星载相机的像移补偿实现英文.docx

motion compensation realization for satellite borne camerayao chengkang, li qinghui(school of technical physics, xidian university, xi an 710071, china)abstract : for the case of image motion in a frame film remote sensing ccd camera as the payloadfor a sun - synchronous orbit satellite , a set of mechanical image motion compensation ( imc ) system was designed . to get the compensation parameters when the remote sensing photographs were captured , a mathematical model for computing the motion velocity was established for a sun - synchronous orbit satellite . based on the characteristics of space remote sensing , the servomotor wasadopted in the system to drive ball screw assembly to accomplish two - dimensional compensation . atlast , the result of the simulation experiment basedby analyzing this specific example and comparing show that the system can compensate the imageon actual remote sensing situation was performed .with the statistical data of this device , the resultsmotion effectively andmeet the requirements forcan be less than one thirdremote sensing image and the target displacement of remote sensing imagepixel after compensation .key words: imc; space remote sensing; triangle mechanical construction;ball screw assemblyclc number: th761.6document code: aarticle id: 1007-2276(2011)06-1090-08星载相机的像移补偿实现姚呈康，李庆辉(西安电子科技大学 技术物理学院，陕西 西安 710071)摘 要： 针对搭载在太阳同步轨道卫星上的画幅 式遥感相机在成像过程中的目标图像存在 相对移动的情况，设计了一套机械像移补偿机构。通过在遥感相机成像过程中获取像移 速度，在理论上构建了 基于太阳同步轨道卫星遥感特点的像移参数计算模型。根据成像特点，该像移补偿机构采用伺服电机 驱动滚珠丝杠的方式实现二维补偿操作。最后，对实际遥感情况进行了数据仿真实验。结果表明：使用 该套补偿系统能够有效地对像移进行补偿，像移残差值可以达到低于 1/3 像素的要求。关键词： 像移补偿； 航天遥感； 三角形补偿机构； 滚珠丝杠收 稿 日 期 ：2010-10-08 ； 修 订 日 期 ：2010-11-16基 金 项 目 ： 中 国 空 间 技 术 研 究 院 cast 创 新 基 金(cast200745)作 者 简 介 ： 姚 呈 康 (1985-) ， 男 ， 博 士 生 ， 研 究 方 向 为 光 学 系 统 设 计 。 email:导 师 简 介 ： 李 庆 辉 (1969-) ， 男 ， 教 授 ， 博 士 ， 研 究 方 向 为 光 学 系 统 设 计 及 光 电 检 测 。 email:第 6 期yao chengkang et al: motion compensation realization for satellite borne camera1091the solar battery board on the satellite and ensure thepreferable irradiating condition of the remote sensing target, the sun-synchronous orbits are always applied as the orbit for the remote sensing satellites, thus the relative geometry relations of the sun irradiation ontothe surface of the orbit would always remain the0 introductionwith the development of aerospace remote sensing technology, the ground resolution of remote sensing camera enhances continuously. since there is relative motion between camera and ground target when taking pictures, the target image recorded in recording media is moved and blurred. the major factors that cause image motion are the course of the carrier s movement along its orbit and the earth rotation when taking vertical pictures. in order to improve the imaging quality of remote sensing pictures and camera resolution, a high resolution system for the earth observation has been designed, especially for a large resolution digital camera, reasonable and effective image motion compensation (imc) device is of great significance to ensure that the definition of remote sensing image is consistent with the requirements of using.the factors that cause the remote sensing image motion are as follow: the movement of the planet along its orbit, the random variation of the satellite gestures and the internal vibration of the moving components in the satellite 1 . in order to compensate the image motion properly and ensure the imaging quality of remote sensing image, the analysis of each factor that can cause image motion is required. the entire scheme for imc operation should be carried out under the limitation of remote sensing demand.the compensation measures in existence mainly include recording medium translation movement, rotating optical components, controlling angle of optical axis, making satellite rotated method and electronic method 2, etc. according to the different remote sensing facilities of different imaging theories, there are relevant imc techniques. based on the study of remote sensing satellite in sun-synchronous orbit, the imc device applies to the way that making the photographic device move along with image motion as compensation technique.to get as much straight sunshine as possible for 3 same . within a working cycle of the satellite , theimage motion direction due to the earth rotation will change compared to the coordinate of the space- based camera; when it goes through the equator, the crab angle would change reversely. the above characteristics of the sun-synchronous orbit satellite require that the compensation device should be able to compensate the two-dimensional image motion.in order to make a reliable and effective scheme for imc, the design of which should be based on the analysis of the practical working environment. during the circulation of the satellite running along its orbit, the most influential factors of image motion are the movement of the satellite and the earth rotation. in the scheme of space-basedcamera, if the image motion issue aroused by theabovetwoelements couldbecompensatedbe accepted. compensationcompletely, the images would betterconcerning the mechanical structure,can be realized by the combination of the crab angleand the parallel motion compensation parts. in the exposure time, the velocity of the imc device should be in accordance with that of the