Observational evidence for massive black holes in the centers of active galaxies

Naturally occurring water vapor maser emission at 1.35 cm wavelength provides an accurate probe for the study of accretion disks around highly compact objects, thought to be black holes, in the centers of active galaxies. Because of the exceptionally fine angular resolution, 200 microarcseconds, obtainable with very long baseline interferometry, accompanied by high spectral resolution, <0.1 km s^-1, the dynamics and structures of these disks can be probed with exceptional clarity. The data on the galaxy NGC 4258 are discussed here in detail. The mass of the black hole binding the accretion disk is 3·9 × 10⁷ M_⊙. Although the accretion disk has a rotational period of about 800 years, the physical motions of the masers have been directly measured with VLBI over a period of a few years. These measurements also allow the distance from the earth to the black hole to be estimated to an accuracy of 4 per cent. The status of the search for other maser/black hole candidates is also discussed.     

卫星雷达干涉测量原理与应用

本文综述卫星雷达干涉测量和差分干涉测量的基本原理及其应用。文章从比较雷达干涉测量与一般航测遥感的基本差别出发 ,介绍各种干涉模式下的雷达干涉测量原理、差分干涉测量原理和数据处理流程。在简要叙述雷达干涉测量应用领域后 ,重点列出 INSAR在地形测量、火山地形测量和 D- INSAR在地形沉降监测中的应用。最后 ,本文还探讨了存在的问题和应用前景。     

The sio masers and dust shell of VX sgr

The circumstellar envelope of the late-type supergiant VX Sagittarii has been studied with VLBI observations of SiO maser emission (J = 1 0v = 1) and interferometric observations of mid-infrared dust emission.     

用InSAR技术提取数字高程模型的研究

合成孔径雷达干涉测量(InSAR,Synthetic Aperture Radar)是一种获取地面数字高程模型(DEM)和探测地面微小形变的新技术。这里概述了干涉测量的发展历史,介绍了干涉测量的基本原理,并利用青藏高原库塞湖地区的数据对干涉测量获得地面数字高程模型进行了详细的阐述,最后对生成的地面数字高程模型进行结果分析。     

Interferometric Baseline Performance Estimations for Multistatic Synthetic Aperture Radar Configurations Derived from GRACE GPS Observations

Recent studies have demonstrated the usefulness of global positioning system (GPS) receivers for relative positioning of formation-flying satellites using dual-frequency carrier-phase observations. The accurate determination of distances or baselines between satellites flying in formation can provide significant benefits to a wide area of geodetic studies. For spaceborne radar interferometry in particular, such measurements will improve the accuracy of interferometric products such as digital elevation models (DEM) or surface deformation maps. The aim of this study is to analyze the impact of relative position errors on the interferometric baseline performance of multistatic synthetic aperture radar (SAR) satellites flying in such a formation. Based on accuracy results obtained from differential GPS (DGPS) observations between the twin gravity recovery and climate experiment (GRACE) satellites, baseline uncertainties are derived for three interferometric scenarios of a dedicated SAR mission. For cross-track interferometry in a bistatic operational mode, a mean 2D baseline error (1σ) of 1.4 mm is derived, whereas baseline estimates necessary for a monostatic acquisition mode with a 50 km along-track separation reveal a 2D uncertainty of approximately 1.7 mm. Absolute orbit solutions based on reduced dynamic orbit determination techniques using GRACE GPS code and carrier-phase data allows a repeat-pass baseline estimation with an accuracy down to 4 cm (2D 1σ). To assess the accuracy with respect to quality requirements of high-resolution DEMs, topographic height errors are derived from the estimated baseline uncertainties. Taking the monostatic pursuit flight configuration as the worst case for baseline performance, the analysis reveals that the induced low-frequency modulation (height bias) fulfills the relative vertical accuracy requirement (σ<1 m linear point-to-point error) according to the digital terrain elevation data level 3 (DTED-3) specifications for most of the baseline constellations. The use of a GPS-based reduced dynamic orbit determination technique improves the baseline performance for repeat-pass interferometry. The problem of fulfilling the DTED-3 horizontal accuracy requirements is still an issue to be investigated. DGPS can be used as an operational navigation tool for high-precision baseline estimation if a geodetic-grade dual-frequency spaceborne GPS receiver is assumed to be the primary instrument onboard the SAR satellites. The possibility of using only single-frequency receivers, however, requires further research effort.Deutsche Forschungsgemeinschaft (DFG) research fellow until Sept. 2004 at the Microwaves and Radar Institute, Deutsche Zentrum für Luft- und Raumfahrt (DLR) e.V., 82234 Weßling, Germany     

Comparative research and its significance of deformation measurements by technologies of laser real-time holographic interferometry and radar differential interferometry

The principles and applications of laser real-time holographic interferometry （LRTHI） and radar differential interferometry （RDI） technologies are described in this paper, respectively. By using LRTHI, we can observe the deformation of samples under pressure in the lab and study the anomaly characteristics relating to different strain fields in different fracture-developing areas; while by using RDI, we can observe the landform and surface deformation. The results of deformation observed before and after the Ms=7.9 Mani earthquake （Tibet） and Ms=6.2 Shangyi-Zhangbei earthquake in China are obtained. It is pointed out that LRTHI and RDi are similar, which study the characteristics of anomalous deformation field by fringe variations for both of them. Therefore, the observation of deformation field in the seismogenic process, especially in the period impending an earthquake by RDI, and the comparative study in the lab by LRTHI are of great significance.     

The 'Pisco' Speckle Camera at Pic Du Midi Observatory

We present a new speckle camera designed and built at Observatoire Midi-Pyrénées. This focal instrument has been used for four years with the 2-meter Bernard Lyot Telescope of Pic du Midi observatory. It can be set in various operating modes: full pupil imaging, masked-pupil imaging, spectroscopy, wave-front sensor and stellar coronagraphy, hence its name 'PISCO' ('Pupil Interferometry Speckle COronagraph'). Restored images of double and triple stars have demonstrated its capabilities in providing close to diffraction limited images (0.06 in V). PISCO has been fully tested and is now ready to be used by the whole astronomical community.     

SAR干涉测量的相干性特征分析及积雪划分

雪盖面积是高山地区和季节雪盖区水文,气象模型的重要输入因子.SAR干涉测量不仅能够生成高精度的DEM,而且能够通过重复轨道雷达信号的相干性来探测地表覆盖的变化.通过分析比较四景重复轨道ERS-1/2的SAR图像发现,裸岩,裸地,灌丛等未受到扰动的地表相干性高,湖面,雪盖等变化明显的地表相干性低.积雪覆盖后的表面,其相干性急剧隐低,利用其相干性特征可以进行积雪划分.根据上述特性,利用地物的散射强度和SAR重复轨道的相干测量进行雪盖划分,分类精度达到82%,结果表明利用SAR干涉测量的相干性特征进行雪盖制图是可行的.     

航天飞机极化干涉雷达数据反演地表植被参数

利用基于极化干涉测量的基本原理和相干散射模型。提出了基于模拟加温-退火算法的极化干涉雷达数据地表植被参数的反演算法，首先，对极化干涉测量的基本原理和一个考虑了地表和植被散射的二层相干散射模型进行了阐述。接着，对模拟退火算法的基本理论和基于模拟加温-退火算法的地表植被参数反演模型进行了论述，最后，利用和田地区1994年10月9日和10日的航天飞机SIR-CL波段单视散射短阵复数据进行了地表植被参数反演的计算，将反演结果与实测数据比较，表明该反演算法能以较好的精度获取地表植被的高度。