  We report the observation of positronium excitation from the 2S to the 20P
state. The Rydberg positronium atoms fly a distance of 40 mm before being field
ionized and detected in a micro-channel plate. The time of flight can thus be
measured and the velocity distribution of the atoms excited in the 2S state is
reconstructed. This is used as an input to the model of the line-shape in order
to properly take into account the second order Doppler shift which is the main
systematic uncertainty in the 1S-2S measurements of positronium.
