Aiming at problem that strapdown inertial navigation system（ SINS） has less reliability promotion with the condition of few redundant inertial device

analyze equivalence of redundancy optimization criteria

and give sufficient and necessary condition of the optimal redundant configuration. Based on contrastive analysis degree of reliability of orthogonal scheme

sideling scheme and cone scheme

a symmetrical and sideling configuration scheme is proposed for four-gyros configuration

this scheme can improve SINS reliability

and it has good consistency when one gyro is failed. Computer simulation results show that it is one of the optimal redundancy schemes for four-gyros configuration. The proposed scheme has a strong reference meaning for little number of shipborne SINS.