Estimating ionospheric reflection height from Doppler measurements

A method is described which allows estimating an ionospheric reflection height from Doppler measurements made over an oblique CW radio transmission. This method requires the presence of multihop signals. The assumptions pertaining to the method's validity are delineated and an example is illustrated.     

Metamer density estimated color correction

Color correction is the transformation of response values of scanners or digital cameras into a device- independent color space. In general, the transformation is not unique due to different acquisition and viewing illuminants and non-satisfaction of the Luther–Ives condition by a majority of devices. In this paper we propose a method that approximates the optimal color correction in the sense of a minimal mean error. The method is based on a representative set of reflectance spectra that is used to calculate a special basic collection of device metameric blacks and an appropriate fundamental metamer for each sensor response. Combining the fundamental metamer and the basic collection results in a set of reflectances that follows the density distribution of metameric reflectances if calculated by Bayesian inference. Transforming only positive and bounded spectra of the set into an observer’s perceptually uniform color space results in a point cloud that follows the density distribution of device metamers within the metamer mismatch space of acqcuisition system and human observer. The mean value of this set is selected for color correction, since this is the point with a minimal mean color distance to all other points in the cloud. We present the results of various simulation experiments considering different acquisition and viewing illuminants, sensor types, noise levels, and existing methods for comparison.     

电离层电波传播相位污染校正

电离层电波传播相位污染使天波超视距雷达（OTHR）接收的回波信号谱扩展。采用合成孔径雷达（SAR）成像中的相位梯度算法（PGA）来校正相位污染，PGA算法要求所处理的各个分辨单元（这里包括方位和距离单元）具有相同的相位误差，但当对于电离怪空间变化剧烈，或OTHR距离和方位分辨率比较粗时，相邻单元的相位污染不完全相同，这里PGA方法不适用。针对这种情况，考虑相位变化不可能非常剧烈。可用低次多项式表示时，采用最小熵搜索方法，分别校正各单元的电离层相位污染。最后采用我国录取的OTHR实测数据验证，表明这两方法的校正效果良好。     

射电成像中电离层扰动的最小熵校正方法

在合成孔径射电成像中,电离层的扰动会在接收信号中引入相位误差,导致图像出现模糊和漂移。本文提出了一种新方法,以校正这种相位误差。使用熵作为衡量射电天文图像质量的指标,熵越小代表图像质量越高,当熵值达到最小时,认为相位误差被校正。相比其他传统方法,本方法仅利用脏图本身就能够校正相位误差。     

电离层加热实验中反射高度下降特征的数值模拟

向电离层中注入大功率电磁波可引发地基非相干散射雷达观测的离子线和等离子体线回波功率增强现象.2010年秋季,在挪威开展了极区电离层离子线与等离子体线增强特征激发实验.实验中首次得到了VHF雷达观测的长生命周期的增强特征, 该现象系由参量衰变不稳定性引起的.另外,增强现象所处高度随加热的进行呈下降趋势,针对该特征文章给出了可能的物理机制,即该现象系由泵波促使反射高度附近的电子扩散效应增强引起的.文章利用修正的IRI和MSISE模型参数对加热实验中的反射高度阶跃性下降特征进行了仿真,仿真结果表明,加热电波引起的电子扩散效应可以使得反射高度下降,并且下降的高度与实验结果吻合较好,验证了该物理机制的可能性.     

影响地面雷达测高精度的因素

针对某型三坐标地面雷达,对影响地面雷达测高精度的主要因素进行了分析。基于阵地地形、电磁环境、杂波环境、大气环境和人为导致的系统异常等因素,分析了地面雷达测高误差产生的原因,评估了多种因素对测高精度的影响程度,提出了在实际应用中规避影响因素以改善地面雷达测高精度的建议。     

高轨道目标电离层折射修正方法研究

为了满足高轨道目标高精度测控的需要,提出了基于双频GNSS的空间投影法和自适应网格法两种电离层距离折射误差修正方法,并对比分析了两种方法在不同仰角、角径时电离层距离折射误差修正精度. 分析结果显示:对于2.0 GHz电波信号而言,在较高仰角（仰角＞45°）处,空间投影法和自适应网格法修正精度较高,均优于0.2 m;在低仰角和大角径时,空间投影法修正剩余快速增大,修正精度快速降低恶化,达到米量级,而自适应网格法修正精度则不受仰角和角径的影响. 这表明:自适应网格法电离层距离修正精度高,适用性强,且便于工程应用,可为提高高轨道目标的测控精度提供有力支撑.     

