Radar Beamwidth Research Articles

The Doppler Characteristics of Sea Echoes Acquired by Motion Radar

The Doppler characteristics of sea surface echoes reflect the time-varying characteristics of the sea surface and can be used to retrieve ocean dynamic parameters and detect targets. On airborne, spaceborne and shipborne radar platforms, radar moves along with the platforms while illuminating the sea surface. In this case, the area of the sea surface illuminated by radar beam changes rapidly with the motion, and the coherence of the backscattered echoes at different times decreases significantly. Therefore, the Doppler characteristics of the echoes would also be affected by the radar motion. At present, the computational requirements needed to simulate the Doppler spectrum of the microwave scattering field from the sea surface based on numerical methods are huge. To overcome this problem, a new method based on the sub-scattering surface elements has been proposed to simulate the Doppler spectrum of sea echoes acquired by a moving microwave radar. A comparison with the results evaluated by the SSA demonstrate the availability and superiority of the new method proposed by us. The influences induced by radar motion, radar beamwidth, incident angle, and thermal noise on the Doppler characteristics are all considered in this new method. The simulated results demonstrate that the spectrum bandwidth of sea surface echoes acquired by radar on the dive staring motion platform becomes somewhat narrower.

Impacts of Vertical Nonuniform Beam Filling on the Observability of Secondary Ice Production due to Sublimation

Abstract Quasi-vertical profiles (QVPs) of polarimetric radar data have emerged as a powerful tool for studying precipitation microphysics. Various studies have found enhancements in specific differential phase Kdp in regions of suspected secondary ice production (SIP) due to rime splintering. Similar Kdp enhancements have also been found in regions of sublimating snow, another proposed SIP process. This work explores these Kdp signatures for two cases of sublimating snow using nearly collocated S- and Ka-band radars. The presence of the signature was inconsistent between the radars, prompting exploration of alternative causes. Idealized simulations are performed using a radar beam-broadening model to explore the impact of nonuniform beam filling (NBF) on the observed reflectivity Z and Kdp within the sublimation layer. Rather than an intrinsic increase in ice concentration, the observed Kdp enhancements can instead be explained by NBF in the presence of sharp vertical gradients of Z and Kdp within the sublimation zone, which results in a Kdp bias dipole. The severity of the bias is sensitive to the Z gradient and radar beamwidth and elevation angle, which explains its appearance at only one radar. In addition, differences in scanning strategies and range thresholds during QVP processing can constructively enhance these positive Kdp biases by excluding the negative portion of the dipole. These results highlight the need to consider NBF effects in regions not traditionally considered (e.g., in pure snow) due to the increased Kdp fidelity afforded by QVPs and the subsequent ramifications this has on the observability of sublimational SIP. Significance Statement Many different processes can cause snowflakes to break apart into numerous tiny pieces, including when they evaporate into dry air. Purported evidence of this phenomenon has been seen in data from some weather radars, but we noticed it was not seen in data from others. In this work we use case studies and models to show that this signature may actually be an artifact from the radar beam becoming too big and there being too much variability of the precipitation within it. While this breakup process may actually be occurring in reality, these results suggest we may have trouble observing it with typical weather radars.

Retrieval of Vertical Structure of Raindrop Size Distribution from Equatorial Atmosphere Radar and Boundary Layer Radar

This work develops an algorithm to retrieve the vertical structure of the raindrop size distribution (DSD) of rain from simultaneous observations of 47 MHz Equatorial Atmosphere Radar (EAR) and 1.3 GHz Boundary Layer Radar (BLR) at Koto Tabang, West Sumatra, Indonesia (0.20°S, 100.32°E, 865 m above sea level). EAR is sensitive to the detection of turbulence, and BLR is susceptible to identifying precipitation echo. The EAR Doppler spectrum broadening effects due to turbulence and finite radar beam width were reduced using the convolution process. The Gaussian function was used to model the turbulence Doppler spectrum. A non-linear least-squares fitting method was applied to retrieve DSD parameters. Subsequently, the equations to estimate DSD using this dual-frequency algorithm assume the gamma DSD model to retrieve the distribution from the Doppler spectrum of precipitation echo. The precipitation events on April 23, 2004 on the Coupling Processes in the Equatorial Atmosphere (CPEA-I) project have been analyzed. Results show that the precipitation spectrum obtained using the dual-frequency method is higher, more precise, and well-fitted than the single-frequency method, meaning the dual-frequency method has great potential to be used in observing the microphysical process and remote sensing application analysis of DSD in Indonesia, particularly at Koto Tabang. The analyses show various microphysical processes that occur in the rain, such as coalescence, evaporation, break-up, and condensation. Furthermore, for the purpose of easier remote sensing application analysis of profile DSD characteristics, we use a DSD ΔΖMP parameter. ΔΖMP is a rain rate insensitive DSD parameter representing mean drop size. The trend of ΔZMP is not totally uniform with regards to rain rate and reflectivity factors, with ΔZMP higher in the first half of the event and becoming lower toward the end. This suggests that we have to use different Z-R relations within the event. Doi: 10.28991/ESJ-2022-06-03-02 Full Text: PDF