targetimage precisely4. meanwhile, because the mentionedcompensationdevice is based on the recordingmediumtranslationmovementastheguidingprinciple, the limitation of remote sensing imaging should also be taken into consideration in systemdesigningactivities.therefore,mechanicalstructure wouldthedesignedaccomplishtheof ofcompensation operation with the requirementprecision without affecting the interval timephotography. in the end, the evaluation of imcfollows the general criterion that the image motionfig.1 schematic diagram for triangle mechanical constructionfig.2 schematic diagram for the parallel motion mechanism construction红 外 与 激 光 工 程第 40 卷1092displacement quantity after compensation should beless than one third pixel 5.the angle betweenaand ccan be changedsynchronously, so as to the crab angle of thephotographic device.1 imc device1.1 compensation construction realizationat present, high accurate two-dimensional motion flat has two kinds of realizations. one is based on the respective control for x, y direction motion, using some gear down construction to improve the precision. this way gives a very high demand for the control system because of the coupling affect between x and y. the other is using rotation and parallel construction, which need high precision on rotation parts. choose the second manner to realize the compensation operation based on the characteristics of satellite remote sensing. the crab angle is mainly caused by the earth rotation. thanks to the large earth radius, when two pictures are taken in turns, the parameters of angle compensation does not need to be modified every time. therefore, for a reasonable period of time, only parallel parts need to be run to accomplish the compensation task. this kind of compensation operation mode would make the whole imc device running more effective and reliable.1.2 rotation parts designbased on the computing results of image motion quantity for a satellite in sun-synchronous orbit, the conclusion that the main reason for the change of crab angle is the earth rotation is obtianed. during the whole remote sensing task, the variation scope of crab angle would be no more than 5 . therefore, there is no need to design a large angle rotation device. a triangle motion mechanism construction is used, which can accomplish accurate compensation under a small range of angle variation. figure 1 is a schematic diagram for this triangle mechanical construction, which consists of unchangeable sides a, b and the length changeable side c. the photographic device is fixed at the junction point of a and c. the central of photographic device is not moved during thecompensation operation. if the length of c is changed,the ball screw assembly is fixed on side c. bycontrolling the amount of length of c, the angle will be compensated. if the lengths of a and b are longenough, a high accurate angle resolutionrealized. can be got by equation (1)can be222a +c -b=arccos(1)2acwhen the length change of c side can be got by equation(2)is x,the new22+(c+x) -b2=arccos a(2)2a(c+x)in the actual practice of remote sensing, the crabangle will not be modified unless the image motion is detected. the parameter that the rotation parts should receive is the amount of length variable x, which is accomplished by the control of servomotor.1.3 parallel parts designfigure 2 is a schematic diagram for the parallel parts. this mechanism construction consists of servomotor, coupling, connection platform, bracket,ball screw assembly and imc base. the photographicfig.3 general flowcharts of control structurefig.4 work flow of imc device第 6 期yao chengkang et al: motion compensation realization for satellite borne camera1093device is fixed on the support of ball screw assembly.the parallel motion compensation operation is driven by the servomotor, which is under the control of imc device. the velocity of the photographic device can be controlled by changing motor speed.before photography, the parallel parts should berun to match the velocity of photographic device withcomputer through the data bus. the parameters ofcompensation would be transmitted to servomotor from the control unit.the specific technological process is as follows: firstly, this imc device parallelizes the photographic device with the direction of image motion by using a triangle mechanical construction assembly before taking space photographs. then, the imc device pushes photographic device by using a parallel motion mechanism during the whole imaging process, so that the image motion of remote sensing picture would be reduced to an acceptable degree, which is based on a corresponding compensation theory. the relatively parameters, such as the angle of the triangle mechanical construction assembly need to be rotatedand the speed of the parallel motion mechanismthatofimagemotionexactly.duringthebe thecompensation operation, the motor speed canmodified to satisfy this demand. the accuracy ofparallel parts is of great significance for imc effect.the original position of photographic device is not in the centre of focal plane. when its velocity is matched, start the exposure operation and reset it after exposure for the next image processing. like the rotation parts, when satellite across the north andsouth poles, the direction of image motion would berequired to be kept during spacebased on