星载光子计数激光测高植被冠层高度误差分析

通过建立植被的单光子探测模型,分析和计算了星载光子计数激光测高植被探测中,因单光子探测原理引起的植被冠层高度误差.实验表明,北方针叶林的星载光子计数测高中单次激光脉冲首次探测到的信号光子的平均高程低于LIDAR数据提供的DSM约2.435 m.仿真结果表明,提高发射脉冲能量,减小激光脉冲发散角有助于减小误差,而拥有更高...     

Estimation of noise error when measuring virtual height during diffusivity of ionospheric F layer

It is shown that during diffusivity of ionosphere its bandwidth of coherence may shrink to hundreds of hertz. Frequency-selective fading appears in these cases and the noise error when measuring ionospheric virtual height increases by 1–2 orders if compared to the normal ionospheric conditions.     

MTD对比相测高的精度影响

比相测高雷达是利用多个天线馈源分别接收目标的回波信号,根据不同馈源接收回波信号时产生的相位差,再结合已知的目标距离,从而获取目标高度信息。但在实际的杂波环境中,信杂比往往很低,使得高度误差太大,影响雷达的测高精度。     

采用SIFT特征的高光谱数据自动几何精校正

针对几何精校正过程中人工选取控制点误差大、未考虑高光谱数据光谱特征一致性等问题,提出了基于SIFT特征的自动几何精校正方法。首先提取图像的SIFT特征,利用高光谱数据的地理坐标定位进行局部特征匹配,然后为了进一步提取高精度、分布均匀的控制点,提出了一种分区域的随机采样一致（Random Sample Consensus,RANSAC）算法。利用航空高光谱成像仪Hymap获取的新疆东天山数据进行算法性能的分析与验证,并采用CE90/CE95以及均方根误差等指标进行定位精度的评价,提出的基于SIFT特征的自动几何精校正方法能够达到0.8像元的定位精度,并且校正前后光谱的光谱角小于0.01 rad。     

空对地雷达精确定位的电波折射误差修正方法

要利用空中飞行器上武器来攻击地面目标,首先必须对地面目标进行精确定位,然后才能实施精确打击.大气介质的不均匀性使得雷达测量定位产生折射误差,从而影响雷达的定位精度.因此对高精度的雷达系统,必须进行电波折射误差修正.这里采用空中雷达处的大气折射率来预测空中雷达电波传播的大气剖面,再经过电波射线描迹方法推出了一种实用于空中雷达对地面目标精确定位的电波折射误差修正方法.仿真计算表明:俯视雷达与同一传播路径上地基雷达的计算结果很吻合.随着俯视角度的增大,电波折射引起的误差逐渐减小,反之,俯视角度愈小,电波折射误差愈大.当雷达在10 km高度时,5°以下俯角的电波折射误差达10 m以上.     

MODIS imagery geometric precision correction based on longitude and latitude information

Moderate resolution imaging spectrum-radiometer (MODIS) imagery is an important remote sensing source, whose goal is to research and forecast the earth changes. In order to provide high precision imagery for MODIS application, geometric correction is one of the most important steps in the image preprocessing. The principle and procedure of geometric correction are introduced in this paper. The appropriate selection method of ground control points (GCPs) is very crucial for geometric correction. Because there are much useful latitude and longitude information in MODIS data, these geographic data are used as GCP to realize geometric precision correction. Through selecting the proper map projection, the original image is transformed from original image coordinate system to map coordinate system. Using the polynomial transformation and the inverse distance weighted interpolation, the corrected imagery can be obtained at last. Experimental results indicate that the quality and precision of the corrected imagery can meet the requirements.     

