CN105093925B - Airborne laser radar parameter real-time adaptive adjustment method based on detected terrain characteristics - Google Patents
Airborne laser radar parameter real-time adaptive adjustment method based on detected terrain characteristics Download PDFInfo
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Abstract
A real-time adaptive adjustment method for airborne laser radar parameters based on the characteristics of a detected terrain comprises the steps that an airplane keeps constant altitude and flies linearly at a constant speed, when the altitude and the two-dimensional spatial frequency of the detected terrain vary, a scanning view angle, pulse repetition frequency and scanning frequency setting values are adjusted in real time through optimization calculation, the bandwidth of a laser scanning point cloud can be kept constant, the point cloud density always meets the requirements of a sampling theorem, and distortion of a reconstructed three-dimensional terrain model and data storage and processing difficulties are avoided. The flight parameters and the characteristics of the measured terrain are measured in real time by a laser scanner, an aircraft flight speed measuring instrument and an aerial camera, the terrain elevation change and the two-dimensional spatial frequency are obtained through comprehensive calculation, then the optimal adjusting values of three parameters of the pulse repetition frequency, the scanning frequency and the scanning angle of view of the laser scanner are obtained through optimized calculation, and real-time adjustment is carried out, so that the real-time self-adaptive optimal measurement aiming at the deformation of the measured terrain can be realized.
Description
Technical Field
The invention relates to a method for optimizing and designing the working parameters of an airborne laser radar and adjusting the working parameters in a real-time self-adaptive manner according to the characteristics of a detected terrain.
Background
The airborne laser radar three-dimensional imaging technology is a new technology in the field of terrain mapping at present, and can generate a digital surface model of a measured terrain so as to reproduce a real measured terrain without distortion.
The measured terrain can be regarded as an elevation function on a two-dimensional plane, so that the scanning measurement process of the airborne laser radar on the measured ground is equivalent to a two-dimensional discrete sampling process.
From the two-dimensional sampling theorem in the reference (Digital Signal and Image processing translation edition/(U.S.) Tamal box, wu zhen, translation of "zhou lin", etc.. Digital Signal and Image processing. advanced education press, beijing, 2006.7, p138), the two-dimensional sampling formula can be expressed as:
Therefore, in order to recover the measured terrain without distortion, the basic premise is that the density distribution of the laser point cloud scanned on the ground meets the requirement of the two-dimensional sampling theorem. But often the terrain spatial frequency is complex, as in literature (smith. spatial data and spatial analysis uncertainty principle. science publishers, beijing, 2005.6, p144), "terrain in nature has two basic types, highly varying terrain (rich in high frequency content) and gently varying terrain (rich in low frequency content). Any complex terrain can be considered a composite of the two situations, except that the two types are contained in different proportions ". Thus, different terrains are composed of different frequencies, and different sampling frequencies should be adopted for different terrains. The airborne laser radar has the following two advantages by adopting different sampling frequencies: on one hand, from the aspect of terrain reconstruction, the real terrain can be reproduced without distortion; on the other hand, the system can achieve the highest use efficiency in terms of the efficiency of the whole laser scanning system, such as the maximum service life of the laser, the minimum data acquisition and storage system data amount, the highest efficiency and the like.
In addition, in the actual operation process of the airborne laser radar, the aircraft is designed to have a fixed absolute flying height (namely, an altitude), but the relative flying height between the aircraft and the measured terrain surface changes along with the elevation change of the measured terrain. The scanning bandwidth of the ground laser point cloud is changed due to the change of the relative flying height of the airborne platform and the terrain surface, and the scanning point number of each line is unchanged, so the density of the point cloud is changed due to the change of the laser scanning bandwidth. When the relative flying height is increased, the scanning bandwidth of the laser point cloud is increased, so that the distance between laser points in each line is increased, the density of the point cloud is reduced, the spatial resolution of laser sampling data is reduced, and the distortion of a reconstructed three-dimensional terrain model is increased; on the contrary, when the relative flying height is reduced, the scanning bandwidth of the laser point cloud is reduced, and although the laser point distance in each row is reduced and the point cloud density is increased, the precision of the subsequent three-dimensional imaging can be improved, the overlapping area between the scanning bands is reduced, and even a missing scanning area appears; meanwhile, too high point density causes data storage and processing difficulty, massive point cloud data can possibly cause hard disk storage, data redundancy is caused, and meanwhile, data processing time is long, efficiency is low, and the like. Therefore, the parameters need to be adjusted in real time according to the elevation change of the terrain to keep the bandwidth and the point density of the scanning point cloud stable and unchanged.
In conclusion, the parameter setting value of the airborne laser radar is adjusted in real time according to the characteristics of the detected terrain, namely the terrain elevation change and the terrain spatial frequency change, so that the airborne laser radar can reach the optimal scanning state, the optimal working efficiency and the accuracy of the reconstructed terrain model are obtained, and the method has important practical significance.
According to the working principle of the airborne laser radar, the factors influencing the laser point cloud density mainly include the flying height and flying speed relative to the ground for an airplane; for laser scanners, mainly the scanning field angle, the scanning frequency and the pulse repetition frequency. Generally, for the purpose of flying efficiency and safety, the flying height and speed of an aircraft are determined and fixed, depending on the performance of the aircraft, and only the set values of the three parameters of the scanning angle of view, the scanning frequency, and the pulse repetition frequency in the laser scanner may be changed.
