Spaced Antenna Method Research Articles

Evaluation of EAR Spaced Antenna Performance Using Multiple Receiving Antennas Orientations

AbstractEquatorial Atmosphere Radar (EAR) is a very high frequency (VHF) Doppler radar with an active phased array antenna system operating at 47 MHz. It had originally been equipped with a single receiving channel system since its establishment in 2001 at the equator in Kototabang, West Sumatra, Indonesia (0.20°S, 100.32°E). Recent progress has enabled the implementation of a spaced antenna (SA) method on the EAR through the development of a multichannel receiver using Universal Software Radio Peripheral (USRP) and GNU Radio. However, this approach exhibits limited height observation and horizontal wind estimation accuracy. Here, we present comparison and analysis of EAR SA performance with five different orientations, taking into consideration the size of the receiving antennas and their separation distance. Experiments with the chosen configurations were carried out between April and September 2019, and the horizontal wind profiles were estimated using full correlation analysis (FCA) and compared with the EAR standard data. In addition, FCA with the least squares method is carried out for all configurations with the aim of improving the estimation accuracy. Based on the results, the configuration with the largest aperture shows a clear advantage over the other four configurations but with limited improvement in terms of maximum observable height.

Spaced-Antenna Aperture Synthesis Using an X-Band Active Phased-Array

Spaced antenna (SA) radars retrieve wind-fields by tracking resolution volume sized bins of scatterers as they advect between two physically displaced antennas. To date, SA methods have been applied for profiling the ionosphere and precipitation free atmosphere. The primary technological difficulty in applying these methods to probe precipitation at microwave frequencies is the requirement for such a short antenna separation that a significant overlap in apertures is necessary. In this article, we synthesize overlapped apertures by segmenting active phased-arrays into subarrays that are multiplexed in time. Antenna pattern measurements are then employed to evaluate beamforming errors on a family of implementations. Based on measurements and Monte Carlo simulations, we find that highly overlapping apertures are most immune to beam squint errors. This is because element-level phase errors are retained on the synthesized SAs and differential beam squint errors are minimized. Finally, we demonstrate a novel method to measure relative phase center displacement between SAs that obviates the need for near-field antenna measurements.

Development of Software‐Defined Multichannel Receiver for EAR

AbstractThe Equatorial Atmosphere Radar (EAR) is a very high frequency Doppler radar operated with an active phased‐array antenna system at 47 MHz. It is located at the equator in Kototabang, West Sumatra, Indonesia (0.20°S, 100.32°E) and was established with a single receiving channel. Here, we present the development of a multichannel receiver system for EAR using a combination of the Universal Software Radio Peripheral X300 (USRP X300) series and GNU Radio software. Two USRP X300 devices, corresponding to four receiving channels, were synchronized using 10‐MHz reference clocks and a one‐pulse per second signal. Received signals were collected by the existing EAR antennas, fed to the USRPs for digital conversion, and then stored on a hard disk drive. Signal postprocessing was carried out to estimate wind velocity profiles. Wind profiles estimated using full correlation analysis indicate that the spaced antenna method from multichannel receivers agrees well with EAR standard observational data.

An atmospheric backscatter model on wind measurements using far-field approximation method

A backscatter model was developed for measuring wind field with the far-field approximation method. The theoretical computation and computer simulations with one spatial dimension show that this model can realistically describe the physical meaning and process of the three methods in wind measurements including the spaced antenna (SA) method, Doppler beamswing (DBS) method, and spaced interferometry (SI). The computational difficulties of the traditional theoretical model cannot only be smoothed away, but common characteristics and differences of the three methods can be compared deeply. The comparison of the numerical results between the Wuhan medium frequency (MF) radar (30° N, 114° E) observation and the computer simulation of the full correlation analysis (FCA) of the SA method indicates that the two results agree very well and this model has practical application.

Wind and turbulence measurements by the Middle and Upper Atmosphere Radar (MUR): comparison of techniques

Abstract. The structure-function-based method (referred to as UCAR-STARS), a technique for estimating mean horizontal winds, variances of three turbulent velocity components and horizontal momentum flux was applied to the Middle and Upper atmosphere Radar (MUR) operating in spaced antenna (SA) profiling mode. The method is discussed and compared with the Holloway and Doviak (HAD) correlation-function-based technique. Mean horizontal winds are estimated with the STARS and HAD techniques; the Doppler Beam Swinging (DBS) method is used as a reference for evaluating the SA techniques. Reasonable agreement between SA and DBS techniques is found at heights from 5km to approximately 11km, where signal-to-noise ratio was rather high. The STARS and HAD produced variances of vertical turbulent velocity are found to be in fair agreement. They are affected by beam-broadening in a different way than the DBS-produced spectral width, and to a much lesser degree. Variances of horizontal turbulent velocity components and horizontal momentum flux are estimated with the STARS method, and strong anisotropy of turbulence is found. These characteristics cannot be estimated with correlation-function-based SA methods, which could make UCAR-STARS a useful alternative to traditional SA techniques.