NEOExchange - an online portal for NEO and Solar System science

Las Cumbres Observatory (LCO) has deployed a homogeneous telescope network of ten 1-m telescopes to four locations in the northern and southern hemispheres, with a planned network size of twelve 1-m telescopes at 6 locations. This network is very versatile and is designed to respond rapidly to target of opportunity events and also to perform long term monitoring of slowly changing astronomical phenomena. The global coverage, available telescope apertures, and flexibility of the LCO network make it ideal for discovery, follow-up, and characterization of Solar System objects such as asteroids, Kuiper Belt Objects, comets, and especially Near-Earth Objects (NEOs).We describe the development of the “LCO NEO Follow-up Network” which makes use of the LCO network of robotic telescopes and an online, cloud-based web portal, NEOexchange, to perform photometric characterization and spectroscopic classification of NEOs and follow-up astrometry for both confirmed NEOs and unconfirmed NEO candidates.The follow-up astrometric, photometric, and spectroscopic characterization efforts are focused on those NEO targets that are due to be observed by the planetary radar facilities and those on the Near-Earth Object Human Space Flight Accessible Targets Study (NHATS) lists. Our astrometric observations allow us to improve target orbits, making radar observations possible for objects with a short arc or large orbital uncertainty, which could be greater than the radar beam width. Astrometric measurements also allow for the detection and measurement of the Yarkovsky effect on NEOs. The photometric and spectroscopic data allows us to determine the light curve shape and amplitude, measure rotation periods, determine the taxonomic classification, and improve the overall characterization of these targets. We are also using a small amount of the LCO NEO Follow-up Network time to confirm newly detected NEO candidates produced by the major sky surveys such as ATLAS, Catalina Sky Survey (CSS) and PanSTARRS (PS1). We will describe the construction of the NEOexchange NEO follow-up portal and the development and deployment methodology adopted which allows the software to be packaged and deployed anywhere, including in off-site cloud services. This allows professionals, amateurs, and citizen scientists to plan, schedule and analyze NEO imaging and spectroscopy data using the LCO network and acts as a coordination hub for the NEO follow-up efforts. We illustrate the capabilities of NEOexchange and the LCO NEO Follow-up Network with examples of first period determinations for radar-targeted NEOs and its use to plan and execute multi-site photometric and spectroscopic observations of (66391) 1999 KW4, the subject of the most recent planetary defense exercise campaign.

The Determination of Effective Beamwidth of Ku Band Profiling Radar Based on Waveform Matching Method in the Boreal Forest of Finland

Radar scientists typically define the radar beamwidth as a half-power beamwidth (HPBW) in the main lobe of the antenna pattern. However, the microwave radiations outside radar HPBW might also backscatter into the radar receiver and change the distribution of the received signal. To determine an actual and effective beamwidth illuminated on the measured targets, we first generate the simulated-waveforms derived from coincident lidar points and radar equation and then develop a waveform matching method to seek out an optimal beamwidth based on the 95% threshold of correlation coefficients between radar waveforms and the simulated-waveforms. The 8565 measurements of a Ku-band profiling radar named Tomoradar and coincident lidar data in a widespread heterogeneous forest area of southern Finland are employed for resolving the effective beamwidth. The results reveal that about 97% of the effective beamwidth are larger than Tomoradar HPBW, but the effective beamwidth could be changeable for each measurement due to variations in the scattering properties of vegetation. Thus, a fixed average effective beamwidth (AEBW) with 0.1-degree resolution is introduced to determine Tomoradar cone according to the effective beamwidth and corresponding proportions. We discover that Tomoradar AEBW is approximately approaching to 8°, which is larger than Tomoradar HPBW of 6°. If we regard AEBW as the actual Tomoradar beamwidth rather than HPBW, the simulated-waveforms have substantially stronger correlation strength with Tomoradar waveforms, and canopy tops derived from lidar data within Tomoradar AEBW are much closer to those extracted from Tomoradar waveforms. The results demonstrate that radar AEBW is a more appropriate reference for designing radar antenna and selecting the region size of validation data such as lidar points or the ground truth. However, considering that radar AEBW is variable for different radar antenna pattern, we suggest that actual radar beamwidth should be defined with a fraction of total radiation energy within radar AEBW, just like the definition of laser divergence of lidar based on the percentage of transmitted laser energy. In this paper, for a forest inventory research case, the fraction of total radiation energy within the AEBW for radar system is supposed to be 91%.