In the current airborne laser radar, the working parameter setting value of the laser scanner is generally fixed, namely, after the flight is started, only one single parameter working mode exists, and the single parameter working mode cannot be changed in a self-adaptive mode.
Therefore, the purpose of the application is to provide an airborne laser radar parameter real-time adaptive adjustment method based on the characteristics of the measured terrain, various information about flight parameters and the characteristics of the measured terrain is obtained by setting various airborne measurement loads, the elevation change and the spatial frequency characteristics of the measured terrain are analyzed, the three parameter setting values of a laser scanner, namely a scanning view angle, a scanning frequency and a pulse repetition frequency, are adjusted in real time according to the fluctuation characteristics and the complexity of the terrain by adopting an optimal design and an adaptive control mode, so that the purpose of adaptively changing the bandwidth density of point clouds is achieved, the terrain sampling always meets the sampling theorem key sphere, the sampling data quantity is not too large, and the data acquisition state of optimal balance is achieved.
Disclosure of Invention
Aiming at the lack of a method and a device for real-time self-adaptive adjustment of airborne laser radar parameters according to terrain change characteristics in the existing airborne laser radar measurement technology, the invention provides a method and a device for real-time self-adaptive adjustment of airborne laser scanning parameters based on the characteristics of a detected terrain, and aims to change three parameter values of an airborne laser radar, such as a scanning field angle, a scanning frequency and a pulse repetition frequency, in real time when the characteristics of the detected terrain change, so that the distribution and the density of laser scanning point clouds reach an optimal state.
The invention provides a real-time self-adaptive adjustment method and device for airborne laser radar parameters based on the characteristics of a measured terrain, which are characterized by comprising a terrain characteristic and flight parameter measuring device (1), a laser scanner parameter self-adaptive design controller (2), a scanner parameter adjusting device (3) and an airborne platform (4). The device (1) for measuring the topographic characteristics and the flight parameters comprises a laser scanner (11), an aircraft flight speed measuring instrument (12) and an aerial camera (13). The scanner parameter adjusting device (3) comprises a pulse repetition frequency adjusting device (31), a scanning frequency adjusting device (32) and a laser scanning field angle adjusting device (33). The airborne platform (4) is an installation platform for various loads, and the terrain characteristic and flight parameter measuring device (1), the laser scanner parameter adaptive design controller (2) and the scanner parameter adjusting device (3) are all fixed on the airborne platform (4). The laser scanner parameter self-adaptive design controller (2) obtains the measurement data of the laser scanner (11), the aircraft flight speed measuring instrument (12) and the aerial camera (13), obtains the characteristics of the measured terrain, such as two-dimensional terrain spatial frequency and elevation information, through optimization calculation, obtains the three working parameters of the laser scanner, namely the optimal adjustment values of pulse repetition frequency, scanning frequency and scanning view angle, outputs control signals, and respectively controls the pulse repetition frequency adjusting device (31), the scanning frequency adjusting device (32) and the laser scanning view angle adjusting device (33) to realize the real-time adjustment of the setting values of the three working parameters. The airborne platform (4) keeps constant altitude and flies in a straight line at a constant speed, when the measured terrain has elevation change, the relative flying height between the airborne platform (4) and the surface of the measured terrain changes, so that the scanning point cloud bandwidth and the point density of the airborne laser radar correspondingly change, and the scanning point cloud bandwidth and the point density can be kept constant by adjusting the scanning field angle and the pulse repetition frequency. When the spatial frequency of the detected terrain changes, the sampling frequency of laser scanning in the two-dimensional direction can be changed by adjusting the scanning frequency and the pulse repetition frequency setting value in real time, so that the point cloud density of the laser scanning always meets the requirement of the sampling theorem, and the distortion of a subsequent reconstruction three-dimensional terrain model is avoided.
The aircraft flight speed measuring instrument (12) adopts a pitot tube airspeed instrument for measurement, wherein the pitot tube (121) is horizontally placed, the potential pressure is zero at the moment, and the aerodynamic pressure measured by the pitot tube is the total pressure minus the static pressure. The differential pressure type pressure sensor (122) drives the dynamic pressure PvConversion to a voltage output, e.g. differential bellows differential self-sensing displacement transducer, by
Where ρ is the air density, the flying speed v can be found. The laser ranging value when the laser scanner is superposed with the nadir line, namely the laser ranging value when the scanning angle is zero, is used as the relative flying height value between the airborne platform (4) and the surface of the measured land. Using laser scanningLaser ranging data of the instrument from the first few seconds and image data obtained by an aerial camera are calculated to predict the two-dimensional terrain spatial frequency of the terrain to be measured in the flight direction and the transverse scanning direction, which is denoted as fgxAnd fgy。
The plane flies in a straight line at a constant speed and keeps the altitude constant during flying, but when the measured terrain has elevation change, the relative flying height between the plane and the measured terrain surface can be changed, so that the bandwidth of the scanning point cloud is changed. Assuming that the scanning frequency and the pulse repetition frequency of the laser scanner are unchanged, when the measured terrain is higher, the relative height between the airplane and the measured terrain is smaller, the scanning point cloud bandwidth is narrower, and the point density of the laser point cloud is higher; conversely, when the measured terrain is lower, the relative flying height is larger, the scanning point cloud bandwidth is wider, and the point density is lower. The narrowing of the bandwidth of the scanning point cloud can reduce the overlapping degree between adjacent bandwidths, and can seriously cause the missing scanning of the tested terrain, thereby causing the sampling failure and the local reconstruction loss of the tested terrain. Too high point cloud density can cause data redundancy, which is not beneficial to improving the storage and processing efficiency of data, and too low density can cause the distortion of a reconstructed terrain surface model to be increased, so that the laser scanning parameters need to be optimized and adjusted in real time according to the terrain features.
