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某航空多光谱相机由基座、扫描稳定平台、相机本体和POS组成。如图1所示,其扫描稳定平台外框架为滚动框架,内框架为俯仰框架。扫描稳定平台内框架上安装多光谱相机、光纤陀螺等负载,框架间转角由感应同步器测量。扫描稳定平台俯仰框架相对惯性空间的运动角速度由光纤陀螺测量,基座通过减振器与载机固连,基座上安装POS,提供载机位置和姿态信息基准。
在无测距信息状态下,相机采用被动无源定位方法实现图像的三坐标定位,原理如图2所示。根据载体、目标在地面投影以及地心的三角形几何关系确定地理位置[6-7],计算流程如图3所示。
由图2~3可见,相机拍摄图像得到像空间坐标系下的坐标(xs,ys,zs),经过归一化后得到单位矢量
${\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {u} _c}$ ,再经过数次坐标变换,分别得到基座坐标系、地理坐标系以及大地直角坐标系下的单位矢量${\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {u} _b}$ 、${\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {u} _N}$ 、${\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {u} _E}$ ,进一步向地球椭球投影,计算可得投影坐标${T_E}$ ,换算得到经纬度坐标$T{\rm{(}}lat,\;lon,\;h{\rm{)}}$ 。根据该计算流程可知,变换过程中的稳定平台角度(θr, θp)、载机姿态角(r, p, y)以及经纬高位置(lat, lon, h)精度决定了目标的三坐标定位精度。由于是远距离成像,角度误差引起的位置误差远远大于位置测量误差,角位置精度对定位精度起决定性作用。因此相机中与角度测量的相关技术,如高精度位姿动态测量技术、扫描稳定平台测角信号处理及辨识补偿、扫描型相机多维外方位元素标定技术以及异构多源传感时钟同步技术是机载相机的三坐标无源定位关键技术,如表1所示。
表 1 误差项与关键技术
Table 1. Error factor and key technology
Error factor Detail Key technology Base position orientation error Position error High precision position orientation measure
technologyOrientation error Stabilized platform angular error Angular rotation error Stabilized angular measurement process,
identification and compensationAngular sensor tilt Circuit electrical error Mounting error POS and stabilized platform mounting error Exterior orientation elements
of multiple spectrum
scannerStabilized platform and multiple spectrum camera mounting error Clock error Sensor measure delay Multisource clock synchronization of heterogeneous sensors Circuit delay Software acquire storage delay -
多光谱相机传感器种类多、含多光谱图像、POS、感应同步器、光纤陀螺及内部电路等,异构多源时钟同步问题是影响相机几何精度的主要因素之一。受传感器输出延迟、传输延迟等因素的影响,图像数据与外方位元素数据存在同步误差,影响图像几何精度、形成观测误差[17]。
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如图6所示,POS采集延迟为t11,多光谱图像数据输出延迟时间分别为
$t_{12}^1$ 、$t_{12}^2$ ······$t_{12}^N$ 。电路传输延迟:管控电路接收到POS数据存在传输延迟时间t21,图像输出控制电路、伺服控制电路接收到管控转发的POS数据存在传输延迟t22,所有数据均发送至图像数据记录设备,发送至图像记录时刻存在传输延迟时间t23。
软件采集数据延迟:采集POS数据及时钟存在延迟时间t31;采集多光谱图像输出数据,存在延迟时间t32;采集稳定平台运动数据,存在延迟时间t33。
其中传输时间延迟t21、t22、t23受波特率、数据量大小制约,由于民用测绘关注事后处理,传输延迟时间t23对数据时钟同步并不影响。实测采集延迟时间t31为几十μs量级,图像输出电路采集延迟时间t32为ns量级,伺服控制电路采集命令延迟t33为ms量级。可见系统的数据之间存在较大时延,约1 ms。
在30 (°)/s扫描速度下,产生的定位误差s=30/57.3×1=0.5 mrad,约为10个像元,因此需要进行时钟同步设计。
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为实现数据对准,读取每一传感器数据时均同时读取本地时钟信息,因此只要实现各个传感器本地时钟信号的对准,即时钟同步,便可解决数据采集延迟问题,实现在后处理时的数据对准。采用各个本地时钟信号后,图像数据输出延迟t11,
$t_{12}^1$ 、$t_{12}^2$ ······$t_{12}^N$ 和采集延迟t31、t32、t33问题已经解决,进一步解决传输延迟t21、t22、t23的问题。采用POS输出的PPS秒脉冲信号时周期进行时钟同步,如图7所示。为了维持秒内时间的连续性,管控电路、图像输出电路及伺服控制电路根据各自晶振产生时间分辨率为ns级的时间计数值,当纳秒时间计数值等于整秒时,向秒计数值进位,计数器清零,秒计数值和纳秒计数值组合起来就是相机本地时间系统输出的完整时间值。
