Results and Future Applications of the ESA Delta-DOR

The Delta Differential One Way Ranging (Delta-DOR) is a powerful interferometric technique that permits to accurately measure the angular position of an interplanetary spacecraft in the celestial sphere. It uses two widely separated antennas (whose connecting line is named baseline) for simultaneous tracking of a transmitting probe in order to measure the delay in the signal arrival time between the two stations. The measurement of this delay is named DOR (Differential One-way Range). This, however, will be affected by several error sources due to the media traversed by the radio wave, to station clock instabilities and other instrumental effects, introducing unwanted terms in the measured delay. Delta-DOR corrects these errors using a quasar as a calibrator. This is possible since the quasar position (its direction) is very accurately known, typically to better than 1 nrad. With Cebreros Deep Space antenna 2 (DSA2), in Spain, coming into operation in September 2005, ESA became potentially able to support Delta-DOR measurements, having two deep space antennas (the first being New Norcia, in Western Australia) forming a baseline of about 11650 km. In order to exploit this possibility, the system had to be upgraded in order to perform such measurements. The system upgrade was done in less than ten months, in order to have an operating and tested Delta-DOR capability before the Venus Express (VEX) orbit insertion at Venus, to augment JPL Delta-DOR dataset. The results of the successful operational campaign on VEX are presented in this paper. After this campaign, the ESA Delta-DOR system was again used (together with JPL Delta-DOR) to provide navigation support prior to the Rosetta Mars fly-by, occurred on the 25th of February 2007. The paper also shows the results of the Rosetta tracking campaigns with comparisons with JPL data. Once ESA Delta-DOR has been fully validated in operation scenarios, interoperability with other Agencies running a network of deep space antennas becomes the natural evolution for the ESA system. For this reason, ESA is currently developing a data translator, in order to be compatible both with JPL and standard VLBI data formats. This has the strategic goal to build cross-support capabilities with JPL and JAXA on one hand, and to enlarge the number of potential baselines on the other, including ESA DSAs in the VLBI network. Moreover, the correlator capabilities have been enhanced with new operational modes. Here, all the results of these activities are discussed in details.