Airborne Optical System Research Articles

Influence of 0°-15° attack angle on aero-optical imaging deviation of a blunt-nose vehicle.

A turbulent flow field is created in a vehicle's head during high-speed flight, and this flow field causes the airborne optical system's receiving target images to be displaced, blurred, and jittered. In this study, we examine the impact of a 0°-15° angle of attack on the aero-optical imaging deviation. With the use of modeling and meshing software, we created a model of a conventional blunt-headed vehicle. Computational fluid dynamics calculations were performed using finite element analysis software; the ray equations were solved iteratively by the Runge-Kutta method. Finally, the imaging deviation data were obtained by using reverse ray tracing and tracing stop criteria. The findings demonstrate that, as the angle of attack increases from 0° to 15°, the thickness of the nonuniform flow field above the vehicle flow field cross-section axis gradually increases. As the density of the nonuniform flow field through which light propagates increases, so does the corresponding refractive index and the aero-optical imaging deviation.

Dynamic mode decomposition for aero-optic wavefront characterization

Aero-optical beam control relies on the development of low-latency forecasting techniques to quickly predict wavefronts aberrated by the turbulent boundary layer around an airborne optical system, and its study applies to a multidomain need from astronomy to microscopy for high-fidelity laser propagation. We leverage the forecasting capabilities of the dynamic mode decomposition (DMD) — an equation-free, data-driven method for identifying coherent flow structures and their associated spatiotemporal dynamics — to estimate future state wavefront phase aberrations to feed into an adaptive optic control loop. We specifically leverage the optimized DMD (opt-DMD) algorithm on a subset of the Airborne Aero-Optics Laboratory-Transonic experimental dataset, characterizing aberrated wavefront dynamics for 23 beam propagation directions via the spatiotemporal decomposition underlying DMD. Critically, we show that opt-DMD produces an optimally debiased eigenvalue spectrum with imaginary eigenvalues, allowing for arbitrarily long forecasting to produce a robust future state prediction, while exact DMD loses structural information due to modal decay rates.

Analysis of data inversion accuracy of airborne optical air data system

The inversion accuracy of three-axis true air speed (TAS), angle of attack (AOA), angle of sideslipe (AOS) was analyzed by analyzing the error transmission rules of multi-beam laser measurement. In addition, experiments were carried out to verify the law of inversion accuracy change, and the experimental and simulation results were well consistent. The results show that the inversion accuracy of three-axis TAS are higher when the number of laser beams increase. The inversion accuracy of TAS in the x, y and z directions has different trends with the elevation angle. In order to ensure the three-axis TAS inversion accuracy less than 2 times the measurement accuracy, the elevation angle should be within the range of 20°-70°. The inversion accuracy of angle is related to the values of TAS and AOS rather than AOA, and it become higher as TAS increase. the range of AOS become larger as TAS increase when the inversion accuracy is given. The conclusions of this paper are useful for the optimal design of optical air data system (OADS).

Influence of LOS angle on aero-optics imaging deviation

When the airborne optical system works in the atmosphere, the image received in the optical system is deviation due to the change of gas flow field outside the optical head cover. This paper studies the effect of target line-of-sight(LOS)angle on aero-optics imaging deviation. We modeled and meshed a typical blunt-nosed high-speed vehicle by GAMBIT software. The CFD calculation was carried out by using the integrated RANS solver in FLUENT software. A reverse ray tracing method and a trace stop criterion were developed for optical calculation. The light propagation path was divided into three segments: the inside of the aircraft, the non-uniform flow field outside the aircraft, and the freestream of the atmosphere. Then we studied the influence in a wide range of LOS angles from 5° to 85°. The results show that as the LOS angle increases, the non-uniform flow field density distribution on the light propagation path becomes more and more flat, the corresponding refractive index becomes smaller, and the aero-optics imaging deviation is also reduced gradually.