Radar imaging and high-resolution array processing applied to a classical VHF-ST profiler

Among the spaced antenna methods used in the field of atmospheric studies, radar interferometry has been of great interest for many authors. A first approach is to use the phase information contained in the cross-spectra between antenna output signals and to retrieve direction of arrival (DOA) of discrete scatterers. The second one introduces a phase shift between the antenna signals in order to steer the main beam of the antenna towards a desired direction. This paper deals with the later technique and presents a variant of postset beam steering (PBS) which does not require a multi-receiver system. Indeed, the data samples are taken alternately on each antenna by means of high-commutation-rate switches inserted before a unique receiver. This low-cost technique is called “sequential PBS” (SPBS) and has been implemented on two classical VHF–ST radars. The present paper shows that high flexibility of SPBS in angular scanning allows to perform radar imaging. Despite a limited maximum range due to the antennas’ scanning, the collected data give a view of the boundary layer and the lower troposphere over a wide horizontal extent, with characteristic horizontally stratified structures in the lower troposphere. These structures are also detected by application of high-resolution imaging processing such as Capon's beamforming or Multiple Signal Classification algorithm. The proposed method can be a simple way to enhance the versatility of classical DBS radars in order to extend them for multi-sensor applications and local meteorology.

Scattering properties of PMSE irregularities and refinement of velocity estimates

Observations of polar mesosphere summer echoes (PMSE) were performed with the ALOMAR SOUSY VHF radar in the Doppler beam swinging (DBS) and the spaced antenna (SA) mode. This radar is located in polar regions at 69°N, 16°E in Andenes, Norway. After a summary of the basic features of PMSE and the system lay-out, we will come to the analysis of PMSE irregularity structures. Differences of the signal power observed with the DBS and the smaller SA antenna configuration point to a dominance of a scattering process of PMSE irregularities rather than to specular reflection. The SA method yields a more pronounced aspect sensitivity than the DBS method which points towards fairly anisotropic PMSE irregularities in the smaller volume sensed by the SA set-up. These anisotropic irregularity layers are frequently inclined towards the horizontal by a few degrees. There are wave and instability events which change the layer structure, their inclination and also the velocity components. When the layer inclination is not horizontal, a leakage of the horizontal velocity into the vertical velocity is manifest. It is shown that this can be corrected by using the measured zenith and azimuth angles of signal arrival and by calculating the leakage of the horizontal velocity components. This procedure yields improved estimates of the velocity components in vertical direction. Finally, some thoughts how to explain such observations of highly anisotropic and inhomogeneous PMSE irregularity structures are summarized.

Tropospheric measurements of turbulence and characteristics of Bragg scatterers using the Jicamarca VHF radar

We apply the Holloway et al. [1997a] spaced antenna (SA) method to data collected with the Jicamarca VHF radar. Measurements are presented of an indicator of turbulent intensity (i.e., the standard deviation of the velocity fluctuation) and the parameters (orientation and correlation lengths) of horizontally anisotropic Bragg scatterers at tropospheric heights. We compare SA indicators of turbulence intensity to concurrent measurements obtained with a very narrow vertical beam (∼0.8°). This comparison results in a very good agreement. We also show that the Jicamarca observations are due to a Fresnel scattering mechanism rather than a reflecting one. Finally, we find an L‐shaped regression between the correlation lengths of Bragg scatterers and the indicators of turbulent intensity. When turbulence is weak (σt < 0.1 m s−1), the horizontal correlation lengths have a large spread of relatively large values (i.e., large aspect sensitivity), but when turbulence is strong, the horizontal correlation lengths are much smaller.

