To improve the low-elevation detection performance of ship-borne phased array radar， the low angle multi-path model， sea surface reflection model and elevation estimation model of phased array radar （PAR） are established. The relationship between low-elevation angle accuracy of ship-borne PAR and angle estimation method， radar frequency， beam direction， sea state and beam width is systematically analyzed through simulation. The results show that sum-sum angle estimation method， lower beam width by improved beam weight， higher beam direction of angle estimation sum beam， multi-frequency smoothing are benefit for low-elevation angle accuracy improvement. A low-elevation angle estimation method which takes advantage of all the factors above is proposed， which has achieved excellent real-world application results and significant engineering practical prospects.

In war， early warning and detection system as a typical complex system， the effective evaluation of its detection effectiveness has always been an important practical problem that needs to be studied and solved. The limitations of traditional effectiveness evaluation methods are break- throughed. On the basis of analyzing the basic principles of knowledge graph technology and Bayesian evaluation method， a new method of detection efficiency evaluation of early warning detection system based on their comprehensive application is proposed. The knowledge graph is constructed by using expert knowledge and experimental data， and the Bayesian network method is further used to implement knowledge reasoning， so as to realize the evaluation of system detection efficiency. The effectiveness of the proposed method by constructing specific combat scenarios is verified， and the evaluation of the proposed method based on deduced data is analyzed.