Statistical Analysis of Microphysical and Dynamical Parameters for Clouds and Precipitation over Nauq Tibetan Plateau in Summertime Using Ka-band Cloud Radar

A Chinese Ka-band solid-state transmitter cloud radar (CR) was employed to investigate clouds and precipitation over the Tibetan Plateau (TP) in summer 2014. After performing quality control and merging the output from the three work modes of CR, the Doppler spectral density data were analyzed to estimate the vertical air motion (Vair) and particle fall speed (Vfall) in clouds. The reproduced radar reflectivity was employed to retrieve the ice water content (IWC) and ice effective radius (Re). The cloud type classification algorithm was developed to classify cloud conditions into eight different categories. The vertical and daily variations of reflectivity, Vair, Vfall, IWC, and Re were then analyzed, and as a result, four conclusions were formulated. First, the clouds with reflectivity less than −10 dBZ were mainly located above 7 km, and the precipitable particles were formed below this layer (rain echo top) due to the abundant supercooled water therein. Second, the weak updraft in the range of 2–4 km caused the high occurrence of weak clouds during 04:00–12:00 Beijing local time (BT), and the rain echo top did not exceed 7 km due to the existing downdraft in 4–6 km. After 14:00 BT, convective clouds quickly developed, and the averaged updraft, reflectivity, and the echo top notably increased. Third, particular cloud types commonly exhibited weak reflectivity, low IWC, and less vertical variations, while the others more obvious vertical variations with larger IWC and Re. Last, compared with the radar sensitivity and range from radar, Vair biases that were introduced by the Doppler spectra broadening due to turbulence, wind shear, and radar beam width, could be neglected.

Concurrent Respiration Monitoring of Multiple Subjects by Phase-Comparison Monopulse Radar Using Independent Component Analysis (ICA) With JADE Algorithm and Direction of Arrival (DOA)

While non-contact monitoring of human respiration has been demonstrated using Doppler radar, the concurrent monitoring of multiple equidistant subjects remains a significant technological challenge. Reported research has so far been limited to maintaining 1-m subject separation, based on the radar antenna beam-width. Proposed here is a hybrid method consisting of an SNR-based intelligent decision algorithm which integrates two different approaches to isolate respiratory signatures of two subjects within the radar beam-width separated by less than 1 meter. Using Independent Component Analysis with the JADE algorithm (ICA-JADE) and Direction of Arrival (DOA), this SNR-based decision algorithm works with an accuracy above 93%. In addition, angular location of each subject is estimated by phase-comparison monopulse and an integrated beam switching capability is demonstrated to optimally extract respiratory information. The proposed method coherently combines two separation methods to overcome multiple-subject monitoring limits which can lead to practical adoption for many respiration monitoring applications.

25 A ZEBRAFISH MODEL OF MYELODYSPLASTIC SYNDROME PRODUCED THROUGH TET2 GENOMIC EDITING

Misovortices frequently occur near the coastline of the Japan Sea during wintertime cold air outbreaks, generally developing over the sea and moving inland. To clarify the behavior of misovortices during landfall, temporal changes in the intensity and tilt of 12 misovortices over the coastal region of the Japan Sea were investigated during the winters of 2010/11 and 2011/12 using an X-band Doppler radar. For 11 vortices whose diameters were more than twice the effective radar beamwidth, the temporal change in the peak tangential velocity at lower levels (averaged below 400 m AGL) was analyzed. It was found that 8 out of the 11 vortices decreased after progressing between 0 and 6 km inland. For the remaining three vortices, the patterns of Doppler velocity couplet became unclear between 0 and 5 km inland, suggesting that these vortices also decayed soon after landfall. For four of the vortices, for which the analysis of the temporal evolution of tilt with height was made possible by several successive volume scans, the forward tilt with height increased after landfall. This study showed that modification to both the intensity and tilt with height of misovortices occurred after landfall.