环境温度对红外辐射测量精度的影响及修正

为了提高靶场红外系统的辐射特性测量精度,找出外场复杂环境温度变化对测量精度的影响并进行相应地修正。首先,建立了红外系统辐射定标的数学模型,模型中包含杂散辐射噪声影响和内部暗电流噪声的影响。接着,通过在高、低温箱内进行辐射定标实验,找出环境温度变化对辐射定标模型中各参数的影响。实验结果表明:环境温度改变引起的杂散辐射变化对辐射特性测量精度有较大影响。同时发现,环境温度变化时,杂散辐射引起的探测器像元灰度输出变化量近似等于实测的全系统像元灰度变化量,最后根据这一结论提出一种简易的适合外场应用的红外系统辐射定标数据修正方法,采用这种方法,以70℃黑体为目标,在0~50℃的温度范围内,黑体温度测量误差的均方根由0.8℃降低到0.095℃。在不增加操作难度的同时,大大地提高了外场红外辐射特性测量精度。     

电离层虚高对超视距雷达多站联合定位精度的影响

在天波超视距雷达系统的短基线多站联合定位中,一般假设多站点电离层反射虚高保持一致,为确定这一假设对定位结果的影响,本文进行电离层探测试验,以研究电离层虚高对多站联合定位精度的影响。试验时分别架设两个接收站模拟短基线超视距雷达系统的接收站点,再在较远距离架设目标站点,利用来自目标站点的发射信号模拟目标的返回信号。本文假设参考站点到目标站点链路的电离层反射虚高和大圆距离是已知的,对于同一工作频率,利用参考站点-目标站点链路上的电离层虚高,去解算定位站点-目标站点之间的大圆距离。参考站点和定位站点相距约90 km情况下,结果显示:目标和定位站(道孚-武汉)大圆距离约为1 260 km时,两条链路的虚高均方根误差约为5.82 km,相应的大圆距离的定位均方根误差约为5.02 km,相对误差约为0.34%;当目标和定位站(乐山-武汉)大圆距离约为1 000 km时,误差分别约为5.5 km, 5.69 km和0.46%。试验结果和理论分析表明,可以从缩短接收站点的布局和降低电离层反射虚高两个方面进一步提高目标定位的精度。本文试验结果可为短基线天波超视距雷达的建设提供较为重要的参考价值。     

TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphererange correction algorithm and pre-launch error budget

For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted     

Adjusting software failure rates that are estimated from test data

Software test environments are often different from field environments. Using test data exclusively to estimate a field failure rate will not usually give a very accurate estimate. In this paper, we extend an empirical calibration methodology for adjusting the failure rate estimate obtained from analysing test data. In addition to scaling the estimated failure rate of a fault, we propose scaling the estimated number of residual faults as well. We also derive likelihood ratio tests to formally determine (from previous releases of the software) if test, and field environments are significantly different. We illustrate our new results with two telecommunications case studies. The combination of the likelihood ratio test, and the calibration methodology offers a practical way to extend the application of software reliability growth models to less formal test environments.     

基于光纤双波干涉的台阶高度在线高精度测量研究

提出一种基于双波干涉的台阶高度光纤在线干涉测量系统。利用光纤光栅(FBG)只反射布拉格波长的特性,使两个波长同时并独立地工作于同一个光纤干涉测量系统中,每个波长的干涉信号分别并同时被探测,将测量量程扩大为半合成波波长。利用反馈电路补偿环境干扰对光纤干涉测量系统的影响,使光纤干涉测量系统适合于在线测量。本光纤干涉测量系统可测量的最大台阶高度达1mm,测量分辨率小于1nm。     

EGNOS test bed ionospheric corrections under the October and November 2003 storms

Two severe geomagnetic storms were experienced on October 29-31 and November 20, 2003, degrading significantly the European Geostationary Navigation Overlay Service (EGNOS) Test Bed (ESTB) performance in Europe. Such storms reached extreme values of Kp=9 during the most severe periods. The analysis of the ESTB ionospheric corrections and their effect on the ESTB integrity and accuracy is presented in this work. The ESTB performance during those storms was monitored from a network of global positioning system (GPS) receivers widely distributed over Europe, including the ESTB reference stations, and the geographical degradation of the accuracy is analyzed in this paper. The correlation between the Kp index and the misleading information (MI) events is also shown. During the most severe stormy periods, the errors in the ESTB ionospheric corrections and its integrity bounds are analyzed to explain the peaks in the navigation system error, which produces MIs. This analysis has been carried out by comparing with direct dual-frequency GPS measurements and global ionospheric maps.