The European ΔDOR Correlator

Over the past decades NASA pioneered the use of VLBI techniques for the determination of the angular position of interplanetary probes. This powerful method, usually called delta-Differential One-way Ranging (ΔDOR), or ΔVLBI, uses a quasar of known celestial coordinates to synchronize clocks at two ground antennas and a phase delay measurement of the spacecraft signal to infer its angular position in the plane containing the baseline vector. By combining observations from multiple baselines (and therefore at least three ground antennas), one could measure the spacecraft celestial coordinates to an accuracy of 10 nanoradians or less . Being almost independent from the dynamical model and a time-localized measurement, ΔDOR is a valuable observable quantity for spacecraft navigation in the interplanetary cruise phase, where gravity gradients are small and single dish Doppler and range measurements are less effective in providing a good determination of the state vector. ESA and the University of Rome "La Sapienza" have undertaken the development of a software correlator for the analysis of ΔDOR data acquired by its 35-m deep space antennas in New Norcia (Australia) and Cebreros (Spain). By using a model of the spacecraft dynamics and earth rotation, the correlator determines the time delay in the arrival of the quasar and spacecraft wavefronts at the two intervening antennas. These quantities are then processed by an orbit determination code to improve the spacecraft ephemerides. The correlator has been validated using observations of the spacecraft Rosetta, Mars Express, Venus Express and SMART-1. By comparing the ΔDOR observations with the known orbit of Mars Express (determined to an accuracy of a few hundred meter from Doppler measurements), the residual delays are less than 0.5 ns, corresponding to a maximum angular error of 15 nanoradians (i.e. 2.25 km at 1 AU). A judicious choice of the reference quasar leads to significantly smaller residual delays. The attained precision is adequate for all foreseen navigational needs of ESA deep space probes