Modification of misovortices during landfall in the Japan Sea coastal region

Misovortices frequently occur near the coastline of the Japan Sea during wintertime cold air outbreaks, generally developing over the sea and moving inland. To clarify the behavior of misovortices during landfall, temporal changes in the intensity and tilt of 12 misovortices over the coastal region of the Japan Sea were investigated during the winters of 2010/11 and 2011/12 using an X-band Doppler radar. For 11 vortices whose diameters were more than twice the effective radar beamwidth, the temporal change in the peak tangential velocity at lower levels (averaged below 400m AGL) was analyzed. It was found that 8 out of the 11 vortices decreased after progressing between 0 and 6km inland. For the remaining three vortices, the patterns of Doppler velocity couplet became unclear between 0 and 5km inland, suggesting that these vortices also decayed soon after landfall. For four of the vortices, for which the analysis of the temporal evolution of tilt with height was made possible by several successive volume scans, the forward tilt with height increased after landfall. This study showed that modification to both the intensity and tilt with height of misovortices occurred after landfall.

간섭계 레이더 고도계용 시뮬레이터 구현

We present an implementation result of a computer GUI-based simulator using MATLAB to verify the performance of interferometric radar altimeter(IRA) which is possible to measure the slant range altitude and the cross-track angle to the nearest point for terrain aided navigation(TAN). After a brief description of the principle of TAN and IRA, we present that the grids are divided for the modeling of the reflected signal in digital elevation map(DEM) and so the radar cross section(RCS) of each grid is calculated and the signal-noise ratio(SNR) of the reflected signal in the radar beam width. And the signal processing procedures of the IRA and the structure of the IRA simulator are shown.

Improved performance over time of integration in momentum flux estimation using Postset Beam Steering technique

Using time series of vertical data collected from middle and upper atmospheric (MU) radar, the adaptive Capon beamforming technique is used to synthesize beams in the desired pointing directions within the radar beamwidth. Beam synthesis has been performed at the tilt angle of 1.5° with different beam configurations (4 beams, 8 beams, 16 beams, 32 beams, 48 beams and 64 beams), which are equally separated azimuth plane. The Power Spectral Density (PSD) is obtained from the synthesized beam using the eigenvector (EV) sub-space based spectral estimation method. The first moment is derived from the EV produced spectrum using the adaptive moment estimation method. From the first order moments derived along equally spaced pointing directions, radial wind velocities are readily obtained in the corresponding directions. From radial velocity obtained in various pointing directions, momentum flux of short duration (<2h) is estimated using the symmetric beam method (SBM) which requires 4 symmetric beams separated by 90° in the azimuth plane. A comparative study has been performed to study the optimum beam configuration, required for the flux estimation, which shows that 32 beam configuration is sufficient to estimate the flux with least error. Study with 32 beam configuration, gives 8 sets of symmetric beams (8×4). Using 8 set of beams, the flux is estimated for each set of beams and averaged. The averaged flux is further integrated over different lengths of time up to 14h. This systematic method for the estimation of momentum flux reveals that spatial averaging of beams in azimuth and integration over different lengths of time have reduced the time of integration from 15 to 16h for the conventional approach to 8–9h for the new approach using Postset Beam Steering (PBS) technique. The flux estimated with spatial-averaging of beam using PBS technique has been compared with other standard methods.

Measurement of Atmospheric Aspect Sensitivity Using Coherent Radar Imaging after Mitigation of Radar Beam Weighting Effect

Abstract The aspect angle, a measurement of the aspect sensitivity of atmospheric refractivity irregularities, was estimated with multiple-receiver coherent radar imaging (CRI) of very high frequency (VHF) atmospheric radar. Two CRI parameters retrieved by Capon’s method were utilized to derive the aspect angle: brightness width from the vertical radar beam and the direction of arrival (DOA) of the echo center from the oblique radar beam. Differing from previous studies with CRI, the radar beam weighting effect on the CRI brightness distribution was considered, and moreover, the radar beamwidth used in study was adaptive to the signal-to-noise ratio (SNR) of data as well as the off-beam direction angle. The study is based on statistical results. It is shown that the brightness width, a representative of the aspect angle, obtained from the modified CRI brightness distribution of the vertical radar beam was generally larger than that without correction, and it was very close to the values derived from the DOA of the 1° oblique radar beam and the power distribution of multiple beam directions. Moreover, the aspect angle derived from the DOA varied with the radar beam direction, which was similar to that obtained from the comparison of echo powers of a radar beam pair; however, the DOA approach yielded a much larger aspect angle in the low-SNR condition. This study recommended a feasibility of improving the measurements of atmospheric parameters with CRI after removing the radar beam weighting effect suitably from the CRI brightness distribution.

