Leveling the Sea Surface Using a GPS-Catamaran Special Issue: Jason-1 Calibration/Validation

In the framework of the TOPEX/Poseidon and Jason-1 CNES-NASA missions, two probative experiments have been conducted at the Corsica absolute calibration site in order to determine the local marine geoid slope under the ascending TOPEX/Poseidon and Jason-1 ground track (No. 85). An improved determination of the geoid slope was needed to better extrapolate the offshore (open-ocean) altimetric data to on-shore tide-gauge locations. This in turn improves the overall precision of the calibration process. The first experiment, in 1998, used GPS buoys. Because the time required to cover the extended area with GPS buoys was thought to be prohibitive, we decided to build a catamaran with two GPS systems onboard. Tracked by a boat at a constant speed, this innovative system permitted us to cover an area of about 20 km long and 5.4 km wide centered on the satellites' ground track. Results from an experiment in 1999 show very good consistency between GPS receivers: filtered sea-surface height differences have a mean bias of ?0.2 cm and a standard deviation of 1.2 cm. No systematic error or distortions have been observed and crossover differences have a mean value of 0.2 cm with a standard deviation of 2.7 cm. Comparisons with tide gauges data show a bias of 1.9 cm with a standard deviation of less than 0.5 cm. However, this bias, attributable in large part to the effect of the catamaran speed on the waterline, does not affect the geoid slope determination which is used in the altimeter calibration process. The GPS-deduced geoid slope was then incorporated in the altimeter calibration process, yielding a significant improvement (from 4.9 to 3.3 cm RMS) in the agreement of altimeter bias determinations from repeated overflight measurements.     

GPS positioning accuracy using precise real‐time ephemeris and clock correction

Global positioning system (GPS) has found applications in various areas including marine geodesy. GPS positioning accuracy, however, is greatly degraded by GPS ephemeris and clock errors, particularly errors due to Selective Availability (SA). Thus, it is crucial to use precise ephemeris and clock corrections for users who require high position accuracy. Presently, precise ephemeris and clock corrections are available only in post‐mission. This paper investigates the generation of precise real‐time ephemeris and clock corrections and the positioning accuracy using them. In this research, precise real‐time ephemeris is generated from accurate dynamic orbit prediction and clock corrections are calculated using instantaneous GPS measurements. Numerical analysis using data from an actual GPS tracking network is performed that indicates use of precise ephemeris and clock corrections can improve the positioning accuracy to the one meter level. This accuracy is attainable in real‐time as the precise real‐time ephemeris and clock corrections become available in the future.     

Accurate GPS measurement of the location and orientation of a floating platform

Abstract

This article describes the design and initial tests of the GPS portion of a system for making seafloor geodesy measurements. In the planned system, GPS antennas on a floating platform will be used to measure the location of an acoustic transducer, attached below the platform, which interrogates an array of transponders on the seafloor. Since the GPS antennas are necessarily some distance above the transducer, a short‐baseline GPS interferometer consisting of three antennas is used to measure the platform's orientation.

A preliminary test of several crucial elements of the system was performed at the Scripps Institution of Oceanography (SIO) in December 1989. The test involved a fixed antenna on the pier and a second antenna floating on a buoy about 80 m away. GPS measurements of the vertical component of this baseline, analyzed independently by two groups using different software, agree with each other and with an independent measurement within a centimeter.

The first test of an integrated GPS/acoustic system took place in the Santa Cruz Basin off the coast of southern California in May 1990. In this test a much larger buoy, designed and built at SIO, was equipped with three GPS antennas and an acoustic transducer that interrogated a transponder on the ocean floor. Preliminary analysis indicates that the horizontal position of the transponder can be determined with a precision of about a centimeter. Further analysis will be required to investigate the magnitude of systematic errors.     

