Image restoration of the asymmetric point spread function of a high-resolution remote sensing satellite with time-delayed integration

The calibration and validation team of the Korea Aerospace Research Institute has calibrated and validated the image data of the KOMPSAT-2 since the launch of KOMPSAT-2 on July 28, 2006. The asymmetric phenomenon of the point spread function of the KOMPSAT-2 image data is evident in both the along and the across direction, most likely because KOMPSAT-2 has a 1 m ground sample distance high-resolution camera with time-delayed integration. Furthermore, because KOMPSAT-2 is in space, the KOMPSAT-2 image data has been corrected with good results by means of modulation transfer function compensation.     

基于移动站的转发式地面站设备时延标校方法

在转发式卫星测定轨系统中,基于伪码测距原理的星地距离测量是实现卫星精密定轨和高精度时间比对的基础。为获得高精度的星地距离,需要将地面站设备时延从伪码测距值中精确扣除。在转发式卫星测轨原理的基础上,提出了基于移动站的转发式地面站设备时延标校方法,实现了对转发式地面站设备时延的标校,标校精度能够优于0.5ns,对提高转发式卫星定轨精度和卫星双向时间比对精度具有重要作用。     

The study of enhanced earth observations on a satellite image chain

The Multi-Spectral Camera (MSC) on the KOrea Multi-Propose SATellite (KOMPSAT)-2 was developed and launched as a main payload to provide a One(1) m panchromatic image and four(4) band four(4) m multi-spectral images at an altitude of 685 km covering a swath width of 15 km. These images, archived around the world, are a useful resource for space applications in agriculture, cartography, geology, forestry, regional planning, surveillance, and national security. The image quality of KOMPSAT-2 depends upon its image chain, which is comprised of an on-board system in the satellite and a processing system at the ground station. Therefore, in this study we determine the factors that have a major impact on the image quality through an investigation of the entire image chain. Consequently, two methods, involving a compression algorithm and a deconvolution technique, were determined as having a significant influence on the KOMPSAT-2 image quality. The compression algorithm of KOMPSAT-2 is rate-controlled JPEG-like algorithm that controls the mismatch between the input and output data rate. The ability to control the input/output data rate may be useful during the operation of the satellite but can also lower the overall image quality. The deconvolution technique may increase the sharpness of images, but it can also amplify the image noise level. Therefore, we propose methods of wavelet-based compression and denoising as an alternative to currently existing algorithms. Satisfactory results were obtained through experimentation with these two algorithms, and they are expected to be successfully implemented into the future KOMPSAT series to yield high-quality images for enhanced earth observation.     

Orbit determination performances using single- and double-differenced methods: SAC-C and KOMPSAT-2

In this paper, Global Positioning System-based (GPS) Orbit Determination (OD) for the KOrea-Multi-Purpose-SATellite (KOMPSAT)-2 using single- and double-differenced methods is studied. The requirement of KOMPSAT-2 orbit accuracy is to allow 1 m positioning error to generate 1-m panchromatic images. KOMPSAT-2 OD is computed using real on-board GPS data. However, the local time of the KOMPSAT-2 GPS receiver is not synchronized with the zero fractional seconds of the GPS time internally, and it continuously drifts according to the pseudorange epochs. In order to resolve this problem, an OD based on single-differenced GPS data from the KOMPSAT-2 uses the tagged time of the GPS receiver, and the accuracy of the OD result is assessed using the overlapping orbit solution between two adjacent days. The clock error of the GPS satellites in the KOMPSAT-2 single-differenced method is corrected using International GNSS Service (IGS) clock information at 5-min intervals. KOMPSAT-2 OD using both double- and single-differenced methods satisfies the requirement of 1-m accuracy in overlapping three dimensional orbit solutions. The results of the SAC-C OD compared with JPL’s POE (Precise Orbit Ephemeris) are also illustrated to demonstrate the implementation of the single- and double-differenced methods using a satellite that has independent orbit information available for validation.     