由表2可见,采用秒脉冲信号作为时钟同步信号后,传输延迟时间t21、t22影响消除。由于是后处理,t23不影响同步性能。此时时钟同步精度受制于1 s内晶振的稳定性以及各传感器输出延迟本身,多源传感器时钟同步精度优于μs级。
表 2 系统时钟及数据统计
Table 2. System clock and data index
Clock signal Data Clock signal accuracy PPS signal UTC time;
POS data<100 ns Management clock POS data and stabilized platform data <100 ns Servo circuit clock Fibro optic gyro <500 ns Industosyn Image output circuit Clock 1 Band 1 <10 ns …… …… Clock N Band N
Three coordinate passive location technology for airborne multiple spectrum camera
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摘要: 航空多光谱相机采用焦平面探测成像系统,可在全天时、准全天候作业的条件下对机下进行大幅宽高分辨率成像探测。针对航空多光谱相机的三坐标无源定位问题,简介了相机组成及无测距信息辅助情况下的三坐标无源定位原理,梳理了航空多光谱相机的三坐标无源定位关键技术。通过载体的位置姿态精确测量,测角电路设计、测角误差标定与补偿,多传感器安装姿态标定,时间同步设计四大关键技术保证相机的定位精度。飞行试验结果表明,图像拼接精度优于10″,验证了技术方法有效。Abstract: A large FOV infrared multiple spectrum camera, which contains advance infrared focal plane, acquires large FOV & high resolution multiple spectrum image toward area below the flight vehicle. It takes images on a whole day and nearly a whole whether condition. The multiple spectrum camera system configuration was proposed. Its angular measurement, three coordinate passive location key technology and solution were described in detail. The angular measure electro-circuit & clock synchronization should be finely designed. When the system had been integrated, angular error calibration and compensation, multi-sensor installation attitude calibration were proposed to improve angular measure accuracy and location accuracy. Flight tests shows that image mosaicking accuracy is 10″ which means the technic is effective.
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表 1 误差项与关键技术
Table 1. Error factor and key technology
Error factor Detail Key technology Base position orientation error Position error High precision position orientation measure
technologyOrientation error Stabilized platform angular error Angular rotation error Stabilized angular measurement process,
identification and compensationAngular sensor tilt Circuit electrical error Mounting error POS and stabilized platform mounting error Exterior orientation elements
of multiple spectrum
scannerStabilized platform and multiple spectrum camera mounting error Clock error Sensor measure delay Multisource clock synchronization of heterogeneous sensors Circuit delay Software acquire storage delay 表 2 系统时钟及数据统计
Table 2. System clock and data index
Clock signal Data Clock signal accuracy PPS signal UTC time;
POS data<100 ns Management clock POS data and stabilized platform data <100 ns Servo circuit clock Fibro optic gyro <500 ns Industosyn Image output circuit Clock 1 Band 1 <10 ns …… …… Clock N Band N -
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