Antenna Design And Digital Control System Development Of A High Performance Ionosonde

The ionosphere is an area of the atmosphere where neutral atoms and air molecules are partially ionized by solar radiation,and it locates more than 60 km above the ground.When electromagnetic waves pass through the ionosphere,phenomena such as reflection and refraction occur,which causing any system that relies on electromagnetic waves to send and receive signals to be affected by the ionosphere.Therefore,the detection of the ionosphere is of great significance in both scientific research and daily life.Among the various methods of ionospheric detection,vertical detection is currently a relatively mature,convenient and widely used ground detection method.The ionosonde acquires echo delays corresponding to different frequencies by emitting high frequency electromagnetic waves of 1-30 MHz,and draws the ionogram to invert the ionospheric characteristic parameters.Height resolution and detection period are two important indicators of the ionosonde.The ionosonde often uses the pulse compression technology to resolve the contradiction between the working distance and the distance resolution,and improve the detection accuracy of the system.In terms of receiving and sending time,the time delay corresponding to the maximum detection height is used as the receiving time,which causes the system's receiving window to be too long and it is difficult to achieve the ionosphere fast detection.Therefore,in order to achieve high-precision real-time detection of the ionosphere,this paper proposes a fast and accurate ionospheric detection system,which is applied to the ground synchronous detection equipment of Zhang Heng-1 satellite and provides relevant scientific data for Zhang Heng-1 satellite.The specific design of the system antenna unit and numerical control system are given: The antenna unit uses two Delta antennas as transmitting and receiving antennas,and uses one-emitting-double-receiving mechanism to transmit mutually orthogonal polarized waves in a time-sharing manner to obtain echo signal scattering information.As the core part of the ionosonde,the numerical control system uses 40-bit Barker-like code as the coding sequence.The pulse width is 10?s,which makes the height resolution of the ionosonde reach 1.5km.In the timing of sending and receiving,the variable delay receiving mode is used to set the delay between receiving and transmitting,which can effectively shorten the receiving time of the echo signal and make the detection period less than 2min.The specific work is as follows:(1)Investigate the main methods of ionospheric detection and the development status of ionosondes at home and abroad.(2)The basic principles of vertical ionospheric detection and the system compositions of the ionosonde are studied in detail.By analyzing the A-H formula,the working process and design parameters of the ionosonde are given.According to the requirements of the antenna unit,two Delta antennas were designed as the transceiver antenna.The size of the Delta antenna is 26m×8m,the pattern is vertically upward,and the standing wave ratio is less than 2.5.(3)Use AX7020 and self-made circuit board as the experimental platform to build the numerical control system.Use the variable delay receiving method to generate the radar transmission and reception timing;under the control of the timing signal,the AD9957 chip is used to generate a cosine signal with a center frequency of 1-30 MHz and a step frequency of 100 k Hz,and then the cosine signal is encoded with 40-bit Barker-like codes to improve the ionosonde's detection accuracy;according to the band-pass sampling theorem,select the AD9238 chip with a sampling rate of 40 MHz to complete the analog-to-digital conversion of the signal,then use digital down conversion(DDC)to complete the frequency conversion and extraction of the signal,and finally use direct memory access(DMA)technology to transfer the signal to the computer.In the design of the entire numerical control system,HDL Designer is used to write the programs of the timing control unit,mixing and decimation in DDC;the filter coefficient file was generated using Matlab,and then imported it into ISE,set the sampling rate and other related parameters to design the finite impulse response(FIR)Filter.Insert IP cores such as First In First Out(FIFO),DMA,and Block RAM(BRAM)to generates the numerical control system circuit,then use SDK to write running programs to achieve system control and data transmission.(4)Use an oscilloscope,signal source,etc.to build the ionosonde's numerical control system test platform.Observe the transmitting signal waveform,and then fix the transceiver interval and transmission frequency to verify the correctness of the transmitter unit.The standard signal source generates a cosine signal with a frequency of 70.04 MHz as the test signal of the receiving unit to verify the correctness of the receiving unit.