The laser pulse emitter (311) emits laser pulses, the laser pulses reach the rotating prism (332) after being reflected by the reflecting mirror (331), the laser pulses are emitted to the ground after being reflected by the rotating prism (332), the rotating prism motor (321) drives the rotating prism (332) to rotate, and the rotating prism photoelectric axial angle encoder (333) measures the real-time rotating angle of the rotating prism (332). The pulse repetition frequency adjusting device (31) generates a square wave signal with a required frequency, and provides the square wave signal to the laser pulse emitter (311), so that the adjustment of the laser pulse repetition frequency can be realized. The scanning frequency adjusting device (32) can output a control signal, and the scanning frequency can be adjusted by changing the rotating speed of the rotating prism motor (321). The laser scanning field angle adjusting device (33) can measure the real-time rotation angle of the rotating prism (332) according to the rotating prism photoelectric shaft angle encoder (333), and the scanning field angle is adjusted by adjusting the size of the scanning angle of the rotating prism (332) for effectively emitting laser pulses.
When the terrain height changes, the relative flying height between the airplane and the measured terrain surface changes, and the purpose of adjusting the parameters of the laser scanner is to keep the scanning point cloud bandwidth and the point density on the ground constant all the time by adjusting the scanning field angle and the pulse repetition frequency. Let the ideal relative flying height between the plane and the ground to be measured be HEThe ideal scanning field angle is +/-thetae(°) when actual flying height is HAAt the time, the scanning angle of view becomes + -thetaa(°) requirement for constant scan bandwidth, HE·tan(θe)＝HA·tan(θa) Then, then
Further, when the rotation speed of the rotating prism (332) is set to n (revolutions per minute), the angle H is set toE·tan(θe)＝HA·tan(θa) The speed is 60n (DEG/s), and the number m of scanning points of each scanning line is constant, then the ideal relative flying height is HEThe scanning field angle is +/-thetaeAt deg. the scan time for obtaining a laser scan line is
The pulse repetition frequency at the ideal relative flying height is
Similarly, when the actual relative flying height is HAThe scanning field angle is +/-thetaaAt an angle of (°) at which the pulse repetition frequency is
Then
Therefore, when the height of the geodetic form to be measured changes, the formula
Real-time adjustment values of the scanning field angles can be obtained; re-routing type
Real-time adjustment values of the pulse repetition frequency can be obtained, so that adverse effects of terrain elevation changes on laser scanning bandwidth and spot density can be eliminated.
According to the sampling theorem, the highest frequency omega of the measured terrain is firstly obtained to reconstruct the three-dimensional image of the measured terrain without distortionhThe parameter design value of the airborne laser radar can enable the ground sampling frequency to meet omegas＞2ωhThe ground sampling frequency depends mainly on the density of the point cloud. Different terrains have different volatility and complexity, namely different terrain spatial frequencies, different sampling frequencies are adopted in the scanning and mapping process of the laser radar, and a three-dimensional model of the measured terrain is reconstructed with optimal sampling efficiency, so that airborne laser radar parameter values need to be adaptively designed according to the spatial frequency characteristics of the measured terrain.
The three parameters of the laser scanner, namely the real-time adjustment values of the scanning field angle, the scanning frequency and the pulse repetition frequency are calculated and obtained according to the flight speed, the relative flight height and the spatial frequency characteristics of the measured terrain. For the rotating prism line scanning method, let the scanning frequency be n, the flying speed of the airplane be v, the relative flying height be h, and the laser pulse repetition frequency be frThe field of view is fov (field of view), and the number of scanning points of a line scanned by the laser is m (i.e. f)rN), the dot pitch in a row of scanning lines is
The line spacing is
Wherein the relative flying height h and the flying speed v are constant, and the variable parameters are the angle of view fov, the scanning frequency of the scanning mirror n and the laser pulse repetition frequency frAt this time, the process of the present invention,the point cloud density is:
when the sampling frequency of the laser point cloud is more than twice of the spatial frequency of the measured terrain, the sampling theorem is satisfied, and the reconstructed terrain three-dimensional model cannot be distorted. If the planar sample is decomposed into two one-dimensional samples, when the sampling theorem condition is satisfied, there are:
wherein f isgxAnd fgySpatial frequencies of the ground under test in the x-direction and the y-direction, respectively, then
Due to the fact that
Therefore, the method comprises the following steps:
then n is not less than 2 v.fgyAnd is and
in practice, to increase the sampling frequency, the sampling frequency is generally 3-5 times the spatial frequency of the terrain to be measured, and if the sampling frequency is 3 times, then when the ideal relative flying height is HEActual relative flying height of HAThe flying speed is v, and the ideal scanning field angle is +/-thetaeThen the actual scan field angle is + -thetaaScanning frequency n, pulse repetition frequency frThe calculation formula of (a) is respectively:
n＝3v·fgy
further, it is known that the pulse repetition frequency adjustment value required for the change of the height of the geodetic object is set to
Thus, the actual pulse repetition frequency may be takenrAnd faThe maximum value of (a) may then satisfy the pulse repetition frequency adjustment required by both terrain elevation changes and terrain spatial frequency changes. In summary, if the flying speed of the airplane, the flying height relative to the ground, and the spatial frequency f of the terrain in two-dimensional directions are obtainedgx、fgyAnd then, the three parameter values of the laser scanner, namely the scanning field angle, the scanning frequency and the pulse repetition frequency, can be subjected to real-time adaptive optimization design.
