Chang’e-3（ CE-3） mission

which includes China’s first lunar lander and rover

was successfully soft landed on theMoon on December 14

2013. After landing

the separation of the rover and the lander is a critical step of the mission operation

and is a lso the basis for subsequent explorations by the lander and the rover. As there exist terrain obstacles and communication blackout

it is important to calculate parameters of obstacles and communication envelope

which will be helpful for rover to avoid obstacles and drive smoothly after separation.To satisfy the requirements of fast and reliable decision making for rover-lander separation

we performed monoscopic measurements of the obstacles and calculated the communication envelope based on an image acuired by the monitoring camera.（ 1） We define three coordinate systems including rover body coordinate system

local coordinate system and monitoring camera coordinate system. These coordinate systems can be transformed to each other by matrix operations according to landing attitudes

rotation matrices and translation vectors.（ 2） Through coordinate system transformation

object coordinates can be calculated from image corodinates by using collinearity equations. In particular

sizes of the obstacles（ e. g.

rocks） and distances to the lander are caculated and labeled on the image.（ 3） Projection transformation is applied to obtain the communication envelope.The monoscopic measurement method developed in this paper is verified with a Digital Ortho Map（ DOM） of 2 mm resolution

which is generated by Navcam stereo images. The measurement accuracy is obtained by comparing monoscopic measurement results with ground truth

which is dervied from obstacle size measurement on the Navcam DOM. Experimental results show that the measurement error in object space is less than 10% and can meet the requirements of decision support.These measurements contributed to evaluation of terrain obstacles and communication blackout directly. The proposed method was successfullly used in CE-3 mission to support decision making for rover-lander separation.