the Venus flyby most closely mimics the situation of prime scientific interest for PRIDE: the flybys at the Galilean moons. During the Venus flyby, the spacecraft constraints are high due to thermal conditions (no nadir pointing in particular) and there will be severe limitations on payload operations [39]. Provided that the MGA is on during the flyby (or very near it), the training experiment during the flyby will also provide VLBI data to improve the Venus ephemeris.

[Figure: Fig. 6 Sky distribution of the total of 21,906 compact radio sources from the RFC catalogue]

[53].
Consequently, the data will also allow us to test our data analysis chain up to and including spacecraft orbit determination and flyby target normal point estimation [26]. As a showcase, Section 4.4 summarizes the results of numerical simulations of PRIDE observations during the Venus flyby to quantify and highlight the potential impact that VLBI data quality, and therefore proper scheduling and preparation of the experiment, can have on the science return. In addition to the VLBI data acquired during the Venus flyby, the Doppler data acquired during ingress and egress of the planet will be useful for occultation studies of Venus’ atmosphere (PRIDE-EXP3, Table 3). An analysis of such data sets using Venus Express was presented by [43], who provided a detailed model for the use of PRIDE in radio occultation measurements and its possibilities for Venus in particular. The Venusian atmosphere makes occultation measurements at Venus fundamentally different than those that could be acquired for the Galilean satellites, although the Venus occultation data could provide useful training data to prepare for PRIDE occultations of the Jovian atmosphere. Possible acquisition of PRIDE Doppler data during the lunar flyby could be used as a more representative test case for radio occultation and/or bistatic radar studies for later observations at the Galilean moons. Training data sets (PRIDE-EXP4, Table 3) obtained with an in-beam mode will be acquired if feasible. Here, we distinguish two types of observations: one where the signal from JUICE and a second spacecraft are tracked simultaneously, and one where the signal from JUICE and the phase calibrator are observed simultaneously (e.g., without nodding). The ‘in-beam with second spacecraft’ mode will be most useful for testing and optimizing PRIDE operational and data analysis systems in advance of simultaneous observations of JUICE and Europa Clipper [46].

Whether the opportunity for such observations will arise is not yet clear (e.g. whether a second spacecraft will be present at a suitable geometry), and will be the topic of a dedicated analysis by the PRIDE team. In addition, opportunities during the ‘regular’ training sessions (EXP1) where a suitable in-beam phase calibrator is available will be used for scheduling and data analysis procedures for in-beam observations. Finally, the interplanetary plasma scintillation (IPS) [47, 54] of the spacecraft radio signal on the solar wind plasma will be accurately evaluated using all cruise phase test data sets. Such observations were first made in 2010 with ESA Venus Express [47], continued with Rosetta, and continue up until the present with numerous ESA/ NASA missions [11]. In the last ten years, thousands of sessions have been undertaken using numerous VLBI radio telescopes. Additionally, an observational campaign has been launched to monitor solar activity, including coronal mass ejections (CME), by initiating radio observations when these events pass between the spacecraft and ground antennas and the JUICE communications system is active. It is anticipated that the signal JUICE transmits during its cruise phase will supplement the most recent observations. In addition to the objectives listed above, PRIDE can also perform short-notice observations under a special protocol established jointly by the PRIDE team and SOC as an exception or contingency. When there is an urgent need for data collection and the typical observation planning approach is insufficient, these observational modes are activated. This contingency operational mode is predicated on the fact that some of PRIDE’s most important operational assets – Earth-based radio telescopes— are extremely sensitive and geographically scattered over the whole range of longitudes. PRIDE can deliver single-dish (signal detection, including Doppler measurements) or VLBI observations in this operational mode. In exceptional circumstances, the former can be carried out with only around 24 hours’ notice.