Abstract: A chirp-coded robust synthetic aperture(CRSA) technique for medical endoscopic ultrasonography was presented.The technique uses chirp-coded excitation and corresponding pulse compression to enhance the robustness of synthetic aperture.The effect of resolution enhancement by the CRSA technique is demonstrated by both FieldⅡsimulation and sample experiment.Data for 6 point targets is simulated and the performance of CRSA is compared with that of conventional barker-coded synthetic aperture technique(BSA).Nylon rope samples are examined by a single element transducer centered at 8 MHz.Both simulation and experimental results indicated that CRSA can improve resolution of ultrasound images with good robustness as compared with the conventional BSA technique.The lateral resolution is upgraded to 1.4 mm from 1.8 mm,and for errors 6%of the correct value,the main-lobe broadening and SNR degradation are 0.04 mm and 0.2 dB respectively,which are 0.95 mm and 3.6 dB lower than those of BSA.It therefore can be concluded that CRSA has more robustness than BSA against the disturbance caused by sound speed errors.