Wherein, the scanning point data of the laser scanner in the first few seconds is combined with the front image data of the aerial camera to obtain the two-dimensional space frequency, i.e. f, of the measured terraingxAnd fgy。
The method for predicting the terrain spatial frequency from the previous data of the laser scanner comprises the following steps: in the laser point line along the x direction or one laser scanning line along the y direction, the topographic variation frequency is represented by the following three indexes: (1) the angle of change: from left to right, the extension line of the connecting line of the previous two points is taken as a datum line, the included angle of the connecting line rotating to the next point is calculated, wherein the anticlockwise rotation angle is set to be positive, the clockwise rotation angle is set to be negative, and the average value of the sum of absolute values of all the positive rotation angles and the negative rotation angles in one scanning line is the sum value, namely
The larger this mean value, the larger the frequency variation of the terrain in this scan line direction. (2) Number of positive and negative mutations in the corner (denoted w): namely, the change times of the turning angle from positive to negative and from negative to positive can reflect the magnitude of the terrain spatial frequency, and the following procedures are used for statistics:
w＝0
for j＝1,n
ifθj·θj+1＝-1
w＝w+ 1
end
(3) maximum value | θ of absolute values of positive and negative rotation angles in two-dimensional directionsx|maxAnd | θy|maxIt reflects the intensity of the terrain fluctuation, and the larger this value, the more intense the terrain fluctuation.
The terrain spatial frequency coefficients obtained from the previous data of the laser scan point cloud can be expressed as:
wherein eta isxAnd ηyIs an empirical scaling factor, wxAnd wyThe positive and negative mutation times of each corner in the laser scanning line in the x direction and the y direction are respectively.
The method for analyzing the terrain spatial frequency by acquiring the terrain texture characteristics from the previous image acquired by the aerial camera comprises the following steps: the gradient map is obtained from the image obtained by the aerial camera, the gradient reflects the steepness of the slope of each point on the image, and if the gradient value at the edge of the object is larger, the gradient map is obtained. According to the gradient, the distribution condition of different objects in the image can be judged. The gradient values of all pixel points in the image are added to obtain an average value, and the gradient average value can reflect the change degree of the image texture. The larger the gradient mean, the larger the image texture variation and the larger the possible spatial frequency of the terrain. In image processing, the gradient calculation of a certain pixel point can adopt the following formula:
|. f (x, y) | ≈ max [ | f (x, y) -f (x +1, y) |, | f (x, y) -f (x, y +1) | ], i.e., the gradient value at a certain pixel point is the maximum of the absolute value of the difference between the adjacent pixels in the horizontal direction and the absolute value of the difference between the adjacent pixels in the vertical direction at the point.
In one image, the mean value of the gradient values of all the pixels is:
wherein, synthesize by laser point cloud data and aerial camera's image data, the topography spatial frequency coefficient who obtains can calculate as:
in practical application, experience estimation is carried out according to a scanning area before flight, and experience scanning frequency and terrain spatial frequency coefficient in two-dimensional direction are obtained, namely
And
and
then, the real-time estimated frequency value of the terrain in the two-dimensional direction in the actual flight process can be obtained, and the calculation is as follows:
the airborne laser radar parameter real-time self-adaptive optimal design system based on the detected topographic features takes an ARM embedded system as the laser scanner parameter self-adaptive design controller (2). The laser scanner (11), the flight speed measuring instrument (12) and the aerial camera (13) obtain flight parameters of an airplane and laser scanning point cloud and photographic images of a detected terrain, the flight parameters and the laser scanning point cloud and the photographic images are respectively sent into the ARM controller through serial ports, elevation values and terrain space frequency characteristic parameter pre-estimated values of the detected terrain are obtained through calculation, real-time optimization design values of three adjustment parameters of the laser scanner are further calculated, the real-time optimization design values of the three parameters are output through a D/A (digital/analog) output port and are respectively provided for the pulse repetition frequency adjusting device (31), the scanning frequency adjusting device (32) and the laser scanning view angle adjusting device (33), and therefore real-time optimal adjustment of pulse repetition frequency, scanning frequency and a scanning view angle can be achieved.
Drawings
FIG. 1 is a composition diagram of an airborne laser radar parameter adaptive adjustment system based on measured topographic features.
FIG. 2 is a schematic diagram of an installation structure of a measuring device of an airborne laser radar parameter self-adaptive adjusting system based on measured topographic features.
FIG. 3 is a graph of the effect of terrain elevation changes on point cloud density.
Fig. 4 is a schematic view of the operation principle and parameter adjustment of the laser scanner.
FIG. 5 is a schematic diagram of a method for adjusting laser scanner parameters during a terrain elevation change.
