Regional Gravity Fields from GOCE and GRACE

地球重力场对海洋学、极地科学、固体地球科学和地球上的高度系统都很重要。如果我们认为海洋是一个虚拟的海洋，它完全是静止的，没有潮汐或洋流，能够在陆地下的狭窄水道中随意流动，那么想象中的水线的表面是平均海平面，科学家称之为大地水准面。因此，大地水准面是一个虚构的表面，接近平均海平面，并随着地球上的位置而变化。从一个地方到另一个地方的变化可以表示为随距离变化的分量的总和，从而产生变化缓慢的项(长波长项)和快速变化的项(短波长项)。科学家们对海洋大地水准面的了解有限，陆地上的情况通常更糟。专门的重力场任务、重力和气候实验(GRACE)以及重力场和稳态海洋环流实验(GOCE)的启动正在开始改变这一点。Grace卫星任务是一对相距约220公里的串联卫星，在地球表面上方约450公里的轨道上运行。它于2001年发射，由GPS提供精确定位，通过微波设备测量卫星之间的距离。如果我们考虑由弹性线连接的卫星，长度将增加和减少，因为卫星的轨道受到地球引力的影响。通过利用这种可变性，就有可能确定引力效应。GOCE是一项计划于2009年发射的欧洲任务。这项任务与Grace的概念类似，但串列卫星的质量相距约1米。这种点质量的三维阵列之间的重力差异由加速度计持续监测，加速度计在一种称为梯度测量的过程中提供重力变化率。梯度法的一个问题是由于空气阻力引起的大扰动，结果是必须构造加速度计以避免饱和。这限制了用梯度计无法测量的长波长的重力场的可观测波长。相反，这些波长将通过GPS跟踪来确定，但精度低于GRACE，因此这两次任务是互补的，为重力场提供了在长波长和短波长下的高精度。GRACE在较长的波长上产生高精度的重力场分量，在较短的波长上产生GOCE。在这项研究中，我们打算合并这两个数据集，以产生比任一特派团本身所能获得的解决方案更好的解决方案。我们还将获得北极和南极洲等地区的重力场，因为这将使我们能够利用Grace提供的全部重力场信号，这些信号在寻求单一的全球解决方案时可能会丢失。当然，区域领域可以结合在一起，给出一个全球解决方案。这项研究确定的高精度大地水准面将使海洋学家能够利用十年或更长时间的卫星测高来测量绝对洋流和涡流等。海洋是世界上最重要的热量输送来源，对这些输送过程和热通量的了解是我们理解全球气候变化的基础。然后，可以对这些运输过程进行建模，使我们进一步了解温室气体和其他人为影响造成的气候变化的影响。

The Earth's gravity field is of importance to oceanography, polar science, solid Earth science and to height systems on the Earth. If we consider the oceans as a virtual sea, that is completely at rest with no tides or currents and with the ability to flow at will in a narrow channel under the land masses, then the surface of that imaginary water line is mean sea-level, which scientists call the geoid. The geoid is thus a fictitious surface that approximates mean sea-level and changes with location over the Earth. The change from place to place can be expressed as a sum of components that change with distance giving rise to terms that change slowly (long wavelength terms) and those that change rapidly (short wavelengths). Scientists have only limited knowledge of the geoid over the oceans with the situation generally worse over land. The launch of dedicated gravity field missions, GRAvity and Climate Experiment (GRACE) and the Gravity field and steady state Ocean Circulation Experiment (GOCE) is beginning to change that. The GRACE satellite mission is a pair of tandem satellites some 220 km apart orbiting about 450km above the Earth's surface. Launched in 2001, precise positioning is provided by GPS with the distance between the satellites measured by a micro-wave device. If we consider the satellites joined by an elastic string the length will increase and decrease as the satellite orbits are affected by the Earth's gravitational pull. By utilising this variability it is possible to determine that gravitational effect. GOCE is a European mission scheduled to be launched in 2009. This mission is of a similar concept to GRACE but with the tandem satellites reduced to point masses about 1m apart. The differences in gravity between this three dimensional array of point masses is continuously monitored by accelerometers which provide the rate of change of gravity in a process known as gradiometry. A problem with gradiometry is the large perturbations due to air-drag with the consequence that the accelerometers have to be constructed to avoid saturation. This restricts the wavelengths of the gravity field observable with the long wavelengths not measurable by the gradiometer. These wavelengths are instead to be determined from the GPS tracking but at a lower accuracy than that of GRACE These two missions are thus complementary supplying the gravity field to high precision at long and shorter wavelengths. GRACE yields the gravity field components to high accuracy at the longer wavelengths and GOCE at the shorter wavelengths. In this study we intend to merge the two data sets to produce solutions superior to that obtainable by either missions by itself. We will also obtain gravity fields over regions such as the Arctic and Antarctica as that will enable us to make use of the full gravity field signals from GRACE which can be lost in seeking a single global solution. Of course regional fields can be combined to give a global solution. A highly accurate geoid as determined by this study will enable oceanographers to use the decade and more of satellite altimetric heights to measure absolute ocean currents and eddies etc. The oceans are the most important source of heat transport around the world and knowledge of these transport processes and the heat fluxes are fundamental to our understanding of global climate change. These transportation processes can then be modelled to give us further insight into the effects of climate change due to green house gasses and other anthropogenic effects.