  Accurate tracking of probes is one of the key points of space exploration.
Range and Doppler techniques are the most commonly used. In this paper we
analyze the impact of the transponder delay, $i.e.$ the processing time between
reception and re-emission of a two-way tracking link at the satellite, on
tracking observables and on spacecraft orbits. We show that this term, only
partially accounted for in the standard formulation of computed space
observables, can actually be relevant for future missions with high nominal
tracking accuracies or for the re-processing of old missions. We present
several applications of our formulation to Earth flybys, the NASA GRAIL and the
ESA BepiColombo missions.
