Velocity changes of the Giotto spacecraft during the comet flybys: on the interpretation of perturbed Doppler data

Upon developing a new theory for the analysis of one-way and two-way Dopple r shifts of the radio carrier signal transmitted by interplanetary spacecra ft, Porsche (1999) urges a reassessment of the radio science data obtained during the Giotto flybys at comets P/Halley and P/Grigg-Skjellerup. We expl ain again the Doppler recording method and the operational strategy of the flybys and present both the two-way and one-way data sets from Giotto's fly by at comet Grigg-Skjellerup as an example. We find no reason to change our most probable flyby scenarios. Furthermore, we assert that Porsche's treat ment of the classical Doppler effect is in error. All quantities derived fr om his analysis are gross overestimates and incompliant with observations a nd cometary physics. (C) 2001 Editions scientifiques et medicales Elsevier SAS.