The time-saving numerical method for GPS/MET observation operator

The global positioning system (GPS) ray-shooting method is a self-sufficient observation operator in GPS/MET (meteorology) data variational assimilation linking up the GPS observation data and the atmosphere state variables. But it cannot be applied to data assimilation and operational prediction so far because of huge computations. In order to reduce the amount of computation, a 2-order time-saving symplectic scheme is used to solve the equations of the GPS ray trajectory, due to its separable Hamiltonian nature, and good results are achieved. Not only does it save 75% of CPU time taken by the old GPS ray-shooting model with 4th-order Runge-Kutta method, but also it improves the simulation accuracy to some extent.