a real - time measurement image motion detecting or gpsphotography, aretechnology 6 for technology. thereversed.thentheoriginalpositionforthephotographic device should be opposed to that before.1.4 general flow for the imc devicefigure 3 is the general flowcharts for controlrotation parts run before the parallel parts every time.this kind of asynchronous operation will ensure the stability of the imc device. furthermore, it is notnecessary for rotation parts to modify its crab anglestructure, fig.4 is the work flow of theimc device.thecontrolcardcancommunicatewithimccontrolbefore photography because the changeis not obvious compared to that of compensation mode will reduce theof crab anglelatitude. this frequency ofcompensation operations, which is of much benefit forreducing vibration on satellite.2 theoretical analysis2.1 image motion quantityaccording to the factors which bring image motion to remote sensing, some parameters during the photography process are needed to know, such as latitude i, height h, speed of satellite v, focus of optical system f , exposure time t and some other parameters about the attitude control of satellite such as average pitching angular speed v 准 , average rolling angular speed v , crab angular speed v , average crabdistance l and some remote sensing constants: rotationfig.5 analysis of t he image motion vector graphfig.6 sketch map of photo overlapping demand红 外 与 激 光 工 程第 40 卷1094angular velocity of the earth , earth radius r.the image motion displacement quantity forthe whole imc device is described as equation(12)the quantities of imagethe following equations7motionarecomputedas= q+(12)33v where q represents the movement error coefficient ofthe imc device, which is based on the accessible accuracy that the device can reach.2.2 limit for the imc deviceas already stated in previous, the existence of imc device should not make any influence on normal remote sensing task. it is necessary to take the limit of remote sensing system into account when the imc device is designed. based on the relative theory of remote sensing, the demand is satisfied that the overlapping extent should not be less than 53% , which means that the imc device should accomplish compensation and reset operation within the relative time. figure 6 showsa sketch map of this demand.1 =f t(3)h2 =fvt 3 =fvt 4 =lv t(4)(5) (6) f5 =rtcosi(7)hwhere 1 represents the image motion caused bysatellite moving along its orbit; 2 , 3 , 4 represent themotions causedbypitching,rollingandcrabthe motionmovement respectively; and 5caused by the earth rotation.these above parametersrepresentscan be compoundedtogether as fig.5 shows. commonly, due to a littleas shown in fig.5, theremote sensing picture isqx =lx /loverlapping ratio forangle between 1 and 2, scalar sum of them is madeas 1, and neglect the value of 4.(13)1 =1 +2(8)the interval time t is described as following ah 252 = 姨1 +5 -215coswhere represents orbital inclinatio n.t= fv （1-qx ）(14)(9)now the crabwhere a represents the size of photographic device, dis the distance that satellite move in the interval time,f is the focus of image system, h is the height of the camera, v is the velocity of satellite, l is the size ofangle and the image motionworked out from the fig.2.quantity3canbe=arcsin 5sin(10)2target, and l is the overlapping size between thesex223 = 姨2 +3 -223cos(11)two pictures.fig.7 relation between the image motion quantity and latitude第 6 期yao chengkang et al: motion compensation realization for satellite borne camera109525 mm, and the pixel size is 12 m. when the photooverlapping demand is 53%, the interval time between two remote sensing pictures based on equation (14) is0.12 s, which is also the time limitation for reset operation of the imc device. in the mean time, the quantity of image motion of the remote pictureshould be less than 4 m (one third pixel level) after3 simulated experimenta satellite for instance 8 -9 is taken and the imagemotion quantity happened on the surface of image plane in all latitudes based on the mathematical modelis computed, which haspreceding chapter. figure 7 computation. the momentbeen discussed in theshows the results of the that the image motion10compensation.the angle of the triangle mechanical construction assembly needs to be rotated at the relative latitude as showed in fig.7 (b), the maximum of crab angle happened at equator area by 3.86 . therefore, during the whole remote sensing task, the variation range of crab angle will be no more than 5 . a device that can accomplish accurate compensation under a small range of angle variation will be enough for imc operation. in addition, it is not necessary for crab angle compensation device to modify the angle before every image operation because the change of crab angle is a slow process. consequently, in a specified period of time, the parallel motion compensation parts could be run with modified parameters every time while the angle compensation parts could maintain still during this process. for the photographic device on the satellite, image motion will be detected if the quantity of which is more than 1 m, according to the current real-time image motion detecting technology. hence, if the detecting system finds image motion more than 1 m, the crab angle compensation parts should be modified.the triangle mechanical construct