FIG. 6 is an analysis chart of the adjustment method of laser scanning parameters according to the spatial frequency variation of the measured terrain.
FIG. 7 is a schematic diagram of a calculation method for predicting the spatial frequency of the measured terrain from previous laser scanning points and aerial camera images.
FIG. 8 is a schematic diagram of a hardware structure of a real-time adaptive adjustment system for airborne laser radar parameters based on measured topographic features.
Detailed Description
The patent embodiments of the present invention are described in further detail below with reference to the accompanying drawings.
FIG. 1 is a composition diagram of an airborne laser radar parameter adaptive adjustment system based on measured topographic features. The invention provides a real-time self-adaptive adjustment method and device for airborne laser radar parameters based on the characteristics of a detected terrain, which are characterized by comprising a terrain characteristic and flight parameter measuring device (1), a laser scanner parameter self-adaptive design controller (2), a scanner parameter adjusting device (3) and an airborne platform (4); the device (1) for measuring the topographic characteristics and flight parameters comprises a laser scanner (11), an aircraft flight speed measuring instrument (12) and an aerial camera (13); the scanner parameter adjusting device (3) comprises a pulse repetition frequency adjusting device (31), a scanning frequency adjusting device (32) and a laser scanning field angle adjusting device (33); the airborne platform (4) is an installation platform for various loads, and the terrain characteristic and flight parameter measuring device (1), the laser scanner parameter adaptive design controller (2) and the scanner parameter adjusting device (3) are all fixed on the airborne platform (4); the laser scanner parameter self-adaptive design controller (2) obtains measurement data of a laser scanner (11), an aircraft flight speed measuring instrument (12) and an aerial camera (13), obtains characteristics of a measured terrain, namely two-dimensional terrain spatial frequency and elevation information, obtains three working parameters of the laser scanner, namely optimal adjustment values of pulse repetition frequency, scanning frequency and scanning view angle through optimal calculation, outputs control signals, and respectively controls the pulse repetition frequency adjusting device (31), the scanning frequency adjusting device (32) and the laser scanning view angle adjusting device (33) to realize real-time adjustment of the three working parameters; the airborne platform (4) keeps constant altitude and flies linearly at a constant speed, when the elevation of the ground to be measured changes, the relative flying height between the airborne platform (4) and the surface of the ground to be measured changes, so that the bandwidth and the point density of the point cloud scanned by the airborne laser radar correspondingly change, and the bandwidth and the point density of the point cloud scanned by adjusting the scanning field angle and the pulse repetition frequency in real time are kept constant; when the spatial frequency of the detected terrain changes, the scanning frequency and the pulse repetition frequency value are adjusted in real time, and the sampling frequency of laser scanning in the two-dimensional direction is changed, so that the point cloud density always meets the requirement of the sampling theorem, and the distortion of a subsequent reconstruction three-dimensional terrain model is avoided.
FIG. 2 is a schematic diagram of an installation structure of a measuring device of an airborne laser radar parameter self-adaptive adjusting system based on measured topographic features. The aircraft flight speed measuring instrument (12) adopts a pitot tube airspeed instrument for measurement, wherein the pitot tube (121) is horizontally placed, the potential pressure is zero at the moment, and the aerodynamic pressure measured by the pitot tube is the total pressure minus the static pressure. The differential pressure type pressure sensor (122) is used for compressing air pressure PvConversion to a voltage output, e.g. differential bellows differential self-sensing displacement transducer, by
Where ρ is the air density, the flying speed v can be found; the laser ranging value of the laser scanner when the laser scanner is superposed with the nadir line, namely the ranging value when the scanning angle is zero, is used as the airborne platform (4) and the measured terrainA relative fly height value between surfaces; the laser point cloud data of the laser scanner in the first few seconds and the previous image data obtained by the aerial camera are adopted to predict the two-dimensional terrain spatial frequency of the measured terrain in the flying direction and the transverse scanning direction through comprehensive calculation, and the frequency is recorded as fgxAnd fgy。
FIG. 3 is a graph of the effect of terrain elevation changes on point cloud density. The airplane flies in a straight line at a constant speed and keeps the altitude constant, but when the measured terrain has elevation change, the relative flying height of the airplane and the surface of the measured terrain can be changed, so that the bandwidth of the scanning point cloud is changed. Assuming that the scanning frequency and the pulse repetition frequency of the laser scanner are unchanged, when the measured terrain is higher, the relative height between the airplane and the measured terrain is smaller, the scanning point cloud bandwidth is narrower, and the point density of the laser foot points is higher; conversely, when the measured terrain is lower and the relative flying height is larger, the scanning point cloud bandwidth is wider, and the point density is lower. The narrowing of the scanning point cloud bandwidth can reduce the overlapping degree of the adjacent scanning point cloud bandwidth, and can seriously cause scanning vacancy, cause the missing scanning of the tested terrain, cause sampling failure and local reconstruction deficiency of the tested terrain. Too high density of point cloud causes data redundancy, which is not favorable for data storage and data processing efficiency, while too low density causes increased distortion of a reconstructed terrain digital model, so that laser scanning parameters need to be optimized and adjusted in real time according to the elevation change of the terrain.