Validation of a UHF spaced antenna wind profiler for high‐resolution boundary layer observations

In this paper we apply a spaced antenna technique derived from the recent work of Doviak et al. [1996a] and Holloway et al. [this issue] to wind measurement with a small UHF boundary layer profiler. We discuss the implementation of the technique, averaging and quality control strategies, and some advantages and limitations of spaced antenna methods over conventional Doppler beam swinging wind profilers in the boundary layer. Such advantages include a relaxation of the assumption of a horizontally uniform wind field and the possibility of high temporal resolution wind profiles. In this regard we present velocity measurements derived from this UHF system with time resolution of about 30 s and compare these measurements with in situ sonic anemometer data taken on a 300‐m tower. Finally, we present an example of a high‐resolution time‐height cross section of atmospheric winds. This example, collected in stratiform precipitation, shows the intriguing situation of a wind speed maximum (jet) which closely follows the height of the melting layer over several hours even as this height changes by several hundred meters.

A note on oblique spaced antenna method

Biases in oblique spaced antenna (OSA) horizontal wind estimates due to flow field inhomogeneities are shown to preclude reliable estimation of the vertical component of wind vorticity with the OSA method.

A comparison of Doppler and spaced antenna radar techniques for the measurement of turbulent energy dissipation rates

There has been concern in the past that spaced antenna radar measurements of energy dissipation rates in the Mesosphere using broad beam arrays may be contaminated by the effects of atmospheric gravity waves. To investigate this possibility, a 1.98 MHz Doppler radar operated by the University of Adelaide was used in two separate configurations; a narrow vertical beam (4.5° half power half width) and 3 smaller spaced antenna systems with broad beams (20° half power half width) were used to simultaneously measure energy dissipation rates in the height region 80 to 100km. The 3 broad beams were also used to determine winds by the spaced antenna method. Although extraction of the energy dissipation rate parameter is a difficult process, temporally and spatially coincident data were obtained enabling some important comparisons to be made. Three years of such data were analysed and the following results noted: (1) the mean energy dissipation rates as measured by the two radars are comparable, (2) the variance of the data as measured by the Doppler radar is larger than that measured by the spaced antenna radar, (3) The spaced antenna radar does not generally over‐estimate energy dissipation rates at moderate and large values, but when turbulence is very weak, it may do so. The degree of over‐estimation is not as serious as has been inferred in the past. However, there is considerable scatter in the short term ratio of measurements using the two methods, which emphasizes the large degree of spatial variability of turbulence strengths in the middle atmosphere.

Radars of the mesosphere, lower thermosphere network: Their capability to measure winds and waves, and their role in step

Three types of radars have demonstrated, during the 1980's, the capibility of measuring winds with useful resolution (2–3 km in the vertical, sampling rates, 1–12 hr −1): medium frequency (MF) using the spaced antenna method, LF systems using similar techniques, and meteor radars, some associated with S.T. radars. The techniques used, assumptions and examples of winds data are shown for each major type of system. The MLT network is now active in the WITS/ CEDAR and STEP (1990–1995) programs, for the latter the project is called MLTCS ( M esosphere, L ower T hermosphere C oupling S tudy). It involves climatologies of mean winds, tides and planetary waves, interactions between these waves, and solar, magnetospheric influences. Some examples of these data are shown.

Analysis of model simulations of spaced antenna/radar interferometer measurements

A computer simulation of scattering from inhomogeneities in the refractive index is used to compare the spaced antenna and interferometer methods for measuring winds in clear air, both with and without turbulent fading. The results show that the spaced antenna analysis which is carried out in the time domain and the interferometer analysis which is carried out in the frequency domain are equivalent in terms of the information that the two methods yield. A data analysis method, equivalent to full correlation analysis, which can be carried out in the frequency domain is presented. The method is applied to model‐generated, simulated data in order to extract the typical full correlation analysis output parameters such as the apparent and true velocity. The results obtained are consistent with the model input parameters.

A model for oblique spaced antenna technique for mesosphere‐stratosphere‐troposphere radars and its applications

The spaced antenna method for VHF/UHF radar applications is discussed for the general observational configurations including the off‐zenith observational mode. On the basis of the assumptions of “locally frozen‐in” and volume scattering a physical model is constructed. Cross‐correlation and autocorrelation functions are derived and applied for the discussion of the various aspects of the full correlation analysis. Spectral width, aspect sensitivity, and characteristic length are compared for different echo mechanisms in this paper. Some observational results based on the model predictions will be discussed.

A dynamic ionosonde design using pulse coding

A research ionosonde with capabilities for measuring drifts by the spaced-antenna method was designed and a limited prototype tested. Using a PC for much of the control and data storage functions and a small quantity of external electronics will make the instrument small and inexpensive. In particular, pulse compression techniques using a 13 bit Barker code gives an improvement in signal-to-noise ratio of about 11 dB. This allows the use of a transmitter with peak power less than a kilowatt, which is compatible with solid-state electronics.

