This paper presents double line-of-sight (LOS) measuring relative navigation technique for far or medium range (tens of kilometers ～ several kilometers) autonomous rendezvous. In this new relative navigation technique, two chase spacecraft with inter-spacecraft crosslink form a measuring baseline relative to the target spacecraft in space and respectively measure LOS angles of the target and their own inertial states. The processing noise model of the relative navigation system is developed based on the spacecraft relative dynamics equations described in inertial frame. The measurement model including double LOS angles of the target and the bearing and length of the measuring baseline is established. And then, a discrete-time extended Kalman filter (EKF) is developed to estimate the inertial relative position and velocity of the target. Numerical simulations are undertaken to verify the new navigation technique. The results indicate that: 1) the relative navigation system is weak observable or even unobservable when only single LOS angle measurements of the target are available, 2) the relative position and velocity of the target can be estimated with sufficient accuracy using double LOS measuring relative navigation technique, but the angle between the two LOS vectors influences the estimation accuracy.
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