Fig. 4 is a schematic view of the operation principle and parameter adjustment of the laser scanner. The laser pulse emitter (311) emits laser pulses, the laser pulses reach the rotating prism (332) after being reflected by the reflecting mirror (331), the laser pulses are emitted to the ground after being reflected, the rotating prism motor (321) drives the rotating prism (332) to rotate, and the rotating prism photoelectric shaft angle encoder (333) measures the real-time rotating angle of the rotating prism (332). The pulse repetition frequency adjusting device (31) generates a square wave signal with required frequency, and provides the square wave signal to the laser pulse emitter (311) to realize the adjustment of the laser pulse repetition frequency. The scanning frequency adjusting device (32) outputs a control signal, and changes the rotating speed of the rotating prism motor (321) to realize the adjustment of the scanning frequency. The laser scanning field angle adjusting device (33) measures the real-time rotation angle of the rotary prism (332) according to the rotary prism photoelectric shaft angle encoder (333), adjusts the scanning angle of the rotary prism (332) which effectively emits laser pulses, and achieves adjustment of the scanning field angle.
FIG. 5 is a schematic diagram of the adjustment method of laser scanner parameters during terrain elevation changes, and the adjustment method is to adjust the laser scanning field angle and pulse repetition frequency to keep the scanning point cloud bandwidth and point density on the ground constant when the relative flying height between the airplane and the measured ground surface changes. Let the ideal relative flying height between the plane and the ground to be measured be HEThe ideal scanning field angle is +/-thetae(°) then when the actual flying height is HAIn time, the scanning bandwidth is required to be unchanged, and H isE·tan(θe)＝HA·tan(θa) From this, it can be found that the scanning angle of view becomes + -thetaaI.e. the adjustment value of the scanning field angle is
When the number of revolutions of the rotating prism (332) is n (revolutions per minute), the angular velocity is 60n (° s) and is constant, and the number of scanning points per scanning line is constant m, the ideal relative flying height is HEThe scanning field angle is +/-thetaeAt deg. the scan time for obtaining a laser scan line is
The laser scanning pulse frequency at the ideal relative flying height is
Similarly, when the actual relative flying height is HAThe scanning field angle is +/-thetaaAt an angle of (°) of the laser scanning pulse frequency of
Then
Therefore, when the height of the terrain changes, the equation
Real-time adjustment values of the scanning field angles can be obtained; re-routing type
The real-time adjustment value of the pulse repetition frequency can be obtained, so that the adverse effect of the terrain elevation change on the laser scanning bandwidth and the point density can be eliminated, and the bandwidth and the point density of the ground laser point cloud are kept unchanged all the time.
FIG. 6 is an analysis chart of the adjustment method of laser scanning parameters according to the spatial frequency variation of the measured terrain. According to the sampling theorem, in order to reconstruct a three-dimensional image of the measured terrain without distortion, the highest frequency omega of the measured terrain is obtained firstlyhThe parameter design value of the airborne laser radar can enable the ground sampling frequency to meet omegas＞2ωhThe ground sampling frequency is mainly determined by the point cloud density. In addition, different terrains have different volatility and complexity, namely different spatial frequencies of the terrains, different sampling frequencies are adopted in the scanning and mapping process of the laser radar, and a three-dimensional model of the measured terrains is reconstructed with optimal sampling efficiency, so that the parameter values of the airborne laser radar are required to be adaptively designed in real time according to the characteristics of the fluctuation frequencies of the terrains. According to the flying speed, the relative flying height and the terrain spatial frequency of the measured terrain, real-time adjustment values of three parameters of the laser scanner, namely a scanning field angle, a scanning frequency and a pulse repetition frequency, are calculated and obtained. For the rotating prism line scanning mode, let the scanning mirror scanning frequency be n, the flying speed of the airplane be v, the relative flying height be h, the laser pulse repetition frequency be frThe field of view is fov (field of view), and the number of scanning points of a line scanned by the laser is m (i.e. f)rN), the dot spacing in a row of scan lines is:
the line spacing is:
the flight height h and the speed v are constant, and the variable parameters comprise an angle of view fov, a scanning mirror scanning frequency n and a laser pulse repetition frequency frThen the point cloud density is:
when the sampling frequency of the laser point cloud is more than twice of the spatial frequency of the measured terrain, the sampling theorem is satisfied, and the reconstructed terrain three-dimensional model cannot be distorted. If the plane scanning is decomposed into two one-dimensional scanning, under the condition of meeting the sampling theorem, the following steps are carried out:
wherein f isgxAnd fgyThe spatial frequencies of the measured terrain in the x-direction and the y-direction, respectively. Then:
because:
therefore, the method comprises the following steps:
then: n is not less than 2 v.fgy，
In practice, in order to improve the imaging quality, the sampling frequency is generally 3 to 5 times the spatial frequency of the terrain to be measured, and if the sampling frequency is 3 times, then when the ideal relative flying height is HEActual relative flying height of HAThe flying speed is v, and the ideal scanning field angle is +/-thetaeThen the actual scan field angle is + -thetaaScanning frequency n, pulse repetition frequency frThe calculation is as follows:
n＝3v·fgy，
in addition, it is known to adjust the pulse repetition frequency from the time of elevation changes in the terrain:
thus, the actual pulse repetition frequency setting may take frAnd faThe maximum value of (a) may then satisfy the pulse repetition frequency adjustment required by both terrain elevation changes and terrain spatial frequency changes. Therefore, according to the above-mentioned calculation formula, if the flying speed of the airplane, the flying height relative to the ground and the two-dimensional space frequency f of the terrain are obtainedgx、fgyThe settings of the three parameters of the laser scanner, i.e. the scanning field angle, the scanning frequency, the pulse repetition frequency, can then be optimally designed.
