According to TanDEM-X satellite program

this paper studies interferometric capabilities of TanDEM-X mission during an orbit period and tries to get the change of relative height error in a global scope.Firstly

based on HILL-equation of the helix formation

effective baseline

including the range and azimuth effective baseline

which has a great impact on interferometry is analyzed in detail during an orbit period.Then the influence of baseline decorrelation imposing on interferometric phase error is discussed detailedly

which is the most important influence on relative height error.At last relative height model is established and relative height error or TanDEM-X system is presented further.The computer simulating result shows the validity of analysis.