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Airborne Optical Imaging and Measurement Technology
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New emerging technologies in airborne optical imaging and measurement
Abstract：Airborne optical imaging and measurement technology has prominent advantages such as high timeliness, low cost, high resolution and easy interpretation. It has been widely applied as an indispensable optical remote sensing method. Due to the influence of aviation flight, in order to obtain clearer optical images and more accurate measurement, the problems should be solved such as aerodynamic, vibration, temperature and pressure changing under the constraints of limited volume, weight and power consumption. To meet the demand of new technologies, new methods and new theories in this field, a special issue of airborne imaging and measurement was organized. A number of excellent research results were introduced in the fields of precision optics, precision mechanics, precision control and image processing. It promotes the further development of relative research from the perspective of theoretical guidance and engineering reference value.Keywords：airborne imaging and measurement;optics;mechanics;control;image processing- 405
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Published：2020-07-13 -
Influence of aviation low temperature on auto-collimating focusing system
Abstract：To analyze the influence of aeronautical low temperature on the self-collimation focus detection system, and enables the design and correction of the actual focus detection system, the relationship among various factors under different imaging conditions was investigated. Based on the principle of autocollimation focus detection and geometry, comparison of single-point and fringe imaging, imaging position and width according to different received light intensities, and presence and absence of astigmatic conditions were analyzed. Thus, a related analysis formula was deduced.Combined with the actual camera working conditions, the corresponding simulation of the optical system was performed, and the simulation results (simulated bimodal voltage values of 1.4 V and 0.47 V) were in accordance with the actual test data (peak-to-focus double-peak voltage values of 1.38 V and 0.56 V). The actual modification measures are carried out to improve the temperature adaptability of the self-collimation detection system.The temperature adaptability analysis is in accordance with the actual demand and has a guiding function for the temperature adaptability of the system.Keywords：aerial camera;auto-collimating focusing;low temperature;single point imaging;fringe imaging- 232
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Published：2020-07-13 -
Theoretical analysis and verification of Ronchi grating auto-collimated focusing for aerial remote camera
Abstract：To address issues such as auto-focusing system fault analysis and component selection, and to improve the focusing accuracy of aerial remote cameras, this article offered insights into the auto-collimated focusing technology based on the Ronchi grating.First, in this paper, several common focusing methods of both domestic and foreign aerial cameras were summarized and compared. Then, the composition and operating principle of optoelectronic auto-collimated focusing systems based on the Ronchi grating were introduced, the mathematical model was established, and the simulation curves of the focusing waveform with different imaging positions were presented. To measure the focal distances, the concept of the defocusing evaluation factor was proposed.Next, the three factors that affect the focusing accuracy of the diffraction effect and the aberration of the optical system, the period selection of the Ronchi grating, and the energy distribution of the light source were analyzed. The influence of the Ronchi grating period selection method was then presented. Finally, the experimental results verify the validity of the simulation results and show that the peak-to-peak ratio of the focusing waveform is 0.82, corresponding to an average focusing detection resolution of 10% at 0.05 mm.Keywords：aerial camera;focusing;auto-collimating;Ronchi grating- 291
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Published：2020-07-13 -
Design of stable aviation platform operated by cable drive
Abstract：Using a tethered balloon as a flight platform for a hyperspectral camera, a two-axis stable platform was designed to compensate the attitude change of the platform in the heading and pitching directions. The azimuth and pitch driving mechanisms of the stable platform were powered by a cable drive. The continuous driving torques of the azimuth and pitch axes were 93.6 N·m and 117 N·m, respectively, and the maximum rotational speeds of the shafts were 25 (°)/s and 20 (°)/s, respectively. The test results show that the transmission accuracy of the stable platform is 5 μrad, and the transmission error is less than 0.7%. The servo bandwidths of the azimuth and pitch axes are 15 Hz and 35 Hz, respectively. The root mean-squared (RMS) errors of the sinusoidal tracking accuracies of the azimuth and pitch axes are 0.0045 and 0.0043, respectively. An outfield flight test was performed with a 35-kg hyperspectral camera mounted on the stable platform pitching frame to conduct air-to-earth observations, and the gyro stability data of the flight test was monitored. The RMS stability accuracy of the azimuth axis (38.83μrad) and the pitch axis (37.26 μrad) met the 50-μrad requirement of the hyperspectral camera.Keywords：aviation imaging;hyperspectral camera;stable platform;cable drive;stability accuracy;tethered balloon- 231
