Design And Implementation Of A C-based Interferometry Software Correlator

Interferometry developed in the field of radio astronomy in the 1960's.Because of its high-accuracy and long-operating-distance,interferometry has become a hot research topic for deep-space navigation since its application in Chang'e missions.Using interferometry,two electromagnetic signals from the same target are received,recorded,and processed so that a high-accuracy angle measurement can be realized.Recording and transmission the data are very important in interferometry.In BACC-CEI platform,RSR(Radio Science Receiver)is employed to record the data.Using RSR,the signals from a spacecraft or a radio source can be received,down-converted,sampled and stored,which is a preparation for correlation processing.Two main problems are faced:Firstly,data need to be transferred after the observation.Secondly,the MATLAB-based correlation software is of low efficiency.This thesis focuses on the design of a C-based interferometry software correlator to solve the two problems.The correlator consists of two parts:the real-time transmission module and the correlation processing module.The main work are as follows:Firstly,the design of the real-time transmission module is carried on.This module can be implemented with TCP protocol or Tsunami protocol.Tsunami is a reliable UDP-based application-level transmission protocol.Requirements of a real-time transmission for interferometry data are introduced,and the congestion control mechanism based on an error-rate feedback applied in Tsunami was analyzed.A retransmission buffer is proposed to improve the retransmission strategy and the mechanism of packet sending of Tsunami,which is implemented in the real-time transmission of the interferometry data.Moreover,the relationship between the extra delay and the transmission rate control is deducted.Finally,a real-time data transmission test is conducted on a 622Mbps link,which shows that Tsunami can achieve much higher bandwidth utilization than that of the TCP protocol with a high bandwidth-delay product.Secondly,the design of the correlation processing module is proposed.Instead of MATLAB,C language is adopted to implement the correlation algorithm.Based on the correlation algorithm(mainly including the time delay compensation,stripe rotation,FFT etc.),the correlation processing module is developed.Finally,a comparison between correlation results of MATLAB-based correlation software and that of C-based module is demonstrated.The results show that the C-based correlation module proposed in this thesis can perform the correlation algorithm correctly and more efficiently.Using the BACC-CEI platform,the work above has been validated in the observations of BeiDou Navigation Satellites and Sino Satellites.Results show that the observation efficiency is significantly increased,and the interferometry data can be transferred in real-time even on poor network conditions.