The variation of ztd is mainly caused by the zenith wet delay ( zwd ) . zenith hydrostatic delay ( zhd ) and zwd from different gps sites exhibit individual features that zhd has a obvious periodic change and zwd is related to weather 总延迟的变化主要是由湿延迟的变化而引起，其中静力延迟变化呈现了明显的周期性，而湿延迟的变化与天气过程相关联。
