In this work we present a software simulator for the performance evaluation of the NDSA (Normalized Differential Spectral Absorption) method at global scale assuming a realistic satellite orbital plane with two counter rotating satellite. The software tool processes atmospheric vertical profiles (pressure, temperature, water and liquid contents). The software simulator accounts for the main disturbance effects: scintillation impairments, thermal noise at the receiver and defocusing. The microwave propagation in atmosphere is simulated by means of the MPM93 model. Performance results about NDSA measurements at global scale for a single day at 17.25 and 20.20 GHZ are presented and discussed.