The LISA Pathfinder mission is a technology demonstrator for a LISA-like gravitational wave observatory in space. Its first results already exceed the expectations. This is also true for the optical metrology system which measures the distance in between the two free-floating test masses with unpreceded precision. One noise source that can possibly affect the measurement is the laser frequency noise. It is measured with a dedicated interferometer and suppressed with a control loop. We measured the laser frequency noise and characterised the control loop in flight. The coupling of laser frequency noise into the measured phase is directly proportional to the path length difference in the respective interferometer. Dedicated experiments have been performed to estimate the path length difference in flight. In addition, this frequency stabilisation scheme is also a possible solution for the LISA mission.