Abstract: Considering that applying the inertial measurement unit (IMU) to low-cost strapdown guided munitions is quite difficult, a line-of-sight (LOS) angle estimation algorithm with ideal trajectory parameters replacing values measured by IMU is designed. According to coordinate transformation and LOS geometrical relationship, ideal trajectory parameters are regarded as uncertain parameters to build the nonlinear filter system. An ideal trajectory robust cubature Kalman filter (ITRCKF) algorithm is proposed to the solve nonlinear system filter problem with uncertain parameters. The filter problem with uncertain system is converted into minimizing the upper bound of covariance error with parameter κ, utilizing the angle of sight with body from the seeker detector and the ITRCKF to estimate states of nonlinear systems. The experimental results show that under the condition of small disturbance, the max error of declination and the root mean square of error (RMSE) by ITRCKF are reduced by 85.57% and 81.93% respectively, compared with the cubature Kalman filter (CKF). And under the condition of large disturbance, the max error of declination and the RMSE by ITRCKF are reduced by 31.64% and 46.39% respectively. The estimated angles of LOS by the proposed algorithm meet requirements for acceptable accuracy, and have a better robust performance than CKF.