Block diagram of the GRAVITY fringe-tracking controller within a single integration time, denoted n. The parameter states, illustrated in green, are central to the fringe tracker. They represent the disturbance conditions (L_n) and actuator positions (X_n). The blue segment indicates the group-delay feedback applied to the disturbance state, the red segment denotes the phase-delay feedback on L_n, and the black segments represent the command send to the actuators.