Abstract: For giant constellations，the space TT&C system needed to serve multiple satellites simultaneously.Due to the broadcasting characteristics of microwave，the information was visible to multiple target satellites and non-target satellites during transmission by the hemispherical coverage multi-target TT&C system.In this process，the confidential messages were easy to be intercepted and eavesdropped by non-target satellites.The directional modulation（DM）technology，as a physical layer security technology，was used to solve this problem.The DM technology was applied for the ground station multi-beam transmitter to enhance the physical layer security.To optimize the beamforming matrices and the superimposed artificial noise vector for transmitted signals，the orthogonal projection method was designed when the directions of the non-target satellites were inaccessible，and the maximum signal-to-leakage-and-noise ratio（Max-SLNR）criterion was applied when the directions of the non-target satellites were available.It follows from the simulation results that both algorithms can distort the received signal constellation pattern in the directions of the non-target satellites，while maintaining the standard constellation pattern in the directions of the target satellites.In contrast，the orthogonal projection algorithm is more suitable without the directions of the non-target satellites，to achieve the undifferentiated interception effect for non-target satellite directions.However，the Max-SLNR algorithm can directionally interfere the receivers of non-target satellites with known directions.

Key words: TT&, C；directional modulation；physical security；orthogonal projection；SLNR