FPGA Implementation of an Adaptive Sweep Algorithm for Spacecraft Radios

Spacecraft communicate with the Deep Space Network (DSN) at a predetermined carrier frequency. However,signals are Doppler shifted away from the original transmittedfrequency due to the high orbital velocities of the spacecraft.This paper describes the implementation of an adaptive sweepalgorithm on a Xilinx Kintex-7 field-programmable gate array(FPGA). This algorithm estimates the carrier Doppler shift andcompensates for it to allow for coherent data demodulation. Thealgorithm is implemented in MATLAB’s Simulink, compliedto Verilog using HDL Coder, and run on the FPGA using theFPGA-in-the-loop Wizard. The FPGA implementation has beenvalidated in the presence of noise, by comparing the standarddeviation of the Doppler residuals at different signal-to-noiseratios, to values obtained via a theoretical analysis of the carriersynchronization loop. Results indicate excellent agreement andthus validate our implementation. We have also tested thealgorithm against flight data obtained from the Lunar Reconnaissance Orbiter (LRO) and the Deep Space Network (DSN),and proven that the algorithm can successfully acquire andtrack the carrier.

航天器以预设载波频率与深空网（Deep Space Network，DSN）进行通信。然而，由于航天器轨道速度极高，信号会发生多普勒频移，偏离原始发射频率。本文阐述了一种自适应扫频算法在赛灵思（Xilinx）Kintex-7系列现场可编程门阵列（Field-Programmable Gate Array，FPGA）上的实现方案。该算法可估算载波多普勒频移并进行补偿，以实现相干数据解调。该算法首先在MATLAB的Simulink仿真环境中完成搭建，随后通过HDL Coder编译为Verilog代码，并借助FPGA在环向导工具（FPGA-in-the-loop Wizard）在FPGA上运行。本FPGA实现方案已通过噪声环境下的验证：将不同信噪比下多普勒残差的标准偏差，与通过载波同步环路理论分析得到的理论值进行对比。实验结果显示二者吻合度极佳，由此验证了本实现方案的有效性。我们还借助月球勘测轨道飞行器（Lunar Reconnaissance Orbiter，LRO）与深空网（DSN）获取的飞行数据对该算法进行了测试，证实该算法可成功捕获并跟踪载波信号。