Effects of radar beam width and scatterer anisotropy on multiple-frequency range imaging using VHF atmospheric radar

[1] Benefiting from the changeable antenna array size and flexible radar beam direction of the middle and upper (MU) atmosphere radar (34.85°N, 136.11°E), the effects of radar beam width and scatterer anisotropy on the performance of multiple-frequency range imaging (RIM) were examined in addition to numerical simulation. First, nine transmitter/receiver modes were employed to reveal that a wider radar beam gives a larger phase bias in the RIM processing. Based on this, layer positions and layer thicknesses were estimated from the imaged powers of various radar beam widths after corrections of phase bias and range-weighting function effect. Statistical examination showed that the imaged layer structure was thicker for a larger radar beam width and this feature became more evident at a higher altitude, thereby demonstrating the influence of radar beam width on the practical performance of RIM. Second, the scatterer anisotropy in the layer structure was examined by means of a vertical and three oblique radar beams (5°, 10°, and 15° north), which were transmitted in company with the RIM technique. The vertical beam observed some single-layer and double-layer structures that were not always detected by the oblique beams, indicating the existence of anisotropic scatterers in the layers. In addition, a comparison of layer positions between the vertical and oblique radar beams showed that anisotropic characteristics of the upper and lower layers of a double-layer structure can be different, demonstrating one more capability of RIM for investigating fine-scale features of the atmospheric layer structures.

Sampling Strategies for Tornado and Mesocyclone Detection Using Dynamically Adaptive Doppler Radars: A Simulation Study

Abstract Increasing tornado and severe storm warning lead time (lead time is defined here as the elapsed time between the issuance of a watch or warning and the time at which the anticipated weather event first impacts the specified region) through the use of radar observations has long been a challenge for researchers and operational forecasters. To improve lead time and the probability of detecting tornadoes while decreasing the false alarm ratio, a greater understanding, obtained in part by more complete observations, is needed about the region of storms within which tornadoes form and persist. Driven in large part by this need, but also by the goal of using numerical models to explicitly predict intense local weather such as thunderstorms, the National Science Foundation established, in fall 2003, the Engineering Research Center for Collaborative Adaptive Sensing of the Atmosphere (CASA). CASA is developing a revolutionary new paradigm of using a network of small, closely spaced, inexpensive, low-power dual-polarization Doppler weather radars to overcome the inability of widely spaced, high-power radars to sample large regions of the lower atmosphere owing to the curvature of earth given that zero or negative beam elevation angles are not allowed. Also, current radar technology operates mostly independently of the weather and end-user needs, thus producing valuable information on storms as a whole but not focused on any specific phenomenon or need. Conversely, CASA utilizes a dynamically adaptive sensing paradigm to identify, and optimally sample, multiple targets based upon their observed characteristics in order to meet a variety of often competing end-user needs. The goal of this study is to evaluate a variety of adaptive sampling strategies for CASA radars to assess their effectiveness in identifying intense low-altitude vortices. Such identification, for the purposes of this study, is defined as achieving a best fit of simulated observations to an analytic model of a tornado or mesocyclone. Several parameters are varied in this study including the size of the vortex, azimuthal sampling interval, distance of the vortex from the radar, and radar beamwidth. Results show that, in the case of small vortices, adaptively decreasing the azimuthal sampling interval (i.e., overlapping beams) is beneficial in comparison to conventional azimuthal sampling that is approximately equal to the beamwidth. However, the benefit is limited to factors of 2 in overlapping. When simulating the performance of a CASA radar in comparison to that of a Weather Surveillance Radar-1988 Doppler (WSR-88D) at close range, with both operating in the conventional nonoverlapping mode, the WSR-88D (with a beamwidth about half that of a CASA radar) performs better. However, when overlapping is applied to the CASA radar, for which little additional processing time is required, the results are comparable. In effect, the sampling resolution of a radar can be increased simply by decreasing the azimuthal sampling interval as opposed to installing a larger antenna.

