  Generic black hole binaries radiate gravitational waves anisotropically,
imparting a recoil, or kick, velocity to the merger remnant. If a component of
the kick along the line of sight is present, gravitational waves emitted during
the final orbits and merger will be gradually Doppler shifted as the kick
builds up. We develop a simple prescription to capture this effect in existing
waveform models, showing that future gravitational wave experiments will be
able to perform direct measurements, not only of the black hole kick velocity,
but also of its accumulation profile. In particular, the eLISA space mission
will measure supermassive black hole kick velocities as low as ~500 km/s, which
are expected to be a common outcome of black hole binary coalescence following
galaxy mergers. Black hole kicks thus constitute a promising new observable in
the growing field of gravitational wave astronomy.
