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质量交换，而这只能通过测量它们的体积如何变化来实现。目前，观测到的1.8毫米/年的海平面上升中，约有50%无法解释。只有使用卫星才能有效地在太空中进行所需的测量，目前有几个任务正在太空中，或者即将部署来解决这个问题。简而言之，可以通过测量从卫星到海面的距离，然后从卫星在地心参照系中的位置减去距离，来推断海和冰的地形及其变化。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。卫星轨道中的误差直接映射到推断的地形中的误差。轨道确定过程和海冰变化时间演变的模拟都依赖于一个“参照系”--简而言之，这是一个用来观察卫星在太空中如何运动的跟踪站的坐标和速度列表。速度是必要的，因为跟踪站位于构造板块上，所有板块都在连续运动。由于这类分析模拟了持续数十年的地球物理效应，因此必须准确地知道跟踪站的这种运动。反过来，用于计算空间站位置(坐标)和速度的方法与轨道确定过程有关-因此，轨道估计中的误差再次产生问题。卫星径向轨道精度要求达到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，这可以通过对卫星部队进行准确建模来实现。这里的主要问题是太阳辐射压力引起的卫星表面力、热效应和地球反射和发射的辐射引起的力(称为反照率效应)，以及大气阻力效应。这些力量，特别是地球辐射效应，具有非常强的季节和纬度特征，如果没有适当地模拟，这些特征在推断的海和冰地形中表现为季节和纬度变化。PI和他的团队已经开发了一套软件实用程序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环境响应的复杂、真实模型的领先技术。该小组已被邀请参加几项国际实验，这些实验涉及以前从未尝试过的建模复杂性，这项提议旨在扩展该小组的技术，并将其应用于当前的任务，以实现1厘米的目标。如果不能解决卫星轨道上系统性偏差的问题，将严重破坏任何以估计的大规模交换为基础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.