Probability of fade for airborne laser communication system over exponentiated Weibull distribution under aperture averaging

In this paper, the probability of fade is performed for the airborne laser communication systems considering both the atmospheric and aero-optic effects. The atmospheric fluctuation is characterized by the exponentiated Weibull (EW) fading channel. The novel analytical expression is derived for probability of fade according to Meijer’s G function. The probability of fade is demonstrated to the simulation data with the Gamma–Gamma and log-normal distributions. The probability of fade is obtained for different flight altitudes and propagation distances. The probability of fade is analyzed in the airborne laser communication systems under the effect of aperture averaging in weak-to-strong turbulence regime. Results suggest that the proposed EW model is valid in airborne laser communication with high altitude, especially in the lower values of the irradiance under atmospheric turbulence and aero-optics effect. The fading of outage performance can be effectively mitigated by aperture averaging. Furthermore, this work is helpful for the compensate technique of system performance on airborne optical communication system.

A Study on Camera Array and Its Applications

The reduced cost of cameras and the complex scenes make it possible and necessary to replace the monocular camera with camera array under certain situations. In this paper, we first provide a review on the existing camera arrays and sort them according to the array arrangement, and then give an overview of the imaging properties that are benefited from the camera array including dynamic range, resolution, seeing through occlusions and depth estimation. At last, a novel camera array-based airborne optical system is proposed to meet the intelligence, surveillance and reconnaissance (ISR) requirements for large field of view (FOV), high dynamic range and resolution, multi-view and multiple dimensions imaging. For the sake of onboard application of this system in practice, some key technologies are highlighted that need to be developed in the future research, such as the self-calibration and the synthetic aperture imaging by camera arrays on mobile platforms.

Experimental investigation on aero-optics of supersonic turbulent boundary layers at different light incident angles

The aero-optical distortion caused by the compressibility of high-speed flow field has a great influence on the development of airborne optical detection system of (hypersonic) supersonic vehicles. The turbulent boundary layer is one of the most important aspects in the aero-optical study, and has become one of the hot research points in the field of aero-optical study. The nano-particle-based planar laser scattering technique is used to measure the density distribution of the supersonic (Ma=3.0) turbulent boundary layers, and the optical path difference, which is quite crucial for the aero-optical study, is obtained by ray-tracing method. The experimental result is verified by being compared with the theoretical result computed by the aero-optical scaling method of turbulent boundary layers. Five different light incident angles (α=60°, 75°, 90°, 105°, 120°) are selected and used to examine the influences of light incident angles on the supersonic turbulent layer, and the underlying flow physics is analyzed. Research shows that the light propagation path in the supersonic turbulent boundary layer contributes to the light incident angle dependence of aero-optics. The different propagation paths lead to the difference between the light propagation distance in the flow field and the correlation results of the corresponding density fluctuation. The oblique incidence of light results in the increase of the propagation distance in the flow field, and then the aero-optics turns worse. The greater the angle between the incident direction of light and the vertical direction of the wall, the more significant the aero-optics is, the difference increases at different times, the difficulty in correcting the aero-optics is also increased. In the supersonic turbulent boundary layer, a large number of vortices with a specific orientation lead to the anisotropy of the aero-optics in the turbulent boundary layer. By calculating the spatial two-point correlation of the density fluctuations at the streamwise plane (x-y plane), the cross-correlation result of density fluctuations at any light incidence angle (α=0°-180°) can be obtained. The local coherent structure scale is nearly 0.20 mm, which is basically consistent with the aero-optical effective scale (≈ 0.18 mm) computed from the formula proposed by Mani et al. When the light is inclined downstream, the direction of light propagation is consistent with the vortex structure in the flow field, and in this direction, the correlation coefficient of density fluctuation is larger, so the aero-optics is more serious. When the light beam is tilted upstream, the correlation coefficient is smaller, so the aero-optics is weaker.

Analysis of the Effects of Thermal Environment on Optical Systems for Navigation Guidance and Control in Supersonic Aircraft Based on Empirical Equations.