On Deriving Vertical Air Motions from Cloud Radar Doppler Spectra

Abstract A method for deriving vertical air motions from cloud radar Doppler spectrum measurements is introduced. The method is applicable to cloud volumes containing small particles, in this case liquid droplets, which are assumed to trace vertical air motions because of their limited size. The presence of liquid droplets is confirmed using multiple ground-based remote sensors. Corrections for Doppler spectrum broadening due to turbulence, wind shear, and radar beamwidth are applied. As a result of the turbulence broadening correction, the turbulent dissipation rate can also be estimated. This retrieval is demonstrated using measurements from the Department of Energy (DOE) Atmospheric Radiation Measurement Program’s (ARM) site in Barrow, Alaska, during the Mixed-Phase Arctic Cloud Experiment (MPACE) of autumn 2004. Comparisons of the retrievals with measurements by research aircraft near Barrow indicate that, on the whole, the retrievals perform well. A small bias in vertical velocity between the retrievals and aircraft measurements is found, based on a statistical comparison of four cases comprising nearly 6 h of data. Turbulent dissipation rate comparisons suggest that the radar-retrieved vertical velocity might be slightly underestimated because of an underestimate of the turbulence broadening correction. However, large uncertainties in aircraft vertical velocity measurements likely impact the comparison.

Variational Analysis of Oversampled Dual-Doppler Radial Velocity Data and Application to the Analysis of Tornado Circulations

Abstract For the detection of severe weather phenomena such as tornados, mesocyclones, and strong wind shear, the azimuthal resolution of radial velocity measurements is more important. The typical azimuthal resolutions of 1° for the Weather Surveillance Radar-1988 Doppler (WSR-88D) radars and of 2° for the planned Center for Collaborative Adaptive Sensing of Atmosphere (CASA) radars are not sufficient for this purpose, especially at far ranges. Oversampling is one strategy that can potentially provide more details about the azimuthal structures of flows, and can be achieved by processing raw data at azimuthal increments smaller than the radar beamwidth. In the presence of dual-Doppler observations, the variational method can be used to effectively recover subbeamwidth structures from these oversampled data, which, combined with the typically higher range resolutions, can provide high-resolution wind analyses that are valuable for, for example, tornado detection. This idea is tested in this paper using simulated data as well as reprocessed level-I data from a research WSR-88D radar, for model-simulated and actually observed tornadoes, respectively. The results confirm that much more detailed, often subbeamwidth, flow structures can indeed be recovered through azimuthal oversampling and a properly configured variational analysis, and the detailed flow analysis is expected to significantly improve one’s ability in identifying small-scale features such as tornadoes from radial velocity observations.

Explicit formulas for two state kalman, H/sub 2/ and H/sub ∞/ target tracking filters

The continuous time, two state, target tracking problem is considered from the Kalman, H/sub 2/, and H/sub /spl infin// filter viewpoint. While previous treatments were numerical in nature, analytic transient responses and infinite horizon solutions with analytic performance expressions are presented here. Tracking indices, involving the maneuver and measurement uncertainties, are shown to have a role for both the steady state and transient responses. In addition, the H/sub /spl infin// tracker has a sensor index involving the performance bound and measurement uncertainty, which, along with the tracking index, plays a significant role in the H/sub /spl infin// tracker expressions. Analytical expressions for the probability of target escape, the probability that the target position will be outside the radar beamwidth (BW), are developed not only to compare the performance of various trackers, but also as a design tool to meet tracking specifications. Examples illustrate the performance of the target trackers as a function of the error gain upper bound.

Two-Dimensional Simulations of Wake Vortex Detection Using Radio Acoustic Sounding Systems

