再分析资料计算中国区域对流层延迟精度

针对欧洲中期天气预报中心(ECMWF)提供的ERA-Interim气压分层再分析资料在中国区域对流层延迟解算的适用性问题,该文基于中国大陆构造环境监测网络(CMONOC)约260个GPS基准站,以GPS PPP事后精密处理结果作为参考,给出了相应的精度评估与时空分布特征分析,结果表明:再分析资料在中国区域对流层延迟的平均偏差和均方根误差分别为0.39cm和1.37cm,时空分布上存在明显差异,另外指出了再分析资料时间分辨率不足,难以反映对流层延迟日内变化的问题。为进一步研究对流层空间关系以及垂直变化特征,精化对流层改正模型提供参考。     

ERA-Interim应用于中国地区地基GPS/PWV计算的精度评估

目的 由于中国绝大多数地基GPS网观测时未作气象观测,致使已积累的大量 GPS观测数据无法在气象领域发挥作用。针对这一情况,研究和分析了利用 ERA-Interim再分析产品获取中国地区气象资料和计算GPS/PWV的方法。以全国分布的24个气象观测站2006、2007年的气压、温度和相对湿度的实测资料为标准,评估了中国地区 ERA-Interim 再分析资料提取这三个参 数和 计 算 GPS/PWV 所能达到的精度,并进行了精度评估。     

利用中国区域MERRA-2资料计算ZTD和ZWD的精度分析

对流层延迟是影响高精度定位与导航的主要误差之一,也是全球导航卫星系统（global navigation satellite system,GNSS）水汽探测的关键参数。美国航空航天局发布了最新一代的大气再分析资料（MERRA-2资料）,其可用于计算高时空分辨率的对流层延迟产品,但是目前尚无文献对利用MERRA-2资料计算天顶对流层延迟（zenith tropospheric delay,ZTD）和天顶湿延迟（zenith wet delay,ZWD）的精度进行分析。因此,联合2015年中国陆态网214个GNSS站ZTD产品和分布于中国区域的87个探空站资料,对利用MERRA-2资料在中国区域计算ZTD/ZWD的精度进行评估。结果表明：（1）以陆态网ZTD为参考值,利用MERRA-2资料积分计算ZTD的年均偏差和均方根误差（root mean square error,RMSE）分别为0.32 cm和1.21 cm,且偏差和RMSE均表现出一定的季节变化,总体上呈现为夏季精度低、冬季精度高;在空间分布上,偏差随纬度和高程的变化趋势并不明显,但RMSE随纬度和高程的增加总体上呈现递减的趋...     

Characterization of diurnal cycles in ZTD from a decade of global GPS observations

The diurnal cycle of the tropospheric zenith total delay (ZTD) is one of the most obvious signals for the various physical processes relating to climate change on a short time scale. However, the observation of such ZTD oscillations on a global scale with traditional techniques (e.g. radiosondes) is restricted due to limitations in spatial and temporal resolution. Nowadays, the International GNSS Service (IGS) provides an important data source for investigating the diurnal and semidiurnal cycles of ZTD and related climatic signals. In this paper, 10 years of ZTD data from 1997 to 2007 with a 2-hour temporal resolution are derived from global positioning system (GPS) observations taken at 151 globally distributed IGS reference stations. These time series are used to investigate diurnal and semidiurnal oscillations. Significant diurnal and semidiurnal oscillations of ZTD are found for all GPS stations used in this study. The diurnal cycles (24 hours period) have amplitudes between 0.2 and 10.9 mm with an uncertainty of about 0.5 mm and the semidiurnal cycles (12 h period) have amplitudes between 0.1 and 4.3 mm with an uncertainty of about 0.2 mm. The larger amplitudes of the diurnal and semidiurnal ZTD cycles are observed in the low-latitude equatorial areas. The peak times of the diurnal cycles spread over the whole day, while the peak value of the semidiurnal cycles occurs typically about local noon. These GPS-derived diurnal and semidiurnal ZTD signals are similar with the surface pressure tides derived from surface synoptic pressure observations, indicating that atmospheric tides are the main driver of the diurnal and semidiurnal ZTD variations.     

