A spacecraft emitting radio signals can be treated as a point-like target by VLBI systems. The latter offer an unsurpassed precision in measuring lateral celestial coordinates of a radio source. However, “traditional” astronomical VLBI systems are based, among other things, on the assumption that the target is infinitely far away, i.e. is located in the “far field”. The Fraunhofer limit defines the “border” between “far field” and “near filed”. For all practical VLBI spacecraft tracking applications, the distance to the target is well within the Fraunhofer limit. Thus, “traditional” VLBI algorithms must be modified for estimating lateral coordinates of a spacecraft.
The value of “near-field” VLBI for planetary and space science has been appreciated since the 1970s. A number of planetary and space science missions benefited from spacecraft VLBI tracking. The technique offered sub-milliarcsecond “positioning” of spacecraft on the celestial sphere. For the ESA’s Huygens Probe, this translated into ~1 km linear precision at the distance to Titan. Recently the technique has been demonstrated “in action” for the ESA’s Venus Express and Mars Express missions. The technique is also accepted as a Planetary Radio Interferometry and Doppler Experiment (PRIDE) for the ESA’s Jupiter Icy Satellites Explorer (JUICE) as a multi-disciplinary enhancement of the scientific suite of the mission, which will provide precise measurements of spacecraft lateral coordinates, radial velocity and its derivatives. The presentation will offer an overview of major approaches to near-field VLBI applications in planetary and space science toward achieving “lateral positioning” of planetary probes relative to ICRF background extragalactic radio sources with the accuracy of 100 to 10 microarcseconds.
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