Precision Doppler Measurements from Low-SNR Signals Experiencing Residual Doppler and Acceleration

The spin-stabilized Juno spacecraft at Jupiter routinely makes radiometric Doppler (range-rate) measurements using the radio link to NASA's Deep Space Network for radio science investigations at the Jovian system, including gravitational measurements and radio occultations. Typically, these measurements are taken using Juno’s high gain antenna where the received signal strength is >40 dB-Hz with simultaneous coherent X- and Ka-band links. During Juno’s Perijove-51 closest approach of Jupiter in May 2023, the spacecraft’s attitude was optimized for other instruments and track was performed using the low gain antenna, which only supports X-band. The signal-to-noise ratio was very low, 4-10 dB-Hz, on average, during the observation. Furthermore, non-zero Doppler rate and accelerations signatures were present, caused by the rotation of the antenna around Juno’s spin axis. Both of these conditions are incompatible for precision Doppler tracking without introducing additional constraints. In this work, we demonstrate a method in which the precision Doppler measurements can be produced utilizing data from open-loop receivers in conjunction with a dynamical model of the signal effects. Frequencies are initially estimated with poor accuracy from the open-loop data, and refined using orbit determination to produce a better estimate of the spacecraft's trajectory. The new trajectory provides a new frequency (or Doppler) estimate to phase-steer the phase-locked loop. This process is iterated until the residual Doppler on the signal is low enough to be completely tracked without cycle slips or residual error. This method resulted in Doppler measurements from Perijove-51 at a precision of 1.7 mHz (X-band) at 60-second integration time. This experiment with Juno demonstrates a real-world example of radiometric tracking between 4-10 dB-Hz. Future missions, such as the upcoming Europa Clipper mission, will benefit from these techniques.

自旋稳定的朱诺号（Juno）木星探测器，通常借助与美国国家航空航天局深空网络（Deep Space Network, DSN）建立的无线电链路，开展木星系统内的射电科学探测，包括引力场测量与射电掩星观测，其核心手段为辐射测距多普勒（range-rate，距离变化率）测量。常规情况下，此类测量通过朱诺号的高增益天线完成，接收信号强度大于40 dB-Hz，并同时启用相干X波段与Ka波段链路。2023年5月的第51次近木点（Perijove-51）过境期间，探测器的姿态被优化以适配其他科学载荷，因此本次跟踪仅能采用仅支持X波段的低增益天线完成。观测时段内，信号信噪比仅为4~10 dB-Hz，整体水平极低。此外，由于天线绕朱诺号自旋轴旋转，观测数据中出现了非零多普勒速率与加速度特征。若无额外约束条件，这两类情况均无法支撑高精度多普勒跟踪任务。本研究提出一种可行方法：结合开环接收机数据与信号效应动力学模型，生成高精度多普勒测量结果。首先从开环数据中以较低精度估算载波频率，再通过轨道确定流程优化参数，得到更精准的探测器轨道解。基于新轨道得到的频率（或多普勒）估计值，可用于对锁相环（phase-locked loop, PLL）进行相位步进校正。该过程迭代执行，直至信号残差多普勒足够低，可实现无周跳与残差误差的完整跟踪。最终，本方法在60秒积分时长下，从第51次近木点过境数据中得到了精度达1.7 mHz（X波段）的多普勒测量结果。本次朱诺号实验，为4~10 dB-Hz低信噪比场景下的射电辐射跟踪提供了真实应用案例。未来任务，如即将实施的欧罗巴快船（Europa Clipper）任务，将可从该技术中获益。