星载SAR亚马逊雨林辐射定标仿真研究

辐射定标是准确、定量描述星载合成孔径雷达(SAR)系统性能的重要手段.星载SAR在轨运行期间,天线方向图较之发射前可能存在较大的失真,从而严重影响所获取的SAR图像数据质量,因此准确测量在轨飞行时天线俯仰向方向图特性并予以补偿是星载SAR辐射定标的主要内容之一.利用具有均匀后向散射系数特性的分布目标进行外定标是测量双程俯仰向天线方向图的最佳方案,而亚马逊雨林是全球最稳定、最均匀、最平坦的分布目标,后向散射系数几乎不随入射角变化,是公认的理想测试场地.鉴于目前我国尚无自行采集的星载SAR图像数据,我们对星载SAR回波功率仿真、天线方向图模型参数估算进行了深入、系统的研究,利用仿真的亚马逊雨林SAR功率图像成功地实现了星载SAR俯仰向天线方向图在轨测试关键环节的模拟,对我国的星载SAR辐射定标研究具有一定的理论意义和重要的工程实用价值.      

Space-borne synthetic aperture radar received data simulation based on airborne SAR image data

Starting from airborne SAR image data, a method of simulating space-borne SAR raw received data is proposed, excluding the conventional method of simulating the targets’ back-scattering coefficients through the model of three-dimensional distributed targets. The airborne SAR image data are regarded as space-borne targets’ back-scattering coefficients, and raw data are generated by using space-borne SAR echo signal model. Raw data’s simulation of space-borne SAR can help us to optimum design of space-borne SAR system scheme, test and evaluate the performance of every imaging algorithm, and research motion error from satellite platform having influence on received data and image quality.     

Transponder calibration of the Envisat RA-2 altimeter Ku band sigma naught

Although the history of spaceborne altimeters goes back to the early seventies, the absolute calibration of the backscattering coefficient has never been deeply investigated. This information has been primarily used to infer the wind speed via an empirical model, and the intercalibration among different satellite altimeters has revealed to be suitable for this purpose, being the wind retrieval based on an empirical relationship. As far as Ku band system is concerned, the sigma naught absolute calibration of the Envisat altimeter (RA-2) has been performed using an active reference target provided by a transponder. This has been exploited during the 6-month Commissioning phase to generate early calibration results. In order to monitor the RA-2 backscatter calibration during the Envisat lifetime, a continuous calibration effort has been carried out by operating the transponder as much as possible. This paper aims to review the entire effort for calibrating the RA-2 sigma naught measurements, which lasted for almost seven years. It presents in detail the adopted methodology and the final outcome of the activity, providing the users with the correction (bias) to get the calibrated sigma naught and analyzing its stability during almost the entire Envisat lifetime. Specifically, it is concluded that the RA-2 backscatter measurements were quite stable, even if a bias of about 1 dB should be considered with respect to the actually released product. Some small changes in the bias as function of time can be identified during most of the Envisat lifetime, consisting in a slight increase in the first two years, followed by a more stable period and a final drop observed at the end of 2009, until the conclusion of the calibration activity (corresponding to the change in Envisat orbit).     

Considerations in the design and deployment of flexible booms for a solar sail

The majority of solar sailing missions utilise semi-rigid boom technology but this places limits on the possible size of the sail. Practical large sails may be constructed using flexible booms on a spinning satellite. The dynamics of the deployment of long flexible booms are investigated and limiting parameters discussed. Deployment strategies are proposed utilising active deployment and a means of passive deployment which requires no direct control of the deployment process. Practical insights into the design, testing and use of a flexible boom solar sail deployment mechanism is presented based on small scale experimental testing, discussing aspects such as boom winding and balanced deployment. Three novel deployment mechanism design concepts are developed; two passive mechanisms using rotational damping to retard the boom deployment and one active concept takes advantage of the forces produced by the spinning nature of the satellite to develop a compact deployment mechanism.     