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Published：2020-07-13 -
Effect and compensation of overlap influenced by flight parameter of oblique aerial camera
Abstract：This study analyzes The effects of aircraft's attitude, speed, and height on the overlap of an obliqueaerial camera was analyzed in this study, and acorresponding compensation method was developed. The calculation of oblique imaging overlap using coordinate transformation was introduced based on ageo-location algorithm and the Gauss-Kruger projection. In the case of single parameter variations, different formulas were provided by the geometric method, and the results were consistent between coordinate transformation and geometric methods. In the case where the three parameters varys imultaneously, the geometric method did not work adequately, and the coordinate transformation method should be used. In the case where the varied angle value of the aircraft attitude was below 1°, the deviating value between real covered area and expected area was attained by thecoordinate transformation method. The usage of camera's azimuth and pitch angles to compensate the effect of aircraft attitude on overlap was introduced. For the variation of aircraft speed and height, changing the imaging period according to the aircraft real time values was suggested, and a new formula regarding imaging period was provided.The validity of the compensation algorithm was verified by simulation and real flight. The result shows that when the camera's field of view is below 0.88°, the deviation between mean and expected values of real overlap is approximately 1%, if the target area is flat.Keywords：overlap;flight parameter;oblique imaging;coordinate transformation;imaging period- 276
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Published：2020-07-13 -
System error corrected ground target geo-location method for long-distance aviation imaging with large inclination angle
Abstract：To solve the problem of large geo-location error in the application of ground target geo-location in long-distance aviation imaging with large inclination angle, a systematic error corrected geo-location was proposed. The systematic error analysis and modeling of the long-distance aviation imaging system with large inclination angle was carried out. In view of the difficulty in obtaining the residual error, a method for estimating residual error parameters based on ground control points was proposed.The simulation results show that the residual error can be reduced to 1/10 by estimating residual error parameters based on ground control points. Flight test data confirm that the mean geo-location error can be reduced from 401 m to 97 m by correcting the systematic error in the positioning of targets in long-distance aviation imaging with large inclination angle. Thus, the systematic error corrected geo-location method can effectively improve the accuracy of ground target geo-location in the application of long-distance aviation imaging with large inclination angle.Keywords：aerial camera;geo-location;error correction;geo-location error- 319
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Published：2020-07-13 -
Design of high integration and miniaturization concentric multiscale optical system
Abstract：A highly integrated optical imaging system with a wide field of view (FOV) and high resolution was proposed to meet the performance requirements of modern photoelectric imaging systems. The system consisted of a concentric spherical lens and an array of micro-cameras arranged in the Galilean concentric multiscale structure. A wide FOV was achieved owing to the structure of the spherical lens. The multiscale micro-camera array enabled capturing of images at high resolution and with a wide field of view. Further, the Galilean structure made the system more compact. In addition, the arrangement of the camera array was designed to reduce the number of cameras and realize a lightweight system. By optimizing the design of the whole system, the modulation transfer function (MTF) can reach 0.3 at a frequency of 270 lp/mm.The root mean square (RMS) radius of the full field of view of the system is also smaller than the detector pixel size of 1.85μm, indicating an excellent imaging effect from the system. Moreover, results of the tolerance analysis indicate that the system is easy to manufacture. Thus, the system not only realizes a wide field of view and high-resolution imaging, but also has low structure complexity and a compact volume structure.Keywords：optical system;multiscale structure;Concentric spherical lens;wide field of view and high-resolution imaging- 323
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Published：2020-07-13 -
Design of infrared dual-band/dual-FOV imaging early warning system