FIG. 7 is a schematic diagram of a calculation method for predicting the two-dimensional spatial frequency of the measured terrain from the previous laser scanning points and aerial camera images. Obtaining the terrain spatial frequency, namely f in the two-dimensional direction of the measured terrain by using scanning point data of the laser scanner in the first seconds and combining the previous image data of the aerial cameragxAnd fgy. First, the method of predicting the spatial frequency of the terrain from the previous data of the laser scanner is as follows: in fig. 7, two laser scanning lines (a) and (b) (including two laser dotted lines along the x direction and two laser dotted lines along the y direction) are depicted, and the spatial frequency of the future detected terrain is predicted according to the more recent historical scanning data. As can be seen from fig. 7, the variation frequency of the graph (b) is obviously higher than that of the graph (a), and is mainly reflected in the following three indexes:
(1) the angle of change: from left to right, the extension line of the connecting line of the previous two points is taken as a datum line, an included angle of the connecting line rotating to the next point is calculated, wherein the anticlockwise rotation angle is set to be positive, the clockwise rotation angle is set to be negative, and the average value of the sum of absolute values of all positive and negative rotation angles in one scanning line is that:
it can be seen that the larger this mean value, the larger the change in frequency of the terrain in this direction. (2) Number of positive and negative mutations in the corner (denoted w): that is, the number of times of change of the rotation angle from positive to negative and from negative to positive reflects the spatial frequency of the terrain, and can be counted by the following procedure:
w＝0
for j＝1,n
ifθj·θj+1＝-1
w＝w+ 1
end
(3) maximum value | θ of absolute values of positive and negative rotation angles in two-dimensional directionsx|maxAnd | θy|maxIt reflects the intensity of the terrain fluctuation, and the larger this value, the more intense the terrain fluctuation. From the previous data of the laser scanning point cloud, the terrain spatial frequency coefficient can be obtained as follows:
wherein eta isxAnd ηyIs an empirical scaling factor, wxAnd wyThe positive and negative mutation times of each corner in the laser scanning line in the x direction and the y direction are respectively.
Secondly, the method for analyzing the terrain spatial frequency by obtaining the terrain texture characteristics from the image of the aerial camera comprises the following steps: the gradient map is obtained from the image obtained by the aerial camera, the gradient reflects the steepness of the slope of each point on the image, and if the gradient value at the edge of the object is larger, the gradient map is obtained. According to the gradient, the distribution condition of different objects in the image can be judged. The gradient values of all pixel points in the image are added to obtain an average value, and the size of the gradient average value can reflect the change condition of the image texture. The larger the gradient mean, the larger the image texture variation, and the larger the possible spatial frequency of the terrain. In image processing, the gradient calculation of a certain pixel point can adopt the following formula:
|. f (x, y) | ≈ max [ | f (x, y) -f (x +1, y) |, | f (x, y) -f (x, y +1) | ], i.e., the gradient value at a certain pixel point is the maximum of the absolute value of the difference between the adjacent pixels in the horizontal direction and the absolute value of the difference between the adjacent pixels in the vertical direction at the point. In one image, the mean value of the gradient values of all the pixels is:
finally, the terrain spatial frequency coefficient obtained by integrating the laser point cloud data and the image data of the aerial camera can be calculated as:
in practical application, experience pre-estimation is carried out according to the measured scanning terrain before flying, and experience scanning frequency and terrain spatial frequency coefficient in two-dimensional direction are obtained, namely
And
and
then, the real-time estimated spatial frequency of the terrain in the two-dimensional direction in the actual flight process can be obtained, and the calculation is as follows:
FIG. 8 is a schematic diagram of a hardware structure of a real-time adaptive adjustment system for airborne laser radar parameters based on measured topographic features. The airborne laser radar parameter real-time self-adaptive optimal design system based on the detected topographic features takes an ARM (LPC2138) embedded system as the laser scanner parameter self-adaptive design controller (2). The laser scanner (11), the flight speed measuring instrument (12) and the aerial camera (13) obtain flight parameters of an airplane and laser scanning point cloud and photographic images of a measured terrain, the flight parameters and the laser scanning point cloud and the photographic images are respectively sent into an ARM (LPC2138) controller through serial ports, elevation values and terrain spatial frequency characteristic parameter pre-estimated values of the measured terrain are obtained through calculation, real-time optimization design values of three adjustment parameters of the laser scanner are further calculated, and the real-time optimization design values of the three parameters are output through a D/A (digital/analog) output port and are respectively provided for the pulse repetition frequency adjusting device (31), the scanning frequency adjusting device (32) and the laser scanning view angle adjusting device (33), so that real-time optimal adjustment of the pulse repetition frequency, the scanning frequency and the scanning view angle can be achieved.
The above description of the invention and its embodiments is not intended to be limiting, and the illustrations in the drawings are intended to represent only one embodiment of the invention. Without departing from the spirit of the invention, it is within the scope of the invention to design structures or embodiments similar to the technical solution without creation.
