Stabilising the Orbital Reference Frame for Ice Cap and Sea Level Observation and Modelling: CRYOSAT and Jason-2

迫切需要量化世界海洋和极地冰盖之间的物质交换，而这只能通过测量它们的体积如何变化来实现。目前，大约50%的观测到的海平面上升1.8毫米yr-1无法解释。所需的测量只能从太空使用卫星有效地进行，几个任务现在已经在太空中，或者即将部署来解决这个问题。简单地说，海洋和冰的地形及其变化可以通过测量卫星到表面的距离，然后从卫星在地心参考系中的位置减去距离来推断。卫星位置是通过轨道确定过程计算的，这需要对作用在卫星上的力进行数学建模。卫星轨道图中的误差直接转化为推断地形的误差。轨道确定过程和海洋和冰变化的时间演变建模都依赖于“参考系”-简单地说，这是用于观察卫星如何在太空中移动的跟踪站的坐标和速度列表。需要速度是因为跟踪站位于构造板块上，所有这些板块都在连续运动。由于这些分析对持续数十年的地球物理效应进行建模，因此必须准确了解跟踪站的这种运动。反过来，用于计算站位置（坐标）和速度的方法与轨道确定过程相关联-因此，轨道估计中的误差再次造成问题。卫星径向的轨道精度需要达到1厘米左右，以减少目标地球物理参数的不确定性。我们认为，这可以通过卫星部队的精确建模来实现。这里的主要问题是太阳辐射压力造成的卫星表面力、热效应和地球反射和发射的辐射造成的力（称为热效应）以及大气阻力效应。这些力，特别是地球辐射效应，具有非常强的季节性和纬度特征，如果没有适当建模，这些特征在推断的海冰地形中会表现为季节性和纬度变化。PI和他的团队开发了一套软件工具来解决这些力建模问题，这些问题被认为是世界上处理航天器对其环境响应的复杂，现实模型的领先技术。该小组应邀参加了几项国际实验，这些实验涉及以前从未尝试过的复杂性建模，该提议寻求扩大该小组的技术，并将其应用于当前的飞行任务，以实现1厘米的目标。如果不解决卫星轨道中的系统偏差问题，将严重破坏任何试图根据估计的物质交换来限制气候变化模型的努力。

There is a pressing need to quantify the exchange of mass between the world's oceans and polar ice caps, and this can only be achieved by measuring how their volumes are changing. Currently circa 50% of the observed sea level rise of 1.8 mm yr-1 cannot be explained. The required measurements can only be made effectively from space using satellites, and several missions are either in space now, or are about to be deployed to attack this problem. In simple terms the sea and ice topography, and how it changes, can be inferred by measuring ranges from the satellites to the surface, and then subtracting the ranges from the position of the satellites in a geocentric reference frame. The satellite position is calculated by the process of orbit determination, which requires mathematical modelling of the forces acting on the satellites. Errors in the satellite orbit map directly into errors in the inferred topography. Both the orbit determination process and the modelling of the time evolution of the sea and ice changes rely upon a 'reference frame' - put simply this is a list of coordinates and velocities of the tracking stations used to observe how the satellites move in space. Velocities are needed because the tracking stations are sited on tectonic plates, all of which are in continuous motion. As these kind of analyses model geophysical effects that last decades this motion of the tracking stations must be known accurately. In turn, the methods used to calculate the station positions (coordinates) and velocities are linked to the orbit determination process - so once again, errors in the orbit estimates create problems. Orbital accuracy in the satellite radial direction of around 1 cm is required to reduce the uncertainty in the target geophysical parameters. We believe this can be achieved by accurate modelling of the satellite forces. The principal problems here are satellite surface forces caused by solar radiation pressure, thermal effects and forces caused by radiation reflected and emitted by the Earth (termed albedo effects), as well as atmospheric drag effects. These forces, particularly the earth radiation effects, have very strong seasonal and latitudinal characteristics which, if not modelled appropriately, appear as seasonal and latitudinal variations in the inferred sea and ice topography. The PI and his group have developed a suite of software utilities to attack these force modelling problems that are recognised as the leading techniques in the world for dealing with complex, realistic models of the spacecraft response to its environment. The group has been invited to participate in several international experiments that involve modelling complexity that has never been attempted before, and this proposal seeks to extend the group's techniques and apply them to current missions to achieve the 1 cm goal. Failure to address this problem of systematic biases in the satellite orbits would seriously undermine any attempt to constrain climate change models on the basis of the estimated mass exchanges.