| The spaceborne laser altimeter is an active remote sensing observation system,which can record the original echo waveform of the laser altimeter,obtain the laser ranging value by processing and extracting various parameters of the received waveform,combined with the attitude of the onboard system,The position and pointing information can correctly obtain the position information and height distribution of the measured object.For example,the digital elevation model(DEM)can be obtained by satellite-mounted laser altimeter and orbiting the earth.In November 2019,China's first sub-meter high-resolution optical transmission stereo mapping satellite Gaofen No.7(GF-7)was successfully launched.It was equipped with two loads of dual line array camera and full waveform laser altimeter.To improve the measurement accuracy of the sparse control point area represented by the western area and the area without control points outside the country,high-precision three-dimensional coordinates of the laser footprint are required as elevation control points.Therefore,in order to improve the GF-7's three-dimensional mapping accuracy and geometric positioning ability,use the waveform matching algorithm to improve the positioning accuracy of the laser footprint,and design a more accurate and faster waveform simulator and matching method to find the true coordinate position of the spot center is needed in this paper.The main problem solved.Based on GF-7 spaceborne laser altimeter,the paper first introduces its system composition and working principle.The simulation accuracy of the echo waveform directly affects the positioning accuracy of the laser matching point of the waveform.The echo waveform simulation is a necessary condition and application premise of the waveform matching;therefore,the paper sequentially discusses the mathematical analytical formulas of flat terrain,stepped terrain,and slope terrain.As well as the characteristics of the digital elevation model DEM,the high-precision DEM data obtained by GF-7 is used as the target response function.The basic equation and specific implementation method of the simulated echo are derived using the convolution relationship between the transmitted pulse function and the target surface model function.Then,the significance of the GF-7 laser elevation control point is discussed,which leads to the importance of the high-precision positioning algorithm of the laser footprint,and the principle of the rough positioning of the laser footprint and the influencing factors of the error are discussed;For the location of the geolocation,a laser footprint geolocation algorithm based on high-precision waveform matching is proposed.The waveform matching algorithm is derived in detail and its accuracy optimization and time complexity optimization are made.Through the experimental verification and analysis of satellite measured data,this paper uses high-precision DEM of different scales as the target response function to simulate the echo waveform.The experimental results show that: To optimize the efficiency of the algorithm and improve the positioning accuracy,DEM in urban areas and mountainous areas It is most appropriate to use a grid interval of 2.0m * 2.0m.The echo waveform simulated by the DEM with an interval greater than 4.0m * 4.0m will cause waveform distortion.Furthermore,the echo waveform simulation was carried out under the condition of fixed DEM grid interval,and the measured waveform of the satellite laser altimeter was used to match the simulated waveform.By setting the matching threshold,using the internal rotation traversal algorithm designed in this paper,the time efficiency has been improved by more than ten times;by comparing with the results of Harding and other scholars,the positioning accuracy of the laser footprint point is within 3m,and the ratio of urban area to single mountain area Complex mountain areas make it easier to find the exact coordinate position of the laser footprint. |
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