Research On The Key Techniques Of Digital Back-End And Signal Processing In Deep Space Exploration

In the past few decades,the deep space exploration has been developed rapidly,and the research on the planetary bodies in the solar system has made huge achievements.Ground deep space stations use radiometric tracking techniques to realize high accuracy orbit determination of the spacecraft and radio scientific experiments,which is one of the key technologies of deep space engineering.Digital back-end（DBE）systems and signal processing are the important techniques of deep space radiometric tracking.In this dissertation,DBE systems and signal processing for deep space exploration are studied,and the main research contents include DBE system design,frequency stability measurement methods about atomic clocks,algorithms and implementations of narrow and broad band signal parameter estimation.The design of a DBE system called SEU-RSR-1G is firstly discussed.The main characteristics of DBE systems for deep space exploration are discussed,followed by a brief description about the hardware platform.Then the analog to digital converter interface at GSps,the multi-channel real-time GSps digital down-conversion（DDC）unit based on polyphase structure and the system reset unit for channel synchronization are realized with other algorithm and logic design.The sampling frequency of SEU-RSR-1G is 1.152 GHz,which has 4 analog intermediate frequency inputs and 8 baseband channels that support wide-narrow bands observations.The lab and real spacecraft experiments results show the system performance could meet design requirements.Subsequently,the high-precision,real-time,narrow-band parameter estimation for deep space exploration is studied and the sequential De-chirp（Seq-De-chirp）algorithm is proposed.The downlink signal is approximated to linear frequency modulation signal in short time.The signal detection and frequency tracking is implemented based on real-time spectrum estimation and DDC.Then the high-precision parameter estimation is obtained by sequential parameter estimator,which includes de-chirp,weighted frequency difference and spec-zooming.The data size and computation are reduced significantly by DDC and sequential processing.The algorithm is tested by the experiment data of Chang’E-3 and Juno’s X-band carrier observations,and the frequency estimation errors are 4.770 mHz and 81.610 mHz,respectively.Based on the research of DBE system and Seq-De-chirp algorithm,a high precision frequency stability measurement system for atomic clocks is proposed.This system uses the SEU-RSR-1G to realize all-digital dual-mixing processing,and the multilevel spectrum zooming algorithm is proposed based on the Seq-De-chirp algorithm,which could obtain high precision frequency and phase measurement with acceptable computation.The measurement capability of this system is better than 10^-16 under 1000 second,which could meet the requirement of hydrogen masers analyzing,so dedicated instruments is no longer required.The discussed Seq-De-chirp algorithm is performed by high performance computers.However,in many applications like spaceborne or lunar-based Doppler receivers,a more compact system structure is required.A hardware integration implementation of Seq-De-chirp algorithm and baseband conversion is designed.Each algorithm module is mapped and implemented in the hardware platform with FPGA and ARM architecture processor.A folding frequency domain convolution Chirp-Z structure is proposed to implement the spec-zooming unit,saves more than 70%of hardware resources,which is implemented with folding technique,look-up table technique and block floating point algorithm.The spacecraft experiment results show the difference of frequency estimation result between hardware integration and floating point arithmetic is less than±0.5 mHz,and those of phase are hardly larger than±2×10^-5 cycles.The proposed system only takes 2.2 ms to process baseband signal under 1 second integration time,with a latency of 130?s which is 28 times faster than software.Moreover,the requirement of space-ground communication links bandwidth is significantly reduced.Therefore,the proposed design could meet the requirements of spaceborne narrowband parameter estimation.Very long baseline interferometry（VLBI）technique is an important boardband radiometric tracking technique.It performs broadband interferometry correlation processing to obtain high-precision difference group delay estimation,based on the cross-power spectrum estimation of two baseband signals.After discusses its principles,two methods for heterogeneous baseband data correlation processing occurred are proposed for joint VLBI observations:the data parameter unity converter（DPUC）and the heterogeneous direct FX（HDFX）correlator.DPUC adopts a unified structure to realize data format and signal parameter conversion,then the processing could be done by ordinary FX correlator.HDFX correlator can directly obtain cross-power spectrum estimation of heterogeneous baseband data,with spectrum alignment and variable point discrete Fourier transform.Both methods have been tested and verified by numerical simulations and VLBI quasar observation data.In VLBI observations,the predicted delay models have big impacts on interferometry measurement results.In order to avoid the problems about predicted delay models,two model-free fringe search algorithms are proposed,namely extended FFT fringe search algorithm and cross ambiguity function-wavelet boosting（CAF-W）fringe search algorithm.The extended FFT fringe search algorithm uses 2D grid searching to increase the present FFT fringe search’s search ranges.CAF-W fringe search algorithm could obtain joint estimation of delay and delay rate by CAF and the interference in the search plane is removed by the wavelet boosting algorithm,which significantly improves the robustness of the fringe detection.The CAF-W algorithm only needs to slide its search window along the delay axis,so its processing speed is1000 times faster than the extended FFT fringe search algorithm.In conclusion,the key techniques about DBE system and signal processing are researched,which includes the software and hardware system design,signal processing algorithms and their implementation.All the proposed algorithms and design have been tested by real experiment data,and their performances could meet the needs of radiometric tracking techniques and radio science experiments.