Assessment of terrain elevation derived from satellite laser altimetry over mountainous forest areas using airborne lidar data

Gaussian decomposition has been used to extract terrain elevation from waveforms of the satellite lidar GLAS (Geoscience Laser Altimeter System), on board ICESat (Ice, Cloud, and land Elevation Satellite). The common assumption is that one of the extracted Gaussian peaks, especially the lowest one, corresponds to the ground. However, Gaussian decomposition is usually complicated due to the broadened signals from both terrain and objects above over sloped areas. It is a critical and pressing research issue to quantify and understand the correspondence between Gaussian peaks and ground elevation. This study uses ～2000 km² airborne lidar data to assess the lowest two GLAS Gaussian peaks for terrain elevation estimation over mountainous forest areas in North Carolina. Airborne lidar data were used to extract not only ground elevation, but also terrain and canopy features such as slope and canopy height. Based on the analysis of a total of ～500 GLAS shots, it was found that (1) the lowest peak tends to underestimate ground elevation; terrain steepness (slope) and canopy height have the highest correlation with the underestimation, (2) the second to the lowest peak is, on average, closer to the ground elevation over mountainous forest areas, and (3) the stronger peak among the lowest two is closest to the ground for both open terrain and mountainous forest areas. It is expected that this assessment will shed light on future algorithm improvements and/or better use of the GLAS products for terrain elevation estimation.     

Improved amplitude- and phase-scintillation indices derived from wavelet detrended high-latitude GPS data

Accuracy and validity of scintillation indices estimated using the power and phase of the GPS signal depend heavily on the detrending method used and the selection of the cutoff frequency of the associated filter. A Butterworth filter with a constant cutoff frequency of 0.1?Hz is commonly used in detrending GPS data. In this study, the performance of this commonly used filter is evaluated and compared with a new wavelet-based detrending method using GPS data from high latitudes. It was observed that in detrending high-latitude GPS data, a wavelet filter performed better than Butterworth filters as the correlation between amplitude- and phase-scintillation indices in S ₄ and ?? _? improved significantly from 0.53, when using a Butterworth filter, to 0.79, when using the wavelet filtering method. We also introduced an improved phase-scintillation index, ?? _CHAIN, which we think is comparatively a better parameter to represent phase scintillations at high latitudes as the correlation between S ₄ and ?? _CHAIN was as high as 0.90. During the analysis, we also noted that the occurrence of the ??phase scintillation without amplitude scintillation?? phenomenon was significantly reduced when scintillation indices were derived using the wavelet-based detrending method. These results seem to indicate that wavelet-based detrending is better suited for GPS scintillation signals and also that ?? _CHAIN is a better parameter for representing GPS phase scintillations at high latitudes.     

联合多代卫星测高数据建立高分辨率浙江近海垂线偏差模型

联合多代卫星测高数据,研究共线平均理论,在时域上削弱测高数据短波误差影响。基于EGM2008重力场模型及DTU10海面地形模型,采用沿轨迹加权最小二乘方法,确定浙江近海2.5′×2.5′分辨率格网点垂线偏差子午分量ξ和卯酉分量η,将所得计算结果与EGM96、EGM2008、ITG-Grace2010s模型值进行比较。结果表明:浙江近海垂线偏差模型与EGM2008模型的精度较为相近,在子午圈及卯酉圈上的RMS分别为±0.15320″、±0.63061″。     

Quality assessment of COSMIC/FORMOSAT-3 GPS radio occultation data derived from single- and double-difference atmospheric excess phase processing

