To achieve the very lowest level of free-fall, a number of disturbances will need to be assessed and their coupling reduced.
Although the system is carefully designed and tested on ground, a number of key aspects won’t be known until LPF is in its final orbit at L1. To achieve the very low target relative test mass acceleration, a number of disturbances will need to be assessed and their coupling reduced. Different investigations are designed which target these sources of disturbance and aim to measure their coupling. For example, one investigation will estimate the spring-like coupling of test masses to the spacecraft by shaking the 2nd test mass and the spacecraft by a few nanometers.