DORIS and GPS monitoring of the Gavdos calibration site in Crete

Due to its specific geographical location as well as its geodetic equipment (DORIS, GNSS, microwave transponder and tide gauges), the Gavdos station in Crete, Greece is one of the very few sites around the world used for satellite altimetry calibration. To investigate the quality of the Gavdos geodetic coordinates and velocities, we analyzed and compared here DORIS and GPS-derived results obtained during several years of observations. The DORIS solution is the latest ignwd11 solution at IGN, expressed in ITRF2008, while the GPS solution was obtained using the GAMIT software package. Current results show that 1–2 mm/yr agreement can be obtained for 3-D velocity, showing a good agreement with current geophysical models. In particular, the agreement obtained for the vertical velocity is around 0.3–0.4 mm/yr, depending on the terrestrial reference frame. As a by-product of these geodetic GPS and DORIS results, Zenith Tropospheric Delays (ZTDs) estimations were also compared in 2010 between these two techniques, and compared to ECMWF values, showing a 6.6 mm agreement in dispersion without any significant difference between GPS and DORIS (with a 97.6% correlation), but with a 13–14 mm agreement in dispersion when comparing to ECMWF model (with only about 90% correlation for both techniques). These tropospheric delay estimations could also provide an external calibration of the tropospheric correction used for the geophysical data of satellite altimetry missions.     

一种基于移动参考站的卫星双向时间频率传递系统时延校准方法CSCD

设备时延是卫星双向时间频率传递过程中的主要误差源,对其进行校准是获得高精度时间比对的关键。从卫星双向时间频率传递的基本原理出发,分析了基于移动参考站的卫星双向时间频率传递链路设备时延校准的方法。工程实践中,限于条件对校准方法进行了调整,实现了对两站卫星双向时间比对链路设备时延的校准。     

星载SAR辐射定标及其精度分析

随着星载合成孔径雷达（SAR)应用需求的发展,为了观测目标的物理特征,可以利用辐射定标技术获取更加精确的目标后向散射系数信息。主要完成单通道单极化条带成像模式下辐射定标及其精度分析,首先从雷达方程出发,进行严格的数学推导研究单通道单极化条带模式辐射定标技术;其次给出通过对已知雷达散射截面积的目标的观测,得到SAR图像上的灰度值与绝对的雷达散射截面积之间的关系;采用测量定标常数的方法,并基于辐射定标流程建立辐射精度误差模型和计算公式,重点研究了短期相对定标和长期相对定标中各因素对精度的影响。而且针对定标常数对绝对定标精度的影响给出了详细说明,指出星载SAR系统需要定期地完成系统修正。最后以TerraSAR-X卫星为例进行了计算验证,结果表明辐射精度预估值与实测值相差小于0.05 dB。     

单通道SAR实现ATI动目标检测的新方法

研究了采用单通道合成孔径雷达(SAR,Synthetic Aperture Radar)多视沿航迹向干涉(ATI,Along Track Interferometry)实现地面慢速运动目标指示的检测方法.分析了该方法所需的步骤,即利用重叠或不重叠的窗构造子视,对子视分别成像,相位误差校正并求干涉.关于其中起关键作用的相位误差校正步骤,提出了利用信号子空间投影进行相位配准的补偿方法.借助该鲁棒的自适应方法可以从某种程度上解决子图像间的去相关问题,降低相位随机性的影响,从2幅子视图像的干涉相位图上敏锐地检测到运动目标.通过仿真实验,验证了该方法可以有效地从强杂波中检测动目标,为单通道SAR实现地面运动目标指示(GMTI,Ground Moving Target Indication)、扩展已有SAR系统功能提供可靠的途径.      

环境C卫星热系统设计与在轨验证

根据环境C卫星的自身特性, 采用以被动式热控制为主和主动热控制为辅的控制方式, 对环境C卫星进行热系统设计. 通过研究环境C卫星的热控设计原则、热设计状态及已解决的关键问题, 对其在轨飞行温度数据进行了分析. 在轨飞行遥测结果表明, 环境C卫星热控系统方案合理, 工作稳定, 性能良好. 星上设备温度环境很好地满足了设计要求.      

星载合成孔径雷达的分辨特性

较全面和详细地论述了星载合成孔径雷达（ＳＡＲ）的分辨特性;导出了距离分辨率、方位分辨率和辐射分辨率的定量关系式;阐述了多种辐射分辨率定义的内涵,并对其错误概率进行了比较;介绍了ＳＡＲ图像的分辨体积和可懂度的概念,并且完成了ＳＡＲ图像分辨特性的方形等效。这些论述对于ＳＡＲ图像质量的评估和比较,以及系统参数的折衷设计具有重要意义。     

卫星姿态指向抖动与SAR成像质量关系研究

建立了卫星姿态指向抖动的数学模型,基于成对回波畸变理论推导了存在姿态指向抖动时点目标扩展函数的解析表达式,提出了卫星姿态指向抖动幅度与星载SAR(合成孔径雷达)成像质量关系的定量分析方法,为SAR卫星总体分析与设计提供了理论依据.计算机仿真结果验证了结论的正确性.      

