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.