Researches On Sidelobe Suppression Technology Of Low Intercept Probability Radar Signal

Low Probability of Intercept(LPI)technique is one of the effective ways in anti-reconnaissance and anti-jamming for radar system.Generally,LPI radar obtain low intercept performance by designing special transmitting waveforms which usually own the following characteristics including large duty cycle,large time-bandwidth product,multiple phases code,irregular pulse(or inter-pulse)repeat frequency,rabid code-pattern changing ability and so on.By using the transmitting signals with aforementioned features,the radar can maintain its detection ability and minimize the transmitting power simultaneously.In other words,it is very difficult for the enemy to detect the radar because of its minimal transmitting power,thus,protecting our radar to the most extent.However,transmitting signals with above features will result in high sidelobes in later signal processing to deal with the echoes.In detection terminology,high sidelobes will cover targets with little powers,thus,leading to targets missing problems for the radar systems.On this base,sidelobe suppression in LPI radar signal processing has been widely researched in recent years.To improve the detection performance of the radar system,this thesis firstly studies the sidelobe suppression methods for typical radar signals such as LFM and PSK.Then,a new combined modulation signal is designed and corresponding sidelobe suppression methods are studied subsequently.In this thesis,we firstly explore the ambiguity function and Doppler tolerance of LFM and phase coded signals.Specifically,we mainly analyze the Barker code,M sequence,Frank polyphonic code,Taylor quadrature code and P4 polyphase code among the multiple phase coding sequences.Targets with different velocities are respectively simulated by using LFM and phase coded signals to study their Doppler tolerances by pulse compress(PC)technique.It can be concluded that LFM signal shows a good Doppler tolerance while phase coded signal is more superior in low intercept performance.Then,the PC sidelobe suppression techniques of LFM and phase coding signals are studied.By applying different cascading window functions combined with PC technique to suppress sidelobes of LFM and phase coding signals,we prove this kind of methods is not applicable for phase coding signals.Further study about a new method named as zero-forcing algorithm shows that it can suppress the sidelobes close to mianlobe effectively.But new unexpected sidelobes come out at the positions out of the original sideblobes,which impedes the final target detection.In this case,we consider another CLEAN algorithm which can eliminate sidelobes effectively.However,target detection results are seriously affected by its threshold of CLEAN method,which is not what we expect.Jointly utilization about the two above methods,a novel algorithm comes out which can suppress the sidelobes and break out threshold limitations at the same time.Our further studies demonstrate its superiority in radar target detection well.Finally,a complementary code signal with low sidelobes is introduced specifically including its generating process and its low sidelobe characteristics.Finally,the design process,corresponding sidelobe suppressing methods and characteristics about a new combined modulation signal(that is LFM and phase coded signals)are well studied.The distance resolution,speed resolution and Doppler sensitivity of the combined signal are compared respectively with different modulation bandwidths.Furthermore,we analyze the influence of different frequency-expanding codes and information codes on the distance resolution,speed resolution and Doppler sensitivity of frequency-expanded phase codes.Next,the proposed method in this thesis is applied to suppress the sidelobes.