Abstract: This paper analyses the design and implementation of an external AGC system for a PN-code ranging radar's receiver in an space vehicle,and focuses on discussing of how to design an all-digital AGC according to the output of the front-end device to extend the dynamic range.The discussion of this paper is valuable for the design of digital AGC in DS-SS(DirectSequence Spread-Spectrum) receiver or in PN-code ranging radar receiver.