This paper introduces briefly the measurement principle of the Radar Altimeter.The altitude measurement error caused by timing jitter

clock accuracy and inherent noise in echo signal is analyzed.The key technique included“Full-deramp”and the method of using filter bank to seperate range returns is discussed.We have developped a Middle-Resolution Ocean Radar Altimeter Receiver System and successed in the experiment combining with the transmitter system. The design requirement of 5 nanosecond system resolution is achieved.