  Residual motion of the arm cavity mirrors is expected to prove one of the
principal impediments to systematic lock acquisition in advanced
gravitational-wave interferometers. We present a technique which overcomes this
problem by employing auxiliary lasers at twice the fundamental measurement
frequency to pre-stabilise the arm cavities' lengths. Applying this approach,
we reduce the apparent length noise of a 1.3 m long, independently suspended
Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control
of the system from the auxiliary laser to the measurement laser.
