Methods and algorithms of angular estimation with super-resolution for aviation airborne radar systems

In the practical application of aircraft, situations are possible when in the on-board radar system (onboard radar) it is not possible to resolve the signals of radar targets in the channels for measuring range, velocity and angular coordinates. In these situations, it is expedient to use superresolution angular estimation procedures in the radar. There are various methods and algorithms for angular estimation with superresolution, each of which has its own advantages and disadvantages. At the same time, in the available scientific and technical literature there is no information on the superresolution angular estimation algorithms used in the radar, the knowledge of which is necessary in the design of promising radar. The article systematizes and analyzes methods and algorithms for angular estimation with superresolution. A classification of methods of angular estimation with superresolution is given. Generalized information about known one-dimensional and two-dimensional methods and algorithms of angular estimation is presented. The algorithms proposed for use in planar antenna arrays (ARs), concentric annular antenna arrays, ring antenna arrays, circular four-quadrant ARs, planar ARs with a total output signal are considered. It is noted that among the features of the construction of an on-board radar is the reduction in the total number of information processing channels, achieved by combining antenna elements into subarrays, the output signals of which are used to estimate the angular coordinates of radiation sources. The specificity of such antenna arrays is that the antenna subarrays have directional patterns that are narrower than those of weakly directional antenna elements. In addition, the distance between antenna modules can significantly exceed the distance between antenna elements in a conventional antenna array and, at the same time, significantly exceed the wavelength size. To determine the possibilities of practical application of the algorithms of angular two-dimensional estimation with superresolution in an on-board radar with a modular AA, considered in the article, special studies are required.

In traditional aerospace telemetry receiving systems, parabolic directional antenna or flat array antenna is generally adopted in the ground telemetry receiving antenna. The antenna array is synthesized into a fixed beam, tracking the target by mechanical scanning. When tracking multi-target is required, it is necessary to deploy multiple ground telemetry antennas to track their respective targets, i.e. one set of antennas tracks one target. Phased arrays antenna is an antenna system and technology developed on the basis of array antenna. Its antenna elements are synthesized into multiple independent beams, which can track one target at the same time, or track each target separately with different beams, so as to achieve the multi-target telemetry and tracking receiving function with a set of phased array antenna. The antenna array element is synthesized into multiple independent beams. This paper mainly discusses an implementation method of multi-target telemetry receiving antenna based on phased arrays technology.

In the general case, the law of distribution of direction finding errors of radio emission sources (RES) against the background of radio interference and internal noise is polymodal and is not described by a Gaussian density. The distribution of these errors depends on the true bearing, the distribution of interference and noise, the structure of the detector-direction finder and its antenna system (AS), the algorithm for processing the mixture of the direction-finding signal and interference. For practice, a more acceptable measure of direction finding efficiency is direction finding sensitivity – the minimum value of the energy signal-to-noise ratio at the input of the direction finding receiver, which ensures that the specified threshold correct detection probability, the values of the normal root-mean-square error (RMSE) and the probability of an anomalous direction finding error on background of white Gaussian noise. In practice, radio direction finders with ring antenna arrays based on a switched two-channel interferometer are widely used, which determine the direction of arrival of a radio wave based on the results of a sequential pairwise "interrogation" of antenna elements (AE). This article considers the direction finding sensitivity of various structures of azimuth direction finders using ring antenna arrays, according to the criterion of normal RMSE of direction finding based on the generalized Cramer–Rao inequality under the assumption of direction finding by the maximum likelihood method. This makes it possible to obtain a generalized expression for the lower bound of the dispersion of the direction finding error of the mentioned set of two-channel interferometers through expressions of their likelihood functions. The purpose of the paper is to analyze the direction-finding sensitivity according to the criterion of normal RMSE direction-finding of radio direction-finders in the case of using a switched two-channel interferometer with ring antenna arrays for various switching options in comparison with the sensitivity of a multi-channel monopulse direction finder. In this paper, generalized expressions for the lower bound of the dispersion of the Cramer–Rao inequality are obtained, which allow one to perform an analytical calculation of the RMSE of direction finding, or to determine the direction finding sensitivity for a given RMSE using a two-channel switched interferometer. Statistical modeling was performed, confirming the reliability of the obtained analytical expressions. Comparison of the potentially achievable direction finding characteristics of a direction finder with a ring antenna arrays of different structure, having different switching options, showed that direction finding by a monopulse direction finder will allow achieving a significant energy gain. The anisotropy of the direction-finding sensitivity of a two-channel switched interferometer with a constant reference element, which is one of the ring elements, is confirmed. Comparison of the results of this work with similar results that the expressions for the lower bound of the dispersion of the direction finding error for various structures of radio direction finders with an annular antenna array are identical to the corresponding expressions obtained by the method of linearization of the phase direction finding algorithm. The practical application of the obtained expressions will reduce the resource intensity of calculations of achievable indicators of the RES direction finding efficiency for the considered options for switching the direction finder with the ring antenna arrays, which will allow not to carry out statistical modeling, but to analyze the functioning of the direction finder depending on the options for switching the direction finder, the characteristics of the AS of the direction finder, number of antennas and characteristics of the radio receiving path.