The thermal environment is an important factor in the design of optical systems. This study investigated the thermal analysis technology of optical systems for navigation guidance and control in supersonic aircraft by developing empirical equations for the front temperature gradient and rear thermal diffusion distance, and for basic factors such as flying parameters and the structure of the optical system. Finite element analysis (FEA) was used to study the relationship between flying and front dome parameters and the system temperature field. Systematic deduction was then conducted based on the effects of the temperature field on the physical geometry and ray tracing performance of the front dome and rear optical lenses, by deriving the relational expressions between the system temperature field and the spot size and positioning precision of the rear optical lens. The optical systems used for navigation guidance and control in supersonic aircraft when the flight speed is in the range of 1–5 Ma were analysed using the derived equations. Using this new method it was possible to control the precision within 10% when considering the light spot received by the four-quadrant detector, and computation time was reduced compared with the traditional method of separately analysing the temperature field of the front dome and rear optical lens using FEA. Thus, the method can effectively increase the efficiency of parameter analysis and computation in an airborne optical system, facilitating the systematic, effective and integrated thermal analysis of airborne optical systems for navigation guidance and control.

Imaging quality evaluation of aerodynamically heated faceted dome based on ray tracing

We investigated the influence exerted by the aerodynamic heating of the faceted dome on imaging quality degradation of an airborne optical system. Optical transmission through the faceted dome was simulated using the ray-tracing program based on a recursive algorithm. To validate the ray-tracing program, a ray trace through the homogeneous faceted dome was performed. The program was shown to possess great accuracy. Three kinds of imaging quality evaluation parameters were presented, wave aberration of the exit pupil, point spread function and modulation transfer function. The results show that the aerodynamic heating of the faceted dome significantly affects the airborne optical system.

An Operational System for Estimating Road Traffic Information from Aerial Images

Given that ground stationary infrastructures for traffic monitoring are barely able to handle everyday traffic volumes, there is a risk that they could fail altogether in situations arising from mass events or disasters. In this work, we present an alternative approach for traffic monitoring during disaster and mass events, which is based on an airborne optical sensor system. With this system, optical image sequences are automatically examined on board an aircraft to estimate road traffic information, such as vehicle positions, velocities and driving directions. The traffic information, estimated in real time on board, is immediately downlinked to a ground station. The airborne sensor system consists of a three-head camera system, a real-time-capable GPS/INS unit, five industrial PCs and a downlink unit. The processing chain for automatic extraction of traffic information contains modules for the synchronization of image and navigation data streams, orthorectification and vehicle detection and tracking modules. The vehicle detector is based on a combination of AdaBoost and support vector machine classifiers. Vehicle tracking relies on shape-based matching operators. The processing chain is evaluated on a large number of image sequences recorded during several campaigns, and the data quality is compared to that obtained from induction loops. In summary, we can conclude that the achieved overall quality of the traffic data extracted by the airborne system is in the range of 68% and 81%. Thus, it is comparable to data obtained from stationary ground sensor networks.

Comprehensive analysis of imaging quality degradation of an airborne optical system for aerodynamic flow field around the optical window

We investigated the influences exerted by the nonuniform aerodynamic flow field surrounding the optical window on the imaging quality degradation of an airborne optical system. The density distribution of flow fields around three typical optical windows, including a spherical window, an ellipsoidal window, and a paraboloidal window, were calculated by adopting the Reynolds-averaged Navier-Stokes equations with the Spalart-Allmaras model provided by FLUENT. The fourth-order Runge-Kutta algorithm based ray-tracing program was used to simulate the optical transmission through the aerodynamic flow field. Four kinds of imaging quality evaluation parameters were presented: wave aberration of the entrance pupil, point spread function, encircled energy, and modulation transfer function. The results show that the imaging quality of the airborne optical system was affected by the shape of the optical window and angle of attack of the aircraft.

