Computation of low degree zonal harmonics of the Earth's gravity field using ROCSAT-1 GPS data

The orbit perturbation theory of Kaula was used to derive an approximate analytical formula for near circular orbit and the order-zero formula. With this theory, we tested with different cases of gravity recovery. For one example, computing perturbation components in the radial, along-track and cross-track directions, and exploring Earth's gravity field determination from orbit perturbations. Secondly, one year GPS data of ROCSAT-1 provided by NSPO was processed with two parts. First part is raw data processing. Second part, data obtained from orbit determination is considered to be referenced to the OSU91A geopotential model. Two processed data used orbital theory computing can be used to estimate geopotential model error and compute time variation of geopotential coefficients. Comparision of the monthly value of ΔC₂₀, ΔC₃₀, ΔC₄₀, ΔC₅₀ estimated from ROCSAT-1 GPS data over one year shows periodic vibration. That is, computing gravity field recovery error in low degree zonal harmonics can effectively present the temporal variation.