On the basis of the analysis of the errors of the monopulse method of direction finding of the target of a radar station with a digital antenna array, a group of errors associated with fluctuations of the equisignal direction when scanning the beam of the antenna system, as well as with the non-identical shape of the generated beams of the monopulse group, was separated into a special cluster. This circumstance is especially critical for large-aperture radars with phased array with electric scanning, where the number of formed receiving beams is significant, in addition, the coordinates of the target usually do not coincide with the coordinate planes along which the coordinates are measured. The nonlinearity of this problem requires unique approaches and solutions. The classical formulation of the problem is associated with obtaining an estimate of the radiation vector of the entire antenna array from the known measured vector of complex signals at the output of the antenna array at a fixed time. To solve this problem and determine the decision rule, a neural network approach was used, while the neural network was trained according to the criterion of the minimum of the selected objective function. The substantiation of increasing the accuracy of measurements of angular coordinates in monopulse systems with scanning digital phased antenna arrays based on the use of neural networks in the process of radar data processing without the formation of direction finding characteristics is given. The results of modeling the characteristics of the antenna array (using the example of a flat antenna array with a dimension of 30 by 30 emitters) under the conditions adopted in the model of the antenna array of assumptions when implementing the classical and original methods are shown using examples of the surfaces of the variances of errors in measuring angular coordinates by the monopulse method in a rectangular spatial domain, which showed reduction of error and expansion of the measurement area. In the future, the practical implementation of this approach is assumed using FPGA in the receiving paths of the frequency-dependent part of the radio direction finder.

A method that ensures superresolution in terms of angular coordinates in surveillance systems with a flat antenna array during synthesis of radio-location and radio-thermal-location images of the Earth surface with the help of an airborne station in the real beam mode is proposed. This method is based on reconstruction of reflection and emission fields.

Planar Beam-forming Antenna Array for 60-GHz Broadband Communication

A 2 × 2 terahertz photoconductive antenna (PCA) array detector with high efficiency synthesis characteristic that improves the signal-to-noise ratio (SNR) of the detected signals has been reported in this paper. By processing the substrate material through a special micromachining process, the current signal generated by the adjacent antenna elements as opposed to that generated by the antenna gap is eliminated. Experiments show that the amplitude of the current signal output by the PCA array detector is consistent with the amplitude of the synchronous superposition of the current signals output by antenna elements, and the synthesis efficiency of the device achieves 93.7%. At the same time, the antenna array detector has low current noise, and its highest SNR is 62 dB under the excitation of different light energy, which is related to the number of antenna array elements.

The performance of an antenna array is considerably affected by mutual coupling effects between antenna elements. When a large number of antenna elements are located close to each other, mutual coupling becomes more significant. In this study, by using a new mutual impedance matrix, a decoupling methodology for compensating mutual coupling effects in a practical very high frequency (VHF)/ultra high frequency (UHF) Yagi‐Uda antenna array is introduced. No previous publications have studied VHF/UHF Yagi‐Uda antenna arrays in this context. In the proposed scheme, extreme care has been taken to account for both self‐ and mutual impedance related to mutual coupling effects. Experimental and simulation results show that using the proposed method, a perfect decoupling is achieved. The application of high‐resolution direction of arrival (DOA) estimation algorithms in decoupled experimental data leads to excellent performance of DOA estimation, in terms of accuracy and resolution. In addition, it is concluded (from experimental and simulation results) that mutual coupling effects between array elements as well as the root‐mean‐square error of estimated parameters depend on the direction of arrival. It is also deduced that in the presence of mutual coupling, estimation of signal parameter via rotation invariance techniques algorithm performs better than other subspace‐based algorithms.

