  We report an accurate measurement of the recoil velocity of Rb atoms based on
Bloch oscillations in a vertical accelerated optical lattice. We transfer about
900 recoil momenta with an efficiency of 99.97 % per recoil. A set of 72
measurements of the recoil velocity, each one with a relative uncertainty of
about 33 ppb in 20 min integration time, leads to a determination of the fine
structure constant alpha with a statistical relative uncertainty of 4.4 ppb.
The detailed analysis of the different systematic errors yields to a relative
uncertainty of 6.7 ppb. The deduced value of 1/alpha is 137.03599878(91).