Joint influences of aerodynamic flow field and aerodynamic heating of the dome on imaging quality degradation of airborne optical systems

We investigated the joint influences exerted by the nonuniform aerodynamic flow field surrounding the optical dome and the aerodynamic heating of the dome on imaging quality degradation of an airborne optical system. The Spalart-Allmaras model provided by FLUENT was used for flow computations. The fourth-order Runge-Kutta algorithm based ray tracing program was used to simulate optical transmission through the aerodynamic flow field and the dome. Four kinds of imaging quality evaluation parameters were presented: wave aberration of the exit pupil, point spread function, encircled energy, and modulation transfer function. The results show that the aero-optical disturbance of the aerodynamic flow field and the aerodynamic heating of the dome significantly affect the imaging quality of an airborne optical system.

Stress-relieved assembly method for a high-resolution airborne optical system

In this manuscript, we report on assembly issues of a new high-resolution airborne optical system (HRAOS), which consists of front-end optics, two after-end optical channels (electro-optical and infrared channels), and a connection module. The beam splitter (BS) plate in the connection module divides the output beam from the front-end optics by 50:50 and feeds into the after-end optical channels. The BS plate has a 116-mm elliptical shape on the major axis while the thickness is only 8 mm to meet the weight limitation of the system. As a result, a small amount of stress on the BS plate causes a relatively large deformation and ultimately leads to a serious deterioration of the image quality. To resolve this problem, we suggest a new assembly method (a four-point-bonding method) and verify it by using a finite-elements analysis. After the proposed method had been applied, the final wavefront error of the entire optical system showed a rms (root-mean-square) value of 66 nm. The error of a previous result was about 317 nm. Thermal effects were also observed.

航空光学成像与测量系统的电磁兼容技术

Electromagnetic compatibility is essential to airborne device.It is directly related to the normal and safe operation of the whole aircraft system.The integrated electromagnetic environment was introduced for the airborne optical imaging and measurement system.Some effective measures were adopted,such as reducing interference source,sorting cables,improving ground system,shielding technology,optimizing the wiring structure of the circuit board and so on,so that the airborne optical imaging and measurement system met the electromagnetic compatibility requirement of the industry standards.All equipment can operate normally,ensuring the safety of the whole aircraft system.

Generic method for aero-optic evaluations.

The effect of aerodynamic flow on the performance of an airborne optical system is becoming a critical issue in the development of electro-optic systems. A novel technique for aero-optic calculations that is based on commercially available software is presented. The optically relevant data from the computational fluid dynamics results are transformed into an index-of-refraction field and introduced as an input to the optical code. The data do not necessarily have to be presented in analytical form; instead it is introduced in a most general form as a discrete set of values located at a nonuniform grid of points. The modified quadratic Shepard method has been adopted for data interpolation, enabling a simple interface with virtually any software output. Several numerical simulations that demonstrate the technique are presented.

Airborne Optical System for Remote Sensing of Ocean Waves

Abstract A compact, turret-based optical system has been designed and constructed for passive imaging of ocean waves using a small aircraft. The purpose of the system is to collect time series of images, which are mapped to a common geodetic surface in order to extract the space–time characteristics of the waves. This is achieved by staring at a fixed geodetic location and by accurately measuring the imaging geometry. The system was designed to be compact and lightweight for future installation on an unmanned aerial vehicle and has been installed on a small, manned, single-engine aircraft for testing and experimentation. Initial tests have confirmed successful operation of the staring capability at moderate distances to a fixed target array on land and to the nearshore region with shoaling gravity waves. Images were mapped to a common reference frame on a geodetic surface at the mean ocean level, and a three-dimensional spectrum of the space–time data cube of the mapped image modulations exhibits the expe...

Multizonal airborne optical system for oceanographic research and ecological monitoring

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text V. V. Murashov, V. G. Lyskov, V. M. Samkov, V. I. Solov'ev, and V. N. Pisarev , "Multizonal airborne optical system for oceanographic research and ecological monitoring," J. Opt. Technol. 68, 52- (2001) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Polarization: A Key to an Airborne Optical System for the Detection of Oil on Water

Skylight polarization, which varies with the position of the sun in the sky, influences the contrast of oil on water. Good contrast is most consistently obtained by viewing in azimuth directions toward or away from the sun. Contrast is enhanced by imaging selected polarization components and by taking the difference between orthogonal polarization components.