Boresight Error Research Articles

Novel Metamaterial-Element-Based FSS for Airborne Radome Applications

A novel metamaterial (MTM) element for radome applications is presented in this paper, which shows negative permittivity and permeability characteristics at distinct frequency ranges. The MTM element is composed of tightly coupled Swastika-shaped conductors printed on both sides of the dielectric substrate. The electromagnetic (EM) analysis of the proposed structure is carried out using the full-wave method-based standard software package and then validated with the experimental results. The MTM structure is further modified by introducing shorting strips at its edges to improve bandpass FSS response with flat-top and sharp roll-off characteristics in microwave and millimeter-wave frequency regimes. In view of airborne radome application of MTM-FSS structure, a multilayered hemispherical radome is designed by embedding it in the mid-plane of the radome wall. The EM performance of the antenna–radome system is analyzed using 3-D ray tracing geometrical optics in conjunction with aperture integration method, incorporating the transmission characteristics of MTM-FSS-based radome wall using CST Microwave Studio. The MTM-FSS-based radome exhibits superior EM performance characteristics as compared to the conventional optimized monolithic radome over the frequency range of 8.5–10.3 GHz, with high transmission efficiency (~0.30) and minimal boresight error (~4 mrad) for the entire beam steering range.

Multidisciplinary design optimization of sandwich-structured radomes

A multidisciplinary design optimization framework is proposed for sandwich-structured radomes. Radomes ensure the functional operation of antenna systems in adverse environment catering for aerodynamic stresses and payload requirements. The existence of radomes can partially degrade the electromagnetic performance of antenna systems. The electromagnetic performance and mechanical responses are taken into account simultaneously in the optimization design. This is more time-saving and economical compared to the traditional separate considerations on these two aspects. Coupled with multi-island genetic algorithm, transmission coefficient and boresight error are identified as the objectives. Lateral deformation, material failure, and structural stability are included in mechanical analysis. Three-dimensional ray-tracing technique and physical optics based aperture integration method are employed to address interactions between the antenna and radome. Tsai–Wu and maximum stress criteria are used to predict material failure. Structural stability is analyzed using the linear perturbation of stiffness matrices. The applicability of the electromagnetic model is validated using examples of a hemispheric air and single-layered radome. A numerical experiment is conducted to investigate the utility and feasibility of the multidisciplinary design optimization model. Results show that the optimal section profile brings about considerable improvement in transmission coefficient and boresight error. Mechanical constrains are reasonably subjected to the preset limits. Hence, the proposed multidisciplinary design optimization model is an effective and feasible alternative in the environment of radome design.

System Self-Calibration Model for Non-Vertical Four-Prism Airborne LiDAR

ABSTRACTSystem errors in light detection and ranging (LiDAR) systems cannot be ignored due to their influence on geo-referencing accuracy. Previous calibration methods are based on the vertical LiDAR system and do not simultaneously consider the bore-sight angles, lever-arm errors, angles, range, trajectory position, and angle correction parameters for each strip. This study proposes a system self-calibration model based on a high-precision positioning model for the non-vertical four-prism airborne LiDAR system. Furthermore, an automatic system calibration process is presented using this model that identifies 12 parameters for overlapping LiDAR data. Also, each strip of trajectory data has three position corrections and three angle corrections. The feasibility and applicability of the method is demonstrated via qualitative and quantitative analyses using real data from plain and hilly areas. The experimental results prove that the proposed method is stable and reliable. Specifically, the proposed method can compensate for system bias, correct original flight data, and provide seamless data. This model can be applied to actual flight engineering datasets, has no terrain limitations, no specific requirements for the trajectory configuration, no specific hypotheses, and no need for control information.

Study on the Electromagnetic Performance of Inhomogeneous Radomes for Airborne Applications—Part II: the Overall Comparison With Variable Thickness Radomes

In the first part of a two-paper sequence, the phase distortion caused by inhomogeneous radomes is studied to reveal the boresight error (BSE) characteristics, and a method is proposed to estimate the insertion phase delay distribution and the BSE. In this second part, the bandwidth performance and the effect of radome coating, thickness errors, and layer number are further analyzed to acquire an overall understanding of the performance of inhomogeneous radomes through the comparison with variable thickness radomes (VTRs). Transmission loss and BSE, the two most important EM characteristics of airborne radomes, are considered. The bandwidth performance of VTRs is, from the authors’ best knowledge, for the first time optimized, and corresponding conclusions are drawn. Results indicate that inhomogeneous radomes have superiorities over VTRs, which makes this kind of radome a potentially popular choice in modern airborne applications.

