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1、专业好文档海潮对高精度GPS网的影响初探赵庆海 郭玉良 方明 任锴(西安测绘信息技术总站 西安 710054)摘 要：叙述了海洋负荷潮改正的基本原理，结合算例分析了海潮对GPS网的影响，指出海潮改正对近海短时间GPS观测高程方向的影响可达数厘米。关键词：GPS 海潮 测站形变1 引言海潮（海洋的潮汐现象）是指在日月等天体引力作用下实际海面相对于平均海平面的周期性涨落，这种涨落主要体现在日周期变化，长期变化相对很小。潮汐现象促使海水质量重新分布，从而产生海洋潮汐的附加位，这一附加位的变化也会引起地面测站的变形，特别是在近海地区，这一变形在垂直方向的瞬时位移可达数厘米1。对于24小时观测的高精度G
2、PS数据处理方案来说，已经在很大程度上消除了周日潮汐的影响。但是对于较短时间的GPS观测（小于4小时），这一影响的量级如何，是否影响GPS网的精度，还需要结合实际数据进行分析。潮汐现象非常复杂，用严格的数学模型进行描述是一个十分困难的事情。早期的海潮模型是利用沿海验潮资料基于流体力学模型建立的，如Schwiderski海潮模型。随着技术的发展，近年来卫星测高技术发展迅速，获得了全球水面地形数据，为精确的全球海潮模型研究奠定了基础，如国际上常用的CRS4.0海潮模型等相继出现。由于缺少海潮资料等原因，许多GPS精密定位软件实际上并没有顾及海潮改正。目前美国麻省理工学院（MIT）研制的高精度GPS
3、分析软件GAMIT/GLOBK已加入海潮改正，即通过两个系数文件实现这项改正，两个文件一个是grid.oct全球范围的格网表（格网间隔：短期项0.5,长期项1.0）；另一个是stations.oct，包括全球300多个VLBL、SLR、GPS站的11个分潮波的振幅和相位，这11个分潮波分别是：，其中为半日潮项，为全日潮项，为长周期潮项。计算海潮改正时，首先查找stations.oct文件中与计算站点最近台站的海潮系数，如果与最近台站的距离小于10km，则采用此台站的系数进行计算；否则，利用grid.oct文件进行内插计算2 3。2 海潮负荷改正基本原理据文献1，根据海潮模型可以计算出地球上任何
4、一点的潮高： (1)式中为天文幅角数，与太阳和月亮的位置有关，可由IERS标准提供的子程序计算。由于海潮负荷极不规则，在潮汐负荷形变计算中一般分两步进行，先求出地球对单位点质量负荷的响应函数，称为Green函数；然后利用Green函数对不规则负荷进行褶积积分。海潮负荷可用潮高与Green函数的褶积表示： (2)式中，为海水密度，为潮高，和分别为测站和流动负荷点的纬度和经度，为格林函数，为测站到点负荷的方位角，为球面角。实际应用时，可以根据计算出的格林函数，将式（2）展开为11阶的调和函数。在测站坐标系中，测站在时刻由于海潮引起的位移按下式计算： (3)式中，和为第j个分潮波在测站处的振幅和相应
5、于格林尼治的相位滞后角，和是第j个分潮波的角频率和初始相位。3 算例分析算例采用某沿海C级GPS网，全网由27点组成，2003年5月观测，每个站点均观测一个4小时的时间段。基线解算时加入6个中国地球运动观测网络工程基准站的同步数据，采用美国MIT的高精度GAMIT软件，并使用IGS后处理精密星历，基线平差采用作者自己编写的软件GPSADJ。首先不考虑海潮负荷改正，即不加入stations.oct和grid.oct海潮系数文件，利用GAMIT软件进行解算，并使用GPSADJ进行基线平差，结果如下：表1 不考虑海潮改正解算结果统计表单位权中误差/m点中误差平均值/cm点中误差最大值/cm基线重复性
6、/mm+ppbNEUNEUNEU1.160.290.741.750.642.554.240.5+0.90.2+1.20.6+1.4考虑海潮改正后，平差结果统计如下：表2 考虑海潮改正解算结果统计表单位权中误差/m点中误差平均值/cm点中误差最大值/cm基线重复性/mm+ppbNEUNEUNEU0.430.220.551.260.370.892.270.4+0.60.3+0.60.2+0.4从表1和表2的比较可以看出，考虑海潮改正后，基线的解算精度明显提高，无论是单位权中误差、点位中误差还是基线重复性均明显变小，甚至高程方向的基线重复性还好于水平方向，说明海潮改正对提高高程方向的精度有较大贡献。
7、下面是海潮改正前后点位分量的变化：表3 加入海潮改正前后点位分量变化统计表点位分量差距平均值/cm点位分量差距最大值/cm点位分量差距最小值/cmNEUNEUNEU-0.080.001.120.210.844.50-0.54-0.660.09图1 加入海潮改正前后点位分量变化从表3和图1可以看出，海潮改正前后点位水平分量的差距绝对值均小于1cm，且正负分布基本均匀，差距平均值小于1mm，说明海潮的影响不明显；高程分量差距明显，且符号相同，平均差距大于1cm，最大为4.50cm，和文献2的24小时GPS观测结果相比，要显著的多，和文献1的分析结果的量级很接近，这说明海潮改正对高精度的GPS测量的
8、影响已不可忽视。4 结束语结合算例分析了海潮对GPS网的影响，初步结果显示：对于近海地区的较短时段的GPS观测，考虑海潮负荷改正可以明显的提高解算精度，特别对改善高程方向的精度贡献明显，这种影响的量级对水平方向不明显，对高程方向的影响可达数厘米，因此对于高精度的GPS测量来说这种影响是不容忽视的。参考文献1 刘根友. 高精度GPS测量中的潮汐改正C. 大地测量与地球动力学进展，2004：398-405.2 段五杏等. Sherneck海潮系数对中国GPS网的适用性分析J. 测绘通报，2001，（3）：18-19.3 Documentation for the GAMIT GPS Analysi
9、s SoftwareR, 2000Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wonder what it was like to huddle around a shortwave radi
10、o and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way before my time.As a kid, I was fascinated with what goes o
11、n in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events, but right now, if you were sitting next to me, youd h
12、ear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch a man take a leap into an unimaginable free fall from
13、the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the winds, knowing that if they would just line up in a ver
14、tical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territory.The guy who is going to do this, Felix Baumgartner, m
15、ust have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere. As soon as the 40-acre balloon, with skin no thicker th
16、an a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communications), Col. Joe Kittinger. He hung his head low in m
17、ission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - winds less than 2 mph, with no precipitation or humidity
18、 and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers), drifting even higher than the cruising altitude of c
19、ommercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles/36.53 kilometers). Here, Fearless Felix will unclip. H
20、e will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell be traveling fast, so despite the distance, it will not
21、 be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting the top of the water, he will begin to slow as he appr
22、oaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Instead, he plans to deploy his 270-square-foot (25-square
23、-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness at mach speeds.Even if everything goes as planned, it w
24、ont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.