Stability of DSS-25 Ka-band Station Delay during the Juno Prime Mission

One of the goals of the Juno mission is to precisely measure the gravitational field of Jupiter. This is accomplished by measuring the Doppler shift of the X-band and Ka-band carrier signals during closest approach periods. Calibration of the station phase delay is crucial to the determination of the spacecraft trajectory and gravitational field as well as their associated uncertainties. The extreme dynamics experienced by the Juno spacecraft make the radiometric measurements sensitive to small errors in the calibration. In 2018, the station delay was measured along the three signal paths used by Juno: X-up/X-down, X-up/Ka-down, and Ka-up/Ka-down. Although the X-band paths were measured consistently with previous estimates, the estimated delay was found to be unexpectedly larger on the Ka-band uplink path. In this work, we estimate the Ka-up/Ka-down round-trip system phase delay using the uplink sweep to the spacecraft. The uplink sweep provides sensitivity to the station delay and allows it to be measured from the carrier frequency observables on a regular basis without the need for specialized ground tests. The Ka-up/Ka-down delay was found to be consistent at 179 microseconds from orbit insertion in July 2016 until the Block 6 Exciter was installed in November 2020, when the delay decreased to 157 microseconds. This change was attributable to the removal of approximately 4.2 km of cable on the Ka-band uplink path during the Block 6 Exciter upgrade.