Study on the Electromagnetic Performance of Inhomogeneous Radomes for Airborne Applications—Part I: Characteristics of Phase Distortion and Boresight Error

Inhomogeneous radomes, characterized by stacked layers of continuously varying dielectric parameters, are potential in yielding excellent electromagnetic (EM) performance. This two-part sequence of papers presents a study on the EM performance of inhomogeneous airborne radomes based on impedance match, including comparison with the conventional constant thickness radomes and variable thickness radomes (VTRs). In this paper, study on the phase distortion of antenna aperture field caused by inhomogeneous radomes gives a physical insight into the cause of radome boresight error (BSE), and explains why the substantial decrease of insertion phase delay (IPD) of inhomogeneous radomes, instead of leading to BSE improvement, increases the BSE. Based on the study, a method is proposed to estimate the IPD distribution which can be used to predict the BSE. The results reveal the EM characteristics of inhomogeneous radomes which is potential in realizing high-performance airborne radomes superior to traditional VTRs. The second part of the paper studies the bandwidth performance and the effect of radome coating, thickness errors, and layer number in order to acquire an overall understanding of the performance of inhomogeneous radomes.

Multibeam Echo Sounders-IMU Automatic Boresight Calibration on Natural Surfaces

ABSTRACTThis paper details a new boresight calibration method for multibeam echo sounder systems This method is based on an automatic data selection algorithm, followed by a boresight least squares adjustment This method, called MIBAC (MultiBeam-IMU Boresight Automatic Calibration), takes in input overlapping survey strips following a simple line pattern over a regular slope. We first construct a boresight error observability criterion, used to select automatically the most sensitive soundings to boresight errors. From these soundings, we perform a 3D adjustment of the boresight angle, thus taking into account the coupling between angles. From a statistical analysis of the adjustment results, we derive the boresight angle precision. Numerical results obtained with four different multibeam echo sounder systems are presented and compared to those of a patch test calibration method. Finally, we demonstrate the performances of MIBAC through a standard deviation along the surface normal approach computed by principal component analysis.

Beam Pattern Analysis of Antenna Array on Complex Platform Using AEP Method Based on Hybrid UTD-ACGF Technique

For accurate beam analysis of a radiating antenna array on a complex platform, the mutual coupling and platform scattering effects are important. We proposed a beam pattern analysis method for an antenna array on a complex platform, which combines the active element pattern method of the receiving mode and hybrid geometrical theory of diffraction antenna current Green’s function technique. This proposed method clearly and mathematically divides the beam pattern of the antenna array on a complex platform into mutual coupling and platform scattering components. For a 12 linear dipole array embedded in a complex aircraft, analyses of the beam pattern and boresight error using the proposed method were verified with the method of moment results, which were simulated with FEKO. With reasonably good accuracy, the proposed method can efficiently analyze a beam pattern reflecting the mutual coupling effect and the platform scattering effect. Additionally, convex optimization with the proposed method is simply applied to synthesize the compensated beam pattern.

Outage Performance of Exponentiated Weibull FSO Links Under Generalized Pointing Errors

Even in clear conditions, free-space optical (FSO) links are impaired by scintillation and dynamic misalignment which result in a slow fading channel. This paper presents the first characterization of outage performance for single-input/single-output FSO links over exponentiated Weibull atmospheric turbulence and generalized misalignment. A novel feature of this paper is that a generalized pointing error model is employed which not only takes into account the impact of different jitters for the elevation and the horizontal displacement but also the effect of different boresight errors for each axis. The developed asymptotic expressions are used to find optimum beam widths that minimize the impact of pointing error effects in a variety of atmospheric turbulence conditions. Obtained results corroborate that the impact of generalized pointing errors is approximately the same over moderate and strong turbulence conditions when an aperture-averaged receiver is considered. Additionally, the use of a transmitter with optimized beam width can result in large gains on the order of 5 dB or even greater.

Integration Multi-Objective Optimization Method of Airborne Radome System

To improve the characteristics of boresight error (BSE) and power transmission coefficient in a phased array antenna-radome system, an integration optimization method, adjusting radome structure and antenna excitation successively, is studied. A multi-objective fitness function is designed for the particle swarm optimization (PSO), which obtain the performance parameters by Aperture Integration-Surface Integration (AI-SI) method. The results obtained are compared with those achieved by pure radome optimization, showing the effectiveness of the proposed method.