This study evaluates the quality of GPS radio occultation (RO) atmospheric excess phase data derived with single- and double-difference processing algorithms. A spectral analysis of 1 s GPS clock estimates indicates that a sampling interval of 1 s is necessary to adequately remove the GPS clock error with single-difference processing. One week (May 2–8, 2009) of COSMIC/FORMOSAT-3 data are analyzed in a post-processed mode with four different processing strategies: (1) double-differencing with 1 s GPS ground data, (2) single-differencing with 30 s GPS clock estimates (standard COSMIC Data Analysis and Archival Center product), (3) single-differencing with 5 s GPS clocks, and (4) single-differencing with 1 s GPS clocks. Analyses of a common set of 5,596 RO profiles show that the neutral atmospheric bending angles and refractivities derived from single-difference processing with 1 s GPS clocks are the highest quality. The random noise of neutral atmospheric bending angles between 60 and 80 km heights is about 1.50e−6 rad for the single-difference cases and 1.74e−6 rad for double-differencing. An analysis of pairs of collocated soundings also shows that bending angles derived from single-differencing with 1 s GPS clocks are more consistent than with the other processing strategies. Additionally, the standard deviation of the differences between RO and high-resolution European Center for Medium range Weather Forecasting (ECMWF) refractivity profiles at 30 km height is 0.60% for single-differencing with 1 and 5 s GPS clocks, 0.68% for single-differencing with 30 s clocks, and 0.66% for double-differencing. A GPS clock-sampling interval of 1 s or less is required for single- and zero-difference processing to achieve the highest quality excess atmospheric phase data for RO applications.     

中国西部区域GRACE卫星重力变化

针对研究中国西部长期重力变化的问题,该文利用GRACE月重力场模型,基于Slepian变换构建中国西部区域局部重力场,并采用GIA和GLDAS等模型扣除相应误差,通过时间序列分析方法得到该区域2003—2013年卫星重力变化。结果表明,东天山呈现负重力变化,可能由区域冰川消融引起的;西天山的正重力变化,应与冰川小幅增加和构造运动引起的壳幔物质积累有关;青藏高原内陆正重力变化可能由区域地壳抬升、粘弹性地壳的构造应变以及壳幔物质的质量迁移与积累引起;青藏高原边界区域的负重力变化,应与冰川加速消融以及地下水抽取有关。Slepian方法较好地克服了滤波平滑处理带来的重力变化信号压制及细节平滑,可为中国西部的地壳运动、地震活动和气候变化等研究提供数据支撑。     

天顶静力延迟模型对GPS可降水量反演的影响分析及改进

利用地基GPS遥感大气可降水量(PWV,precipitable water vapor)过程中,天顶静力延迟(ZHD,zenith hydrostatic delay)模型精度直接影响PWV反演精度。文中将常用天顶静力延迟模型(Black模型、Hopfield模型和Saastamoinen模型)计算所得ZHD与基于湖南省2016-04-01—2017-03-31高空气象探测秒级数据计算所得探空ZHD进行对比,发现模型存在较明显系统偏差,其中Black模型ZHD平均偏低40mm以上。利用探空实测ZHD对天顶静力延迟模型进行回归建模,订正模型系数后可明显减小模型系统偏差和均方误差。改进后的ZHD模型可明显减小GPSPWV反演的系统偏差,并略微降低均方误差,其中采用改进后SA模型反演所得GPSPWV与探空PWV相比,平均偏低不超过0.230 7mm,反演准确性有较显著改善。     

Combined GPS/GLONASS data processing

To obtain the GLONASS satellite position at an epoch other than reference time,the satellite‘s equation of motion has to be integrated with broadcasting ephemerides.The iterative detecting and repairing method of cycle slips based on triple difference residuals for combined GPS/GLONASS positioning and the iterative ambiguity resolution approach suitable for combined post processing positioning are discussed systematically.Experiments show that millimter accuracy can be achieved in short baselines with a few hours‘ dual frequency or even single frequency GPS/GLONASS carrier phase observation,and the precision of dual frequency observations is distinctly higher than that of single frequency observations.     