In this paper, a compact two-port MIMO antenna array system is described. The antenna array spans the range from 27 to 40 GHz, whereas the impedance of the antenna element is matched at 50 Ω. The gain of the antenna element is between 5.5 and 8.5 dBi, and its radiation efficiency is between 65 and 90%. With high impedance matching for 5G MMW (Millimeter Wave), particularly at 28 GHz and 38 GHz, the 2-port antenna array operates in the frequency range of 27–40 GHz. The suggested MIMO array operates effectively with a gain of approximately 10 dBi and a radiation efficiency of approximately 95%. The antenna array's overall dimensions are a length of 55.27 mm, a width of 27.635 mm, and a depth of 1.6 mm with partial ground. A FR-4 substrate is used in the antenna's fabrication, greatly reducing the cost. In the antenna array, a decoupling surface is used between the antennas, with orthogonality being maintained between the ports to reduce mutual coupling. The results of the modelling show a reduction in the measured mutual coupling between array ports of less than − 35 dB. An envelope correlation coefficient (ECC) of less than 1 × 10–4 is preferable. Additionally, the channel loss capacity is less than 0.3 bits/s/Hz, the mean effective gain is approximately − 6 dB, and the total active reflection coefficient is upgraded to be less than − 30 dB. Moreover, a diversity gain of approximately 10 dB is achieved. The proposed construction was created using CST Microwave Studio 2019. When the antennas are constructed and tested, the experimental outcomes surpass those of the simulation. Each antenna element is 27.635 mm long, 27.635 mm wide, and 1.6 mm thick. Slots in the radiating circular patch antenna element could be used to improve the radiation characteristics across the intended bands. The parametric study specifies that the distance between the antenna elements should be 0.5 λ, where λ is the operational wavelength.

Ceramic microstrip patch antenna formed using Barium Strontium Titanate (BST) has been investigated. The Computer Simulation Technology (CST) Microwave Studio is used for the simulation. We measured the gain of antennas with two, four, and six elements array of BST antenna and found that the gain of an six elements of BST array antenna was enhanced by about 2.126 dB gain over the four elements of BST array antenna at 2.3 GHz. The impedance bandwidths of these BST array antenna for VSWR <; 2 were in the Worldwide interoperability for Microwave Access (WiMAX) application range which is 2.3 GHz to 2.5 GHz. Compared with the conventional array antenna with the same aperture size, the performance of the antenna is improved obviously and the design is suitable for array applications for example base station.

This paper deals with a 2×2 Integrated Filter Antenna (IFA) array designed at 5.2 GHz. By incorporating the filtering action in the conventional antenna array, the need for an additional filter for suppressing the out-of-band signals has been avoided. In order to achieve this the last resonator of the filter has been replaced by a slot antenna, operating at the same band as that of the filter, in each arm of the array. The overall dimension of the subsystem has been optimized to 1.56λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 1.56λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> . It has got a fractional bandwidth of 3.0% at 5.2 GHz. The sub-system provides a maximum gain of 12.43 dBi and HPBW of 33° in broadside direction. The proposed structure was designed, simulated and fabricated. The measured reflection co-efficient plot shows a shift of 0.09 GHz of the overall band which may be due to the fabrication tolerances.