Knoware and hardware of boresight error compensation in «antenna – radome systems»

We consider solutions to improve the bearing and tracking accuracy of on-board radar objects by radars with “antenna – radome” systems. We developed algorithms for instrumental boresight error compensation according to the nodal data. We propose a method of forming the nodal data using the results of experimental measurements performed with the help of automated systems. Moreover, we give practical recommendations for the method implementation. On the basis of the difference matrices we developed a design procedure for errors in presenting the data by different methods and obtained analytical relations for the informed choice of the form of compensating arrays.

Novel approximation of misalignment fading modeled by Beckmann distribution on free-space optical links.

A novel accurate and useful approximation of the well-known Beckmann distribution is presented here, which is used to model generalized pointing errors in the context of free-space optical (FSO) communication systems. We derive an approximate closed-form probability density function (PDF) for the composite gamma-gamma (GG) atmospheric turbulence with the pointing error model using the proposed approximation of the Beckmann distribution, which is valid for most practical terrestrial FSO links. This approximation takes into account the effect of the beam width, different jitters for the elevation and the horizontal displacement and the simultaneous effect of nonzero boresight errors for each axis at the receiver plane. Additionally, the proposed approximation allows us to delimit two different FSO scenarios. The first of them is when atmospheric turbulence is the dominant effect in relation to generalized pointing errors, and the second one when generalized pointing error is the dominant effect in relation to atmospheric turbulence. The second FSO scenario has not been studied in-depth by the research community. Moreover, the accuracy of the method is measured both visually and quantitatively using curve-fitting metrics. Simulation results are further included to confirm the analytical results.

The calibration method of phased array seeker with the phantom-bit technology

This paper studied the influence of beam point error of Phased Array Strap-down seeker (PASS)and the calibration method. The Last End-off Phase Feeding method and the Round-off Phase Feeding method have been illustrated and analyzed to discuss the reason of the bore-sight error (BSE). The calibration error would influence the bore-sight because of oscillation of BSE. With ignoring the system noise, a method which used the digital phase shifter code has been put forward. The study showed that using the digital phase shifter code to calibrate can reduce the bore-sight error which induced by virtual phase shifting technology effectively.

Aberration and boresight error correction for conformal aircraft windows using the inner window surface and tilted fixed correctors.

A static solution to aberrations and boresight error for tilted conformal aircraft windows at different look angles is reported. The solution uses the inner window surface to correct the window aberrations at a 0° look angle and uses fixed correctors behind the window to correct the residual window aberrations at other look angles. Then, the boresight error for the window at different look angles is corrected by tilting the fixed correctors. The principle of the solution is discussed, and a design example shows that the solution is effective in correcting the aberrations and boresight error for a tilted conformal aircraft window at different look angles.

Impact of nonzero boresight pointing error on ergodic capacity of MIMO FSO communication systems.

A thorough investigation of the impact of nonzero boresight pointing errors on the ergodic capacity of multiple-input/multiple-output (MIMO) free-space optical (FSO) systems with equal gain combining (EGC) reception under different turbulence models, which are modeled as statistically independent, but not necessarily identically distributed (i.n.i.d.) is addressed in this paper. Novel closed-form asymptotic expressions at high signal-to-noise ratio (SNR) for the ergodic capacity of MIMO FSO systems are derived when different geometric arrangements of the receive apertures at the receiver are considered in order to reduce the effect of nonzero inherent boresight displacement, which is inevitably present when more than one receive aperture is considered. As a result, the asymptotic ergodic capacity of MIMO FSO systems is evaluated over log-normal (LN), gamma-gamma (GG) and exponentiated Weibull (EW) atmospheric turbulence in order to study different turbulence conditions, different sizes of receive apertures as well as different aperture averaging conditions. It is concluded that the use of single-input/multiple-output (SIMO) and MIMO techniques can significantly increase the ergodic capacity respect to the direct path link when the inherent boresight displacement takes small values, i.e. when the spacing among receive apertures is not too big. The effect of nonzero additional boresight errors, which is due to the thermal expansion of the building, is evaluated in multiple-input/single-output (MISO) and single-input/single-output (SISO) FSO systems. Simulation results are further included to confirm the analytical results.

