17 radio telescopes observing the Huygens signal coming from 1,200,000,000 kilometer distance

The Huygens Probe landed 14 January 2005 on Titan, the largest moon of Saturn. During and after the landing, Huygens transmitted data to Cassini, which acted as data relay station flying nearby Titan. There were two transmission channels. One of the two channels was very stable in frequency: it was locked to an on-board ultra stable oscillator (USO). Reception of its carrier signal of this transmission from Huygens has been possible with Earth-based radio telescopes at 1,200,000,000 kilometre distance. Frequency stability permitted to process signals received at different radio telescopes with very long baseline interferometry (VLBI) techniques. Exploitation of the latter techniques permits to retrieve the Huygens trajectory through the Titan atmosphere and actual landing location with superb resolution. The scenario for the VLBI experiment is outlined, the direct signal reception at two large radio telescopes is indicated and related descriptive information is given. Post-processing of VLBI data is ongoing at the time of writing in Joint Institute for VLBI in Europe (JIVE). Good progress can be indicated.