The development of an aperture coupled microstrip dual-band and dual-polarization 6×6 antenna array system on RO4003C is presented. The antenna array has a multilayer structure that consists of square microstrip elements, placed on each substrate layer to provide dual frequency operation at 14 and 35 GHz. Dual polarization is achieved by using two right-angled microstrip lines and two perpendicular slots to feed the antennas. The microstrip feed line is placed orthogonally from the center of the slot on the ground plane between the substrates. The energy from the microstrip feed line is aperture coupled to each square antenna element through the slot. Two right-angled non-overlapping slots are used on each antenna to enable the two different polarization states. The antenna array has 14 GHz patches on the top substrate layer and 35 GHz patches on the lower substrate layer in order to maintain a similar percentage bandwidth. The microstrip feed lines are located at the bottom of the feed substrate. A symmetric feed structure is followed for the antenna array. The aperture coupling feeding network has the advantage of reduced spurious radiation compared to edge or probe feeds as it utilized highly linearly polarized feed lines and slots, and a large ground plane to prevent backside radiation. The designed antenna array has linear polarization at 45-degrees and 135-degrees at the Ku- and Ka-frequency bands. The array is designed using a hybrid simulation scheme in Keysight Advance Design System (ADS). The ADS Schematic is used for the feed network while the ADS Momentum is used for the antenna elements and the entire array structure simulations. In this way, all aspects of the design (microstrip antenna elements and feed lines) can be optimized independently. This array is suitable for airborne (small remotely piloted), and small-satellite NASA flight programs.

A modular dual-polarized Vivaldi antenna array design for 18–30 GHz frequency is presented. The array module consists of the antenna and RF modules. The antenna module comprises <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 4$ </tex-math></inline-formula> dual-polarized antenna elements with element spacing of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> /2 at 30 GHz. The RF module contains the amplifiers and phase shifters that control all the elements using commercial off-the-shelf (COTS) flip-chip components. The footprint of the RF module is the same as that of the antenna module allowing assembly of antenna arrays of almost any size and shape. Additionally, the interface between the antenna and the RF modules is connectorless, decreasing the number of components required in the assembly and decreasing the overall cost of the system. An array of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times 8$ </tex-math></inline-formula> dual-polarized antenna elements is constructed from two array modules to prove the seamless operation of the modular design. The prototype uses Anokiwave chips with a frequency range from 26.5 GHz to 29.5 GHz. The measured amplitude and phase of the electric field in front of the antenna aperture is uniform so as to equally feed the elements. Additionally, the demonstrated beam steering up to ±60° in the plane of the larger array dimension matches well with the simulations, proving the feasibility of the design.

In time-modulated antenna arrays (TMAAs), as a result of periodical switch-on and switch- ofi of the antenna elements, including operating frequency (termed as center frequency) signal, sideband signals are appeared at either sides of the center frequency in integer multiples of the modulation frequency. In this paper, it is shown that without using phase shifters, just by suitably controlling the on-time instants (OTIs) and on-time durations (OTDs) of a time-modulated linear antenna array (TMLAA) elements, simultaneously, along with a pencil beam pattern at the operating frequency, a shaped beam pattern can be obtained at both the flrst positive and negative harmonics of the time- modulation frequency. The important advantage of such a technique is that realization of multi-beam pattern in conventional antenna array (CAA) system generally requires complex feed network, whereas by using simple radio frequency (RF) switching circuit in the feed network of TMLAA, by virtue of the properties of harmonic radiations, synthesis of a shaped pattern at either (positive or negative) harmonic results in generating the same pattern in its opposite harmonic, and the synthesized patterns at difierent harmonics can be simultaneously used as independent communication channels. By employing a difierential evolution (DE) based optimization method, numerical results for a 16-element TMLAA with uniform excitation show that in conjunction with a pencil beam pattern at the center frequency, a ∞at-top or a cosec square pattern at flrst positive and negative sidebands of side-lobe levels (SLL) i20dB can be synthesized by suppressing the higher sideband level (SBL) to below i10dB.

This paper discusses the comparative effect of Electromagnetic Band Gap (EBG) structures on two element Microstrip antenna array. The dielectric substrate used is FR-4 glass epoxy with dielectric constant 4.2 and loss tangent 0.0245. The conventional antenna array is resonating at a frequency of 5.53 GHz with a mutual coupling of -17.83 dB. The first proposed antenna array with square slot EBG structure in the ground plane and fractal patch EBG structure on the surface produces a mutual coupling of -50.21 dB and a gain of 8.78 dB at 5.53 GHz. The second proposed antenna array with star slot EBG structure in the ground plane and diamond patch EBG structure on the surface produces a mutual coupling of -51.63 dB and a gain of 7.20 dB at 5.53 GHz. Both the proposed antenna arrays are producing appreciable decrease in back lobe radiation. The antenna arrays are designed using Mentor Graphics IE3D software and measured are obtained using vector network analyzer.