基于倾斜光学元件的共形光学窗口瞄视误差校正

基于倾斜光学元件的共形光学窗口瞄视误差校正

Alignment of a three-mirror anastigmatic telescope using nodal aberration theory.

Most computer-aided alignment methods for optical systems are based on numerical algorithms at present, which omit aberration theory. This paper presents a novel alignment algorithm for three-mirror anastigmatic (TMA) telescopes using Nodal Aberration Theory (NAT). The aberration field decenter vectors and boresight error of misaligned TMA telescopes are derived. Two alignment models based on 3rd and 5th order NAT are established successively and compared in the same alignment example. It is found that the average and the maximum RMS wavefront errors in the whole field of view of 0.3° × 0.15° are 0.063 λ (λ = 1 μm) and 0.068 λ respectively after the 4th alignment action with the 3rd order model, and 0.011 λ and 0.025 λ (nominal values) respectively after the 3rd alignment action with the 5th order model. Monte-Carlo alignment simulations are carried out with the 5th order model. It shows that the 5th order model still has good performance even when the misalignment variables are large (-1 mm≤linear misalignment≤1 mm, -0.1°≤angular misalignment≤0.1°), and multiple iterative alignments are needed when the misalignment variables increase.

극초음속 비행체의 공기광학 조준오차 예측을 위한 전산해석 연구

Aero-optic phenomena cause the image position displacement on an imaging plane of the airborne optical/IR systems. Particularly, the aero-optic boresight error(BSE) is important factor for homing, positioning and aiming applications of hypersonic flight interceptor missile. In this paper, an estimating method of aero-optic BSE for a hypersonic flight vehicle is studied. A ray tracing method and a transform method of refractive index fields from flow density fields are combined with computational fluid dynamics(CFD) method.

Experimental research of boresight error correction using returning signals from the illuminated targets through atmosphere

The relationship between boresight error and returning signal from the illuminated target is studied theoretically. The operating principle of a new-style adaptive-optics device, which is named adaptive fiber-optics collimator with the abilities to correct both laser beam transmission error and pointing error, is introduced. The laser beam pointing experimental setup with a laser propagation distance of 200 m through horizontal atmosphere is established. Based on returning signals from the illuminated two-dimensional and three-dimensional targets, the beam pointing closed loop using stochastic parallel gradient descent algorithm (SPGD) algorithm is achieved under different initial boresight errors. The experimental results indicate that the evaluation parameters of the residual boresight errors are less than 6% and 10.8% for the two-dimensional target and three-dimensional target, respectively, which are within the tolerance of theoretical analysis. Finally, the influence of SPGD parameter on moving-target pointing is analyzed.

Multiobjective Particle Swarm Optimization of Boresight Error and Transmission Loss for Airborne Radomes

The streamlined shapes of airborne radomes tend to cause severe degradation of the electromagnetic (EM) performance, which could be compensated by properly designing the thickness profile of the radome. Boresight error (BSE) and transmission loss (TL) are the two most important EM characteristics that need to be improved. Previous researches focused on the single-objective optimization of either BSE, or the linear combination of BSE and TL through the conventional weighted aggregation (CWA) method. However, the single-objective optimization is limited in reaching mutual balance between BSE and TL, and could not explain why the optimal solution obtained by optimizing BSE usually has a better TL than the constant thickness radomes. In this communication, the multiobjective particle swarm optimization (MOPSO) combined with 3-D ray-tracing method is employed to optimize the BSE and TL simultaneously for airborne radomes. The simulation results explain why the optimal solution on BSE usually has a better TL; the TL wall phenomenon in the multiobjective space is also explained. Both problems are due to the variation of mean incident angle of the radomes during antenna scanning, which is general for streamlined radomes. Significant conclusions are drawn and could benefit the radome design in engineering applications.

Principle and performance analysis of coherent tracking sensor based on local oscillator beam nutation.

By integrating coherent tracking sensor functionality into a coherent communication receiver, a dual-function receiver with coherent boresight error sensing is developed for tracking in coherent free-space optical communication systems. The sensor principles are analyzed according to optical interference theory, and the boresight error detection algorithm and small signal linear model are derived. Analysis of local-oscillator beam nutation on system performance shows that the best nutation half-angle is 0.5-1 μrad, the noise equivalent angle is less than 0.02 μrad, and the communication sensitivity degradation is 0.2-0.6 dB. This technology opens new avenues for free-space optical communication system design.