Detecting Juno's ‘Heartbeat’: Communications Support during Critical Events of the Juno Mission

Since launch, radio science has been a key component of the Juno mission to Jupiter. The prime objective of the radio science investigation is to estimate the gravitational field of Jupiter from the Doppler shift on the radio link between the spacecraft and the Earth-based observing antennas of NASA's Deep Space Network (DSN). In addition to estimation of the gravitational field, radio science has provided critical engineering support to the Juno mission. Utilizing high-sensitivity open-loop receivers and real-time signal processing, radio science is able to detect the ‘heartbeat’ of the Juno spacecraft and determine the current state of the spacecraft. The Juno spacecraft utilizes two frequencies for communication with Earth: the telecommunications system X-band link and the radio science Ka-band link. Radio science has provided communications monitoring support for the spacecraft launch in 2011, spacecraft main engine firings (including Deep Space Maneuvers in 2012 and Jupiter Orbit Insertion in 2016), the Earth gravity assist flyby in 2013, and times when the spacecraft was off-Earth point during Jupiter closest approach with a weak signal level. By measuring the signal-to-noise ratio, received carrier frequency, and subcarrier frequency of the X-band downlink signal in real-time, radio science is able to determine the state of the spacecraft in scenarios where the link margin is not sufficient to support telemetry. An off-nominal spacecraft state will change the signal-to-noise level, subcarrier frequency, and spin modulation of the carrier frequency which are detectable in the open-loop receiver of the DSN. With the addition of multiple frequency shift keying (MFSK) ‘tones’ encoding, the subcarrier frequency can be changed onboard the spacecraft for determination of selected events by the flight team. Tones were utilized during main engine firings on Juno, including Jupiter Orbit Insertion (JOI). Tones are decoded in near real-time by the Entry, Descent, and Landing (EDL) Data Analysis (EDA) system downstream of the DSN open-loop receivers. Robust implementation of hardware, software, and operations planning has ensured successful data collection and real-time status reporting of spacecraft state to the Juno mission. Lessons learned from communicating with Juno in this way while in the harsh environment of Jupiter are documented and discussed in the context of upcoming missions to Jupiter.