is fed with the Taylor distribution. The results indicate that a region exists just above the sea surface where the sea surface clutter masks the backscattered power from the rain. The results also demonstrate the feasibility of quantitative measurements of rainfall in regions above this interference zone if a planar array antenna fed with the Taylor distribution of peak sidelobe level -30 dB is used.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>