Combined GPS/GLONASS data processing

To obtain the GLONASS satellite position at an epoch other than reference time, the satellite's equation of motion has to be integrated with broadcasting ephemerides. The iterative detecting and repairing method of cycle slips based on triple difference residuals for combined GPS/GLONASS positioning and the iterative ambiguity resolution approach suitable for combined post processing positioning are discussed systematically. Experiments show that millimeter accuracy can be achieved in short baselines with a few hours' dual frequency or even single frequency GPS/GLONASS carrier phase observations, and the precision of dual frequency observations is distinctly higher than that of single frequency observations.     

Land surface phenology from VEGETATION and PROBA-V data. Assessment over deciduous forests

Land surface phenology has been widely retrieved although no consensus exists on the optimal satellite dataset and the method to extract phenology metrics. This study is the first comprehensive comparison of vegetation variables and methods to retrieve land surface phenology for 1999–2017 time series of Copernicus Global Land products derived from SPOT-VEGETATION and PROBA-V data. We investigated the sensitivity of phenology to (I) the input vegetation variable: normalized difference vegetation index (NDVI), leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR), and fraction of vegetation cover (FCOVER); (II) the smoothing and gap filling method for deriving seasonal trajectories; and (III) the method to extract phenological metrics: thresholds based on a percentile of the annual amplitude of the vegetation variable, autoregressive moving averages, logistic function fitting, and first derivative methods. We validated the derived satellite phenological metrics (start of the season (SoS) and end of the season (EoS)) using available ground observations of Betula pendula, B. alleghaniensis, Acer rubrum, Fagus grandifolia, and Quercus rubra in Europe (Pan-European PEP725 network) and the USA (National Phenology Network, USA-NPN). The threshold-based method applied to the smoothed and gap-filled LAI V2 time series agreed best with the ground phenology, with root mean square errors of ˜10 d and ˜25 d for the timing of SoS and EoS respectively. This research is expected to contribute for the operational retrieval of land surface phenology within the Copernicus Global Land Service.     

Quantifying winter wheat residue biomass with a spectral angle index derived from China Environmental Satellite data

Quantification of crop residue biomass on cultivated lands is essential for studies of carbon cycling of agroecosystems, soil-atmospheric carbon exchange and Earth systems modeling. Previous studies focus on estimating crop residue cover (CRC) while limited research exists on quantifying crop residue biomass. This study takes advantage of the high temporal resolution of the China Environmental Satellite (HJ-1) data and utilizes the band configuration features of HJ-1B data to establish spectral angle indices to estimate crop residue biomass. Angles formed at the NIR_IRS vertex by the three vertices at R, NIR_IRS, and SWIR (ANIR_IRS) of HJ-1B can effectively indicate winter wheat residue biomass. A coefficient of determination (R²) of 0.811 was obtained between measured winter wheat residue biomass and ANIR_IRS derived from simulated HJ-1B reflectance data. The ability of ANIR_IRS for quantifying winter wheat residue biomass using HJ-1B satellite data was also validated and evaluated. Results indicate that ANIR_IRS performed well in estimating winter wheat residue biomass with different residue treatments; the root mean square error (RMSE) between measured and estimated residue biomass was 0.038 kg/m². ANIR_IRS is a potential method for quantifying winter wheat residue biomass at a large scale due to wide swath width (350 km) and four-day revisit rate of the HJ-1 satellite. While ANIR_IRS can adequately estimate winter wheat residue biomass at different residue moisture conditions, the feasibility of ANIR_IRS for winter wheat residue biomass estimation at different fractional coverage of green vegetation and different environmental conditions (soil type, soil moisture content, and crop residue type) needs to be further explored.     

