Abstract: Method for determining high-throughput-satellite (HTS) system user downlink signal-to-noise-plus-interference ratios (SNIRs) and SNIR distributions is provided, including a procedure for estimating parameters of one spot beam and that for multiple co-color spot beams, and the projection of the satellite antenna radiation patterns onto Earth to form the spot beam power spatial distributions that lead to the SNIRs and SNIR distributions of stationary or mobile user terminals (UTs) in clear sky (CS) line-of-sight (LOS) propagation channel conditions. The estimation precision is controllable. The method is implemented in a programming language run on computational hardware with built-in or connected user interface (UI) devices including desktops, laptops, severs or super-computers. It can be used alone or as part of a simulation package or part of a system operation management software of the control gateway (GW) in the HTS system for system downlink designs.