LDG-400 radio altimeter works in C-band, which is frequency modulated continuous wave working system. The antenna adopts the mode of separation of transmitter and receiver. Before the missile is launched, power is supplied to the height fixing device, and the height fixing device is self checked and installed through the communication interface; after the missile is launched, the RF module is turned on, and the frequency change is set according to the installation height. When the installation height is met, the height fixing device outputs the height fixing instruction information and telemetry information. LDG-400 type height fixing device is mature in technology and adopts frequency modulated continuous wave technology, which has the advantages of small volume and large height measuring range.