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高度系统都有重要意义。如果我们把海洋看作是一个虚拟的海洋，它完全静止，没有潮汐或水流，并且能够在陆地下的狭窄通道中随意流动，那么这个虚拟水线的表面就是平均海平面，科学家称之为大地水准面。因此，大地水准面是一个近似于平均海平面的虚拟表面，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之和，这些分量产生缓慢变化的项（长波长项）和快速变化的项（短波长项）。科学家们对海洋上的大地水准面只有有限的了解，陆地上的情况通常更糟。专门的重力场飞行任务、重力和气候实验以及重力场和稳态海洋环流实验的启动正在开始改变这种状况。GRACE卫星使命是一对相距约220公里的串联卫星，在地球表面上空约450公里的轨道上运行。该卫星于2001年发射，由GPS提供精确定位，卫星之间的距离由微波设备测量。如果我们考虑用弹性绳连接卫星，随着卫星轨道受到地球引力的影响，其长度将增加或减少。通过利用这种可变性，有可能确定引力效应。GOCE是欧洲的一项使命，计划于2009年发射。这一使命与GRACE的概念相似，但串联卫星减少到相距约1米的点质量。点质量的这个三维阵列之间的重力差由加速度计连续监测，加速度计在称为梯度测量的过程中提供重力的变化率。梯度测量的一个问题是由于空气阻力引起的大扰动，其结果是必须构造加速度计以避免饱和。这就限制了重力场的波长，而长波长的重力场则不能用梯度仪测量。这些波长是由GPS跟踪确定的，但精度低于GRACE。因此，这两个任务是互补的，在长波长和短波长处提供高精度的重力场。GRACE在较长波长处产生高精度的重力场分量，而GOCE在较短波长处产生高精度的重力场分量。在这项研究中，我们打算合并这两个数据集，以产生解决方案上级，可获得的任务本身。我们还将获得北极和南极等地区的重力场，因为这将使我们能够利用GRACE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，区域领域可以结合起来，提供一个全球解决方案。一个高度精确的大地水准面，确定这项研究将使海洋学家使用的十年和更多的卫星测高高度来测量绝对洋流和涡旋等海洋是最重要的来源，在世界各地的热量输送和这些运输过程的知识和热通量是我们对全球气候变化的理解是至关重要的。然后，这些运输过程可以被模拟，使我们进一步了解气候变化的影响，由于绿色房屋气体和其他人为影响。

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.