Aparallelcodeisdevelopedtosimulatenumericallywakevortexdetectionusingaradioacousticsoundingsystem (RASS). The code is written in FORTRAN 90 with the message passing interface for parallel implementation. The numerical simulation solves simultaneously the linearized Euler equations for a nonuniform mean e ow and the Maxwellequationsforanonhomogeneousmedium.Theradartransmitterandreceiverantennasaremodeledusing an array of point sources and a beam-forming technique, respectively. Many features of the RASS are explored using thenumericalsimulation.First,a uniformmeane owisconsidered,and theRASSsimulation isperformedfor two different types of incident acoustic e eld: a short single-frequency acoustic pulse and a continuous broadband acoustic source. Both monostatic and bistatic cone gurations are examined, and their results are compared. Taylor and Oseen vortex velocity proe les are used as samplemodels, and their mean e owe eldsare reconstructed from the backscattered electromagnetic signal using the Abel transform. The effect of radar beam width is also considered, as are the issues of nonaxisymmetric and interacting vortices. I. Introduction T HEcapacity of airports is constrainedseverelyby the airtrafe c control system’ s consideration of aircraft wake vortices. The present Instrument Flight Rules (IFR) restrictions are based on aircraft weight. The “ 3‐ 4‐ 5‐ 6 rule” sets the separation distances of aircraft by categories from small, less than 41,000 lb (18,600 kg), to heavy, greater than 255,000 lb (116,000 kg). These separations are viewed as very conservative. However, because there remains signie cant uncertainty about wake vortex behavior under different atmospheric conditions, there are considerable technological barriers to improvements in terminal area productivity. In an attempt to enhance airport capacity, an aircraft vortex spacing system (AVOSS) has been under development at NASA Langley Research Center. The AVOSS will provide the means to allow air trafe c control to reduce spacing safely in instrument operations when the appropriate weather conditions exist. A key element in AVOSS is the development of a real-time reliable wake vortex detection system (WVDS). A radio acoustic sounding system (RASS) is a promising candidate for a WVDS. The basic concept of RASS is based on the tracking of sound waves with radar. When acoustic waves are transmitted, they produce pressure perturbations, which lead to e uctuations in the atmospheric permittivity. An incident electromagnetic e eldisthenscatteredduetothepermittivityvariationsandgenerates an echo that can be detected by a receiver antenna. The spectrum of this echo shows a Doppler shift proportional to the local speed of sound. However, the backscattered signals are so weak that no measurable signal would be received unless the Bragg condition is satise ed.The Bragg condition is a relationshipbetween the incident electromagnetic and acoustic wavelengths that ensures a constructive interference between the two waves. The Bragg condition may be written as ¸e D 2¸a sin®

Estimation of Rainfall Drop Size Distributions from Dual-Frequency Wind Profiler Spectra Using Deconvolution and a Nonlinear Least Squares Fitting Technique

Abstract This paper compares two different analysis approaches for dual-frequency (VHF and UHF) wind profiler precipitation retrievals. The first technique is based on a general deconvolution process to remove broadening effects from the Doppler spectra due to turbulence and finite radar beamwidth. The second technique is based on a nonlinear least squares fitting approach, where an analytical function that describes the drop size distribution is convolved with a clear air distribution to model the broadening of the observed spectra. These techniques are tested with simulated data, where the true drop size distribution is known and represented by a gamma or exponential distribution, and with observations. The median volume diameter D0, which is based on the third moment of the drop size number distribution, is used to test these retrieval techniques. This parameter is closely related to the rain rate, which is approximately the 3.67th moment of the drop size number distribution. Methods for extracting D0 ...

Signal processing of wide bandwidth and wide beamwidth P-3 SAR data

This research is concerned with multidimensional signal processing and image formation with FOliage PENetrating (FOPEN) airborne radar data which were collected by a Navy P-3 ultra wideband (UWB) radar in 1995. The digital signal processors that were developed for the P-3 data commonly used a radar beamwidth angle that was limited to 35 deg. Provided that the P-3 radar beamwidth angle (after slow-time FIR filtering and 6:1 decimation) was 35 deg, the P-3 signal aperture radar (SAR) system would approximately yield alias-free data in the slow-time Doppler domain. We provide an analysis here of the slow-time Doppler properties of the P-3 SAR system. This study indicates that the P-3 database possesses a 50 deg beamwidth angle within the entire [215, 730] MHz band of the P-3 radar. We show that the 50-degree beamwidth limit is imposed by the radar (radial) range swath gate; a larger beamwidth measurements would be possible with a larger range swath gate. The 50-degree beamwidth of the P-3 system results in slow-time Doppler aliasing within the frequency band of [444, 730] MHz. We outline a slow-time processing of the P-3 data to minimize the Doppler aliasing. The images which are formed via this method are shown to be superior in quality to the images which are formed via the conventional P-3 processor. In the presentation, we also introduce a method for converting the P-3 deramped (range-compressed) data into its alias-free baseband echoed data; the utility of this conversion for suppressing radio frequency interference signals is shown.