一种改进的低纬度地区NWP反演ZTD积分模型

针对数值天气预报(NWP)模型在低纬度地区反演对流层延迟(ZTD)精度较差的问题,该文在原有积分模型的基础上,从考虑重力变化、统一高程系统、补偿缺失数据和分段积分4个方面着手提出了一种改进的积分模型,并采用低纬度地区108个IGS站点2018年全年的ERA-interim大气等压面数据及IGS ZTD数据进行实验以评估此改正积分模型的性能。结果表明,改进的积分方法较传统的积分方法精度高,具体表现为测站ZTD平均残差减小约80.93%,RMS平均减小约17.57%。此外,本文比较了5种插值方法估计NWP反演的GNSS ZTD的精度,结果表明在低纬度地区利用克里金插值和反距离权重插值得到的ZTD精度优于其他插值方法。     

一种GPS/GLONASS/INS数据融合算法

针对卫星导航系统和惯性导航系统(INS)的不同特性,提出了一种GPS/GLONASS/INS数据融合算法。采用差分自适应检测算法、改进码平均相位算法以及位置联合解算方法实现了GPS/GLONASS数据融合,借助于改进的粒子滤波器、INS误差模型建立系统状态方程和观测方程,完成GPS/GLONASS系统速度值和INS系统速度值数据融合,提高组合导航系统精度和可靠性。使用真实数据对数据融合算法性能进行仿真分析,结果表明所设计算法是有效的,能够处理非线性非高斯条件下的滤波估计,提高滤波精度和系统可靠性。     

Analysis of the EUREF-89 GPS data from the SLR/VLBI sites

In May 1989, the IAG Subcommission for the European Reference Frame organized a GPS measurement campaign, called EUREF-89, to establish a common European Reference Frame. During a 2-week period various types of GPS receivers were deployed at about 100 different locations in Europe, which included many national geodetic first order points and most of the well-known SLR and VLBI sites. In this study, the measurements from those SLR and VLBI sites, and three additional points in The Netherlands, have been analyzed adopting a fiducial network approach. In the first place, the study provided valuable experience in the use of the GIPSY software for the analysis of GPS data from large networks equipped with a mixture of receiver types. Furthermore, this analysis represents an independent check of the SLR/VLBI network, used as the reference frame for the official EUREF solution. Daily solutions of baselines up to 2500 km in length have been obtained with a repeatability of 0.5–2.0 parts in 10⁸, while the agreement with SLR results is at about the same level. The accuracy of the estimated coordinates is at a level of about 4.0 cm in the horizontal and 6.0 cm in the vertical direction. Of particular interest are the results for some baselines in Greece, which have also been measured by mobile SLR in the framework of the WEGENER/MEDLAS project. The GPS results seem to confirm the trends in the baseline length changes emerging from those SLR studies.     

Geocenter variations derived from GPS tracking of the GRACE satellites

Two 4.5-year sets of daily geocenter variations have been derived from GPS-LEO (Low-Earth Orbiter) tracking of the GRACE (Gravity Recovery And Climate Experiment) satellites. The twin GRACE satellites, launched in March 2002, are each equipped with a BlackJack global positioning system (GPS) receiver for precise orbit determination and gravity recovery. Since launch, there have been significant improvements in the background force models used for satellite orbit determination, most notably the model for the geopotential, which has resulted in significant improvements to the orbit determination accuracy. The purpose of this paper is to investigate the potential for determining seasonal (annual and semiannual) geocenter variations using GPS-LEO tracking data from the GRACE twin satellites. Internal comparison between the GRACE-A and GRACE-B derived geocenter variations shows good agreement. In addition, the annual and semiannual variations of geocenter motions determined from this study have been compared with other space geodetic solutions and predictions from geophysical models. The comparisons show good agreement except for the phase of the z-translation component.     

以GPS资料研究中国南北地震带深部构造应力场变化特征

本文利用中国南北地震带2004-2007年间观测的239个GPS站数据,对中国南北地震带地壳内15km深处的最大主应力和最大剪应力进行了计算分析,并将计算结果与同期发生在中国南北地震带的地震活动进行了对比分析,结果表明观测同期发生的中强震大都位于应力变化大并且有深大断裂的地区,汶川地震进一步说明,四川地区地壳内的局部应力场变化与最大剪应力变化在空间分布上与强震活动区域存在较好的一致性。