Technical Analysis And Research On The Instability Of LFMCW Radio Altimeter

It is a basic requirement for radio altimeter that the height of the aircraft can be measured stably and accurately.There are two states during the height measurement of the radio altimeter.The first is the tracking state.Within the altimeter range,the altimeter should be able to measure and display the true height of the aircraft from the ground or the water surface.The second is the search state,when the height has exceeded the altimeter range or the altimeter receiver cannot detect the echo signal,the altimeter cannot give the normal height value.A LFMCW radio altimeter is a high altitude radio altimeter with a range of 0m to 15000 m,it is equipped on a certain type of bombers.In actual flight height,the altimeter indicates “0m” or “1m” in the range of the height measurement,and display “FFFF”(at this time the altimeter is in the search state),which should be normal in the altimeter range.The height discontinuity of the altimeter and height display which does not accord with the actual flight condition,called radio altimeter altimetry instability.It is absolutely inadmissible that the instability of a certain LFMCW radio altimeter height measurement seriously affects the safety of pilots and aircraft and will affect the successful completion of combat missions.In this paper,the altimetry principle of LFMCW radio altimeter is analyzed and studied,the perigee sensitivity parameter settings of the altimeter and theory of electromagnetic interference and the measures adopted are discussed,and the reasons for the instability of the LFMCW radio altimeter are analyzed.The high altitude echo signal is weak,the ability to suppress the electromagnetic interference and the ability of the antenna leak signal will affect the quality of the height echo signal received by the altimeter and make the altimeter unstable.Though the analysis and study of the circuit and actual condition of a certain LFMCW radio altimeter equipped with a bomber,the perigee sensitivity parameter and the EMI filter are added,the filtering mode of the power supply module is improved,and the function of eliminating the abnormal data is added in the software,improves the altimeter internal grounding mode,make a LFMCW radio altimeter sensitivity curve setting more realistic,enhances the ability of altimeter to resist electromagnetic interference and antenna leakage.According to the actual application of a LFMCW radio altimeter,the perigee sensitivity test and the improved power sensitivity test are designed,the unstable phenomenon of a LFMCW radio altimeter is reappeared on four LFMCW radio altimeters.According to the theoretical analysis,the corresponding measures were formulated and adopted.The LFMCW radio altimeter with the measures adopted has been verified by internal field test and field flight,which proves that the measures are effective and the theoretical analysis is in place to ensure the stability of the altimeter height measurement.