Abstract：To address the demand for detection of point/dim targets in complex environments, an infrared dual-band dual-field of view (FOV) imaging early warning system was designed. To improve the target detection capability and environmental adaptability, the system used high-order aspheric surfaces to reduce the number of system lenses and improve the system transmittance. At the same time, it corrects on-axis/off-axis aberrations and advanced aberrations to improve the imaging quality of the system. An optical passive compensation method was adopted to realize an athermalized design in the range of -40 ℃ to 60 ℃. A rotating electromagnet was used as the driving element to complete the zooming in process in a duration of 80 ms to ensure that the target is not lost during the FOV switching process. Using the electric limit, mechanical limit, and magnetic locking mechanism as limit components, the stabilization accuracy of the optical axis wobble is less than two pixels.The design results show that the optical-mechanical structure of the infrared imaging early warning system is reasonable and compact, the imaging quality is good, and it meets the requirements of target detection.The system has potential application prospects in the field of infrared imaging in early warning systems.Keywords：optical design;dual-band system;dual-FOV system;rotary electromagnet drive;optical axis accuracy- 267
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Published：2020-07-13 -
Modeling and application analysis of operating range of air-based infrared system
Abstract：In the existing action distance model of the infrared system, the influence factors are not thoroughly considered.Moreover, it is difficult to accurately evaluate the detection ability of the system and the infrared stealth effect of the target.Therefore, to address these issues, the action distance model and an application case analysis of the air-based infrared system were presented in this work.Based on the analysis of the influencing factors of the typical working distance of the infrared system, the implicit function of the atmospheric spectral transmittance and the working distance were defined. The distribution model of the spectral radiation intensity was clarified based on the temperature difference between the vertical layers of the atmosphere.The weighted correction coefficient of the atmospheric transmittance was deduced, considering the characteristics of the infrared spectral radiation line of the target.Next, the model of the distance between the air-based infrared system and the point target was built, which included the characteristics of the target spectral radiation, the atmospheric temperature, and the height of the infrared system. Based on this model, the detection capability of different types of air-based infrared systems and the infrared stealth performance of different types of targets were compared and analyzed. The results show that for aircraft and missile targets, the design of a large-aperture long-wave infrared system is helpful in enhancing the detection ability of the air-based infrared system regarding the head-on direction of the target.Furthermore, the large-aperture mid-wave infrared system has a stronger detection ability regardingthe rear direction of the target.The infrared stealth and penetration ability of a small-size, low-surface-emissivity, and low-speed flying target is better.Finally, the stealth performance of a target in the stratosphere is better than that of a target in the troposphere and that of a middle-layer flying target.Keywords：air-based infrared system;detection range;model;stealth- 238
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Published：2020-07-13 -
Infrared polarization characteristics on sea surface based on bidirectional reflection distribution function
Abstract：Aiming to determine the polarization characteristics of the sea surface in the infrared wave band, a model of the polarization bidirectional reflection distribution function (pBRDF) for sea surface micro-elements was developed based on the micro-facet BRDF. Considering the effects of emission and reflection from a sea surface on the radiation received by the detector during radiation transmission, a novel model of sea surface infrared polarization characteristics was proposed. The elevation and slope of the sea surface could be captured using the Elfouhaily wave spectra and fast Fourier transforms (FFTs). Then, the degrees of linear polarization of sea surface emission and reflection radiation for different observation and incident zenith angles and for different wind speeds were calculated.Infrared polarization images of the sea surface and a ship were simulated. Through a comparative analysis of simulated data and the data obtained from the literature, the results show that the model established in this paper is suitable for investigating the infrared polarization characteristics of the sea surface. Compared with infrared images based on light intensity information, the infrared polarization images provide more details about the sea surface. Meanwhile, the differences in polarization characteristics between the target and sea surface are clearer, and the contrast between the sea surface and target is higher. The model proposed in this paper is of significance in the detection of maritime targets in the infrared wave band.Keywords：polarized imaging;infrared wave band;sea surface;polarization characteristics;polarized bidirectional reflectance distribution function- 273
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