Claims (3)
1. A real-time adaptive adjustment method for airborne laser radar parameters based on the characteristics of a detected terrain is characterized by comprising a terrain characteristic and flight parameter measuring device (1), a laser scanner parameter adaptive design controller (2), a scanner parameter adjusting device (3) and an airborne platform (4); the device (1) for measuring the topographic characteristics and flight parameters comprises a laser scanner (11), an aircraft flight speed measuring instrument (12) and an aerial camera (13); the scanner parameter adjusting device (3) comprises a pulse repetition frequency adjusting device (31), a scanning frequency adjusting device (32) and a laser scanning field angle adjusting device (33); the airborne platform (4) is an installation platform for various loads, and the terrain characteristic and flight parameter measuring device (1), the laser scanner parameter adaptive design controller (2) and the scanner parameter adjusting device (3) are all fixed on the airborne platform (4); the laser scanner parameter adaptive design controller (2) obtains the measurement data of the laser scanner (11), the aircraft flight speed measuring instrument (12) and the aerial camera (13), obtains the characteristics of the measured surface of the terrain, namely two-dimensional spatial frequency and elevation information, through real-time calculation, and obtains the pulse repetition frequency of the laser scanner through optimized calculationThe optimal adjustment values of the scanning frequency and the scanning field angle are output, and control signals are output to respectively control the pulse repetition frequency adjusting device (31), the scanning frequency adjusting device (32) and the laser scanning field angle adjusting device (33) to realize the real-time adjustment of three working parameter values; the airborne platform (4) keeps constant altitude and flies in a straight line at a constant speed, when the measured terrain has elevation change, the relative flying height between the airborne platform (4) and the surface of the measured terrain changes, so that the bandwidth and the point density of the laser scanning point cloud correspondingly change, and the bandwidth and the point density of the scanning point cloud are kept constant by adjusting the values of a scanning field angle and pulse repetition frequency in real time; the two-dimensional terrain spatial frequency of the measured terrain in the flying direction and the transverse scanning direction is predicted and recorded as f through comprehensive calculation by adopting laser ranging data of the laser scanner in the previous seconds and previous image data obtained by an aerial cameragxAnd fgy(ii) a When the spatial frequency of the detected terrain changes, the scanning frequency and the pulse repetition frequency value are adjusted in real time, and the sampling frequency of laser scanning in the two-dimensional direction is changed, so that the point cloud density always meets the requirement of the sampling theorem, and the distortion of a subsequent reconstruction three-dimensional terrain model is avoided.
2. The method for real-time adaptive adjustment of airborne laser radar parameters based on measured topographic features as claimed in claim 1, wherein when the ideal relative flying height is HEActual relative flying height of HAThe flying speed is v, and the ideal scanning field angle is +/-thetaeThen the actual scan field angle is + -thetaaScanning frequency n, pulse repetition frequency frAre respectively calculated as
n＝3v·fgy，
In addition, the pulse repetition frequency is adjusted according to the elevation change of the terrain
The actual pulse repetition frequency may be takenrAnd faThe maximum value of (1) can simultaneously meet the pulse repetition frequency adjustment required by the terrain elevation change and the terrain spatial frequency change; thus, when the flying speed of the airplane, the flying height relative to the ground and the spatial frequency f of the terrain in two-dimensional directions are obtainedgx、fgyAnd then, the three parameter setting values of the laser scanner, namely the scanning field angle, the scanning frequency and the pulse repetition frequency, can be subjected to real-time adaptive optimization design.
3. The method as claimed in claim 1, wherein the scanning point data of the laser scanner in the first few seconds is used to obtain the two-dimensional spatial frequency of the measured terrain by combining the image data of the aerial camera, and the spatial frequency in the x direction is fgxSpatial frequency in y-direction of fgy(ii) a From previous point cloud data of the laser scanner, according to three indexes, namely the mean value of the sum of absolute values of all positive and negative rotation angles in a scanning line
Number of positive and negative sudden changes of corner w, maximum value | θ of absolute value of positive and negative corner in two-dimensional directionx|maxAnd | θy|maxObtaining a two-dimensional terrain spatial frequency prediction coefficient, wherein the prediction coefficient in the x direction is calculated as
The prediction coefficient in the y direction is calculated as
Eta thereinxAnd ηyIs an empirical scaling factor, wxAnd wyRespectively the positive and negative mutation times of each corner in the laser scanning line in the x direction and the y direction; image acquisition using aerial cameraThe method comprises obtaining gradient map from image obtained by aerial camera, wherein the larger the mean value of gradient is, the larger the variation of image texture is, the larger the possible spatial frequency of terrain to be detected is, and in one image, the mean value of gradient values of all pixel points is recorded as
The calculation formula of the prediction coefficient of the two-dimensional terrain space frequency in the x direction by integrating the laser scanning point cloud data and the aerial camera image data is
The calculation in the y direction is
In practical application, experience estimation is carried out according to a scanning area before flight, the experience scanning frequency in a two-dimensional direction is obtained, and the experience scanning frequency in the x direction is recorded as
In the y direction
In addition, a two-dimensional terrain spatial frequency prediction coefficient is obtained, and is recorded in the x direction
In the y direction
The calculation formula of real-time pre-estimated terrain spatial frequency in the two-dimensional direction of the terrain to be measured is in the x direction
In the y direction of
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