| The streak array detecting lidar has many advantages,such as long working distance,wide field of view,large depth of field and high data rate,consequently it has important application prospect in long distance target 3D(three-dimentional)reconstruction and high altitude airborne wide mapping.A completed theoretical model of range accuracy has not been established,meanwhile either the technical means to improve the range accuracy or the optimization of the three-dimensional mapping technology based on the imaging characteristics of the system is lacking in the existing research work.For these problems,this paper makes theoretical and experimental research on the range accuracy and 3D mapping technology of the streak array detecting lidar.Based on the ranging principle and cascade imaging process of the streak array detecting lidar,a theoretical expression of the signal distribution function in a single time-resolved channel was deduced in company with the lidar function and the line spread function of the detector in this paper.The expression shows that the distribution function of the signal intensity follows a Gaussian distribution,and its streak width is determined by the laser pulse width and the line spread function of detector.The noise characteristics of the radar system were classified and discussed according to the correlation between the noise and the signal intensity.The theoretical expressions of mean and variance of multiplicative noise and additive noise were given respectively,and then the theoretical model of the total signal was established.These would provide the necessary theoretical basis for the range accuracy analysis and detection process research.Considering the imaging characteristic of the streak array detecting lidar,the constant output power and near saturation imaging modes are established in the signal detection process to obtain ideal original streak images with different detection distance and application requirement.According to the signal and noise characteristics of the system,the theoretical model of the range accuracy is established by the error transfer theory using the centroid weight algorithm as the signal identification.The expression of range error caused by additive noise and multiplicative noise and the expression of sampling error were deduced in two different working modes.The theoretical model of range accuracy was simulated and verified experimentally,using the re sidual jitter analysis method of the plane target distance imaging.In the simulation verification,the relationship between the three kinds of main errors and the key parameters of the system was discussed by simulating the process of laser emission,rece ption and the signal acquisition process of the detector.In the experimental verification,a streak array detecting lidar principle verification system was set up which could select and control the noise source,and the influence of the streak width on th e range accuracy was emphatically discussed.The results showed that in the constant output power mode,the error caused by multiplicative noise increases linearly with the increase of streak width while the error caused by additive noise does not change w ith the streak width;in the near-saturation imaging mode,the error caused by the multiplicative noise is proportional to the square root of the streak width while the error caused by the additive noise is inversely proportional to the streak width.Based on the theoretical model of range accuracy,four kinds of range accuracy optimization methods were proposed to improve the range discrimination capability and target recognition ability of the system.(1)The method of parameter optimization based on optimal streak width was studied.The theoretical expression and numerical results of optimum streak widths under different operating modes were given.The time and spatial axes separation method of focusing voltage was used to achieve the independent control of the time axis extension function under the premise of ensuring the highest spatial resolution of the detector,further obtain the echo signal with the optimal streak width in the experiments.After the optimization,the system could reconstruct the distance image of the Long-distance target at 1.7 km under 173 m range gate in decimeter level.The range rms error was reduced to 0.19 m.(2)An optimal threshold method was proposed in the distance extraction of streak images.The effect of threshold value on the accuracy of range was discussed under different streak widths and noise intensities based on the simulation results.The empirical formula of optimal threshold was established.The influence of strong background noise to the range image was suppresse d by the optimal threshold method in the field mapping.(3)The optimal value of the time bin size was discussed.The simulation and experimental results showed that if the distance gate width is enough to cover the measured depth of field,the range accuracy could be increased by decreasing the time bin size.In the indoor small depth of field measurement experiment,the system range rms error was able to be reduced to 3 mm using the time bin size of 0.07 ns.(4)A super-resolution signal discrimination method based on iterative weighted centroid algorithm was proposed.The mechanism of the algorithm in distance extraction was also analyzed.By introducing the Gaussian weighting factor,the algorithm effectively suppressed the boundary blurring effect caused by the streak blurring of the adjacent channel,and identified the feature of the target at 1.4 km accurately.When the streak width was 4.4 ns,the range rms error of the system was reduced to 0.15 m after 15 iterations.The range accuracy of the mapping result is superior to the minimum distance resolution of the system,so the super resolution imaging of the streak array detecting lidar was achieved.Finally,a set of 3D surveying and mapping system based on aircraft platform was established.At the base of range accuracy optimization method and echo intensity calculation results,the choice of the system working mode and the value of the key parameters at different flight heights were discussed.In order to solve the problem that the mapping width was restricted by the field of view of the detector in airborne linear array scanning system,a new type of sweep scanning system was proposed.The coverage ability of the laser foot on the surveying area under this scanning system was analyzed combined with the digital elevation maps.The range error caused by the inhomogeneity of detector scanning electric field and the horizontal positioning deviation caused by the non-linearity of the laser foot-scanning trajectory were corrected.After the optimization and calibration,the high-precision airborne wide mapping with range gate of 173 m and data rate of 500 k Hz was achieved by the system.The range rms error could reach 0.11 m and 0.16 m under the altitude of 3000 m and 5800 m respectively.The required mappi ng detecting time for 30 km2 plain areas was 2 min and 13 s,which mean that the mapping efficiency was improved by nearly 10 times compared to the traditional push-broom scanning system. |
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