VHF Radar Measurements of In-Beam Incidence Angles and Associated Vertical-Beam Radial Velocity Corrections

Abstract Atmospheric backscatter or partial reflections observed with VHF radars are strongly enhanced for angles within a few degrees of the vertical, although the strongest echoes are not necessarily exactly from the vertical direction. Consequently, a nominally vertical beam can actually receive the strongest echoes from a direction slightly off-vertical. The radial velocities measured in the vertical beam, then, are not true vertical velocities but have contributions from both the projection of the vertical and the horizontal velocity along the effective beam direction. We present measurements of the tropospheric and lower stratospheric incidence angles obtained over a period of four days with a spaced antenna radar system and calculate the true vertical velocities by means of a correction derived by combining the incidence-angle measurements, the radial velocities in the vertical beam, and the horizontal winds obtained by the standard spaced antenna method. Typical values for the incidence angles are...

The Oblique Spaced Antenna Method for Measuring the Atmospheric Wind Field

Abstract A novel extension of the spaced antenna method for VHF/UHF radar and wind profiler applications is introduced in this paper. It is proposed that instead of pointing the spaced antenna beams vertically, off-vertical oblique configuration should be used. It will be, shown that this technique can be used to obtain horizontal wind at independent “points” in the atmosphere without making assumptions about homogeneity or variation of the wind field between these points. Good experimental confirmations are achieved by comparing the results with those derived from the conventional spaced antenna method and the Doppler method. Applications of the proposed technique in measuring kinematic properties of the wind field such as divergence, vorticity, etc., in the atmosphere and the accuracy of these measurements are discussed.

The Chung‐Li VHF radar: Technical layout and a summary of initial results

The particular characteristics of the VHF mesosphere‐stratosphere‐troposphere (MST) radar and wind profiler system, operated at the National Central University in Chung‐Li, Taiwan, Republic of China, are described. These are the dual‐mode applications of the Doppler and the spaced antenna technique, where the latter is used in the conventional mode to determine the wind velocity as well as in the novel interferometer mode. A brief outline of the scientific rationale and the technical layout of this particular configuration of the Chung‐Li VHF radar is given. A generally applicable method for system performance checks by deducing principle radar data parameters before the data analysis has been developed and is briefly described. Receiver system and antenna calibration measurements with celestial radio sources are reported and prove the appropriateness of the antenna system. Radar reflectivity profiles, measured with vertical as well as with off‐vertical antenna beams, are presented and discussed. We also present height‐time‐reflectivity plots for quiet and disturbed weather conditions and reflect on their peculiarities. We show that the wind profiles measured with the conventional Doppler method and the spaced antenna method are in good agreement with radiosonde measurements. The spatial correlation parameters, deduced with the spaced antenna method, are consistent with results from other radars. We also discuss measurement capabilities to deduce the tropopause height with the Chung‐Li VHF radar. We also describe echo power variations due to focusing from reflectivity structures modulated by gravity waves and a brief case study of Kelvin‐Helmholtz instability. We finally outline future research directions which could be performed with the Chung‐Li VHF radar.

The Spaced Antenna Technique for Radar Wind Profiling

Abstract The Doppler method for radar wind profiling of the troposphere and stratosphere has been the standard technique used for operational and most research applications of the instrumentation. An alternative approach for measuring winds, by making use of scattering and reflection from variations in the refractive index, is what is known as the spaced antenna technique. This technique has received little attention so far, and few comparisons of the two techniques have been carried out, in spite of the fact that results to date indicate that there can be some advantages of the spaced antenna method over the Doppler method, especially when small-antenna and low-power VHF systems are being considered. We describe the technique and summarize the diverse results that relate to a comparison between the Doppler and spaced antenna methods. The expected advantages of the spaced antenna method for small radar systems are also described.

Heights of MF radar scatter (1986/87) and the wind field (55–95 km): Saskatoon, Canada

The Saskatoon MF radar (2.2 MHz) at 52°N, 107°W, has been used to measure the heights of occurrence of radar scatter during four seasons, and twelve months of 1986/87. Mean winds, and gravity waves are also available, by the spaced antenna method and from the same radar echoes. Certain heights, called elsewhere ‘preferred heights’, are identified near 60km, 70km, 75km in summer, and 80–86 km. Several layers have seasonal and diurnal variations. Associations with electron density gradients (rocket data), mean wind shear in summer, and gravity wave amplitude-minima in the equinoxes are effectively demonstrated. Case studies, involving 3 h data sets of radar scatter and wind elaborate the comparison: gravity waves of long period (τ > 6 h) are shown to modulate the scattering process.