天基视频SAR系统设计及成像算法研究

视频合成孔径雷达（SyntheticApertureRadar,SAR）作为一种新型微波遥感体制,能够实现对热点区域动态目标的持续监视,目前对视频SAR的研究主要集中在机载平台的视频SAR系统,未对天基视频SAR系统设计及成像算法展开深入研究。文章设计了一种基于双站模式的天基视频SAR系统,其中以静止轨道SAR卫星作为发射源,并利用低轨SAR卫星接收目标区域的回波信号。研究了视频SAR成像性能与各主要工程参数间的关系,并提出一种适用于天基视频SAR的成像算法,该算法能够有效解决双站天基视频SAR成像中大斜视角应用与数据重叠等问题,成像结果正确反映了运动目标的散焦、移位现象,同时静止目标取得了良好的聚焦效果。     

基于平行光管与二维旋转平台的长焦相机内参标定

长焦镜头能够采集位置更远的图像,而长焦镜头成像的特殊性对传统相机标定方法提出了挑战。针对这一问题,阐述了一种基于平行光管和二维旋转平台的相机内参标定方法。长焦相机被架设在二维旋转平台上,对放置在平行光管内的透过式棋盘格成像。通过旋转二维平台,改变相机的位置,可以得到不同位置下清晰的棋盘格图像。采集了棋盘格格点的像素坐标以及二维平台的角度数据,建立了合适的模型,优化求解出了长焦相机的内参参数,实现了实验室内对长焦镜头的标定。由模型计算得到的重投影棋盘格角点的图像坐标与在实际图像中检测到的棋盘格角点坐标相比,平均误差约为0.5个像素,这验证了算法的可行性。     

气象卫星扫描辐射计的辐射定标

阐述了气象卫星扫描辐射计的辐射定标。包括卫星发射前的地面定标,飞行中的星上定标及星地对比定标。指出飞行中的可见光波段的定标至今还未能投入业务使用,仍处于研究阶段,而静止气象卫星的飞行中定标,无论红外或是可见光波段只能做到相对定标。为解决绝对定标问题,必须寻求星体外的客观目标作为定标源。采用这一途径,即使星上定标设备失效,也不致影响卫星数据的使用。     

Statistical description of low-latitude plasma blobs as observed by DMSP F15 and KOMPSAT-1

The global distribution of low-latitude plasma blobs was investigated by in-situ plasma density measurements from the Korea Multi-Purpose Satellite-1 (KOMPSAT-1) and Defense Meteorological Satellite Program (DMSP) F15. In the observations, blobs occurred in the longitude sector where the activity of the equatorial plasma bubble (EPB) was appreciable, and additional blobs were found at the lower (KOMPSAT-1) altitude as in the EPBs. However, several notable differences exist between the distributions of EPBs and blobs. First, KOMPSAT-1 found few blobs around 0°E in March and June, as did DMSP F15 from 30°W to 120°E for every season. Second, the overall occurrences in December and March at the DMSP F15 (840 km) altitude were somewhat lower than expected from those of the EBPs. Third, at the DMSP F15 altitude, the occurrence probability of plasma blobs was less controlled by yearly variations in the solar activity. These results imply that topside ionospheric conditions as well as the existence of EPBs control further development of blobs. Additionally, it was found that the blob latitudes became higher as the yearly solar activity increased. Moreover, most of the blobs were encountered in the winter hemisphere, possibly due to the low ambient density.     

GPS授时校频方法研究与试验结果

为了解决多目标综合测量系统各测站之间时间同步和频率校准问题,提出了利用GPS(Global Positioning System)单星或多星共视方法进行站间时间同步与校频,给出了这两种方法的计算公式,分析了星历误差、星钟误差、电离层折射误差、对流层折射误差、多径效应和接收机硬件延迟对时间同步精度的影响.为了验证GPS授时校频精度,进行了相关试验.通过与铯原子钟比对,表明利用GPS可实现纳秒级时间同步,校频精度也优于5.0×10-11,多星共视具有更高的同步校频精度.