Very Long Baseline Interferometry (VLBI) was developed as a radio astronomical technique in the 1960s to sharpen angular resolution for studies of natural celestial radio sources primarily in the interests of astrophysical studies [1, Chapter 1]. The method turned out to be so powerful and versatile that its applications had been developed quickly in other areas of astronomy and space science, ranging from astrometry and geodesy [1, Chapter 12] to fundamental physics [e.g., 2? ], studies in heliophysics and interplanetary medium [3–5] and deep space navigation. A brief review of the latter VLBI applications in which spacecraft are observing targets is given in [6]. In most cases of VLBI observations of spacecraft within the Solar System, the distance to the target is smaller than the so-called Fraunhofer criterion [7].

In this case, it is appropriate to use the term near-field VLBI observation.

[Figure: Fig. 1 A schematic configuration of PRIDE (not to scale) supporting two Jovian missions, JUICE (the lower left spacecraft) and Europa Clipper (the upper right spacecraft). The spacecraft are in two-way radio contact (blue arrows) with mission-specific Earth-based tracking stations (blue Earthbased antennas). Earth-based VLBI radio telescopes (three magenta Earth-based antennas) receive radio signals from the spacecraft (yellow arrows) and emission of distant natural celestial radio sources (red arrows). The configuration can operate at X- and/or Ka-bands, (8.4 GHz and/or 32 GHz, respectively).]

The Planetary Radio Interferometry and Doppler Experiment [PRIDE, 6, 8] utilizes data recording and processing technology developed for ‘traditional’ astronomical VLBI but with the target source being a spacecraft (see Fig. 1). The main measurables of PRIDE are the spacecraft’s lateral celestial position and radial velocity. The former is provided by VLBI phase-referencing, the latter – by estimating the Doppler shift of the spacecraft carrier tone. The PRIDE methodology is described in [6, 9, 10], and corresponding software implementation in [11]. PRIDE has been selected by the European Space Agency (ESA) as one of the eleven science experiments of the Jupiter Icy Moons Explorer [JUICE, 12–16, and a special issue of Space Science Reviews 2023–2024, and references therein], the first ESA’s Cosmic Vision 2015––2025 largeclass flagship science mission. It was launched on 14 April 2023. JUICE will conduct investigations of Jupiter and its Galilean moons, particularly Ganymede, but also the other moons Callisto and Europa.

One of the scientific focuses of the mission is the potential habitability of these icy worlds. Jupiter in situ studies began in the 1970s with NASA’s Pioneer program [17], Voyager program [18, 19], succeeded by the Galileo mission [20, 21] in 1995–2003. NASA’s Juno mission [22] has been exploring the Jovian system since July 2016. Its goals are thorough studies on the planet’s core and magnetic field, the composition of Jupiter’s atmosphere, and the planet’s auroras. NASA’s Europa Clipper mission [23, 24] is expected to launch in 2024 and join JUICE for simultaneous in situ studies of the Jovian system from 2030. The Juno, JUICE and Europa Clipper missions are highly synergistic and will considerably deepen the understanding of Jupiter and its system. The primary science objective of PRIDE for the JUICE mission is to support the improvement of the Jovian system ephemerides [25–27], which will enhance the science return for studies of the origin, evolution and potential habitability of the Galilean moons. The detailed descriptions of PRIDE–JUICE science objectives are provided in [6].

[Table: Table 1 The world VLBI arrays. The number of telescopes in the arrays and their X- and Ka-band capabilities are indicative and might vary with a tendency toward increase. Some telescopes might be members of more than one array. See [6] for more details.
VLBI Array Number of telescopes X-band Ka-band
European VLBI Network (EVN) 22 22 6 U.S. Very Long Baseline Array (VLBA) 10 10 10 Japanese VLBI Network (JVN) 9 9 – Chinese VLBI Network (CVN) >6 6 1 Korean VLBI Network (KVN) 3 – 3 East-Asia VLBI Network (EAVN) 15 3 – Australian Long Baseline Array (LBA) 5 5 2 University of Tasmania Array 5 5 –]