测量船动态吃水测量方法研究

针对测量船特别是中大型测量船的动态吃水问题,提出了单船和双船载波相位差分GPS测定方法,给出了动态吃水计算公式。根据装备现状,对比分析了三种测量方法的适用范围,例举了典型参数值的动态吃水观测误差,旨在为测量作业人员提供借鉴和参考。     

GPS单点测速技术在海浪测量中的应用研究

分析比对了GPS单点测速技术与其他已有测波手段的不同,讨论了基于多普勒频移的GPS测速方法,并分析了各类误差对测速精度的影响。分别利用原始多普勒值和导出多普勒值进行了静态测速试验和动态测速试验,将导出多普勒测速结果与RTK（RealTimeKinematic,实时动态测量系统）位置差分测速进行了比测。结果表明,使用静态数据采用高频导出多普勒值测速的精度可以达到亚厘米级每秒,而采用原始多普勒观测值进行测速因接收机型号的不同,结果差异较大;动态测速试验中,采用导出多普勒测速的方法与RTK位置差分测速的符合精度可达cm／s。     

Potential Seismic Risk in Taiwan Strait Region

GPS observation indicates that the Fujian coastal region of China mainland, the region of Taiwan Strait and northern Taiwan island all show a generally homogenous horizontal motion with weak deformation. Historical earthquake record over more than 800 years and modern instrumental data reveal that there is potential seismic risk in and around Taiwan Strait region. After the National Seismic Zoning Map of China (2001) the expected seismic risk in northern part of the Taiwan Strait is lower than that in middle and southern part. The suggested northern route of the Taiwan Strait passage project seems to be relatively save seismically.     

沪宁高速公路一次冬季浓雾过程的数值模拟分析

利用由多个美国研究部门及大学的共同参与研发的新一代中尺度预报模式和同化系统——WRF（Weather Research Forecast）模式,对2006年12月24—27日沪宁高速公路及其周边地区出现的一次罕见的持续性大雾过程进行数值模拟研究。结果表明,此次大雾过程属于较典型的平流雾,其生成和维持的主要原因是大气层结稳定、系统的下沉运动、充足的水汽;同时也有辐射作用,地面的大气长波辐射冷却是雾维持的重要因素。大范围下沉运动使中低层大气增温与地面辐射降温配合,形成深厚的逆温层,对大雾长时间维持起决定性作用。所以这是一次以平流雾为主伴随辐射雾的过程。27日14时后干冷空气南下才使大雾消散。      

Access数据库在地质资料整理中的应用

利用GPS手持机对地质观察点进行坐标数据的采集,通过GPS手持机应用软件（GISOffice、MapSource）将所采集的原始数据传输保存到电脑中,使用Microsoft Office办公软件（Access、Excel）对数据进行整理、应用,或存贮、共享应用于其他地质资料整理.     

上海短历时强降水的雷达和闪电活动特征

用雷达反射率及其导出产品(分层垂直液态水含量(VIL)、分层平均反射率等)与闪电和大气温度层结资料,通过对降水系统时间演变、反射率垂直结构和闪电活动的分析,将上海2010年6—11月的短时强降水(1 h雨量≥20 mm)分为层状暖云型、对流暖云主导型和对流冷云主导型。(1) 与雨强密切相关的关键因子是:低层(0 ℃层以下、高度在0～1 km、0～2 km、0～3 km和0～4 km)的VIL、低层(高度在1～2 km、2～3 km和3～4 km)的平均反射率和整层最大反射率;(2) 不同类型强降水中,高层(高度在0 ℃、-10 ℃和-20 ℃层以上)的VIL、整层VIL、高层(高度在5～6 km、6～7 km和7～8 km)的平均反射率、总地闪和负地闪频次这5个因子对雨强的作用各有不同;(3) 对应不同降水类型的平均反射率垂直廓线具有不同的垂直结构和发展演变特征;(4) 在三类强降水中,对流冷云主导型的雨强与总地闪和正地闪的相关性最好。      

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