An apparatus for controlling a physical system, comprising: a controller configured to output a command signal to control a state of a platform of a physical system and to receive a feedback signal providing information about the state of the platform; and an integrity detection system, wherein the integrity detection system is configured to: output an excitation signal into the closed loop system; receive the feedback signal; determine whether a component associated with the excitation signal is present in the feedback signal; and identify whether or not the closed loop control system is experiencing a failure based on the determination; wherein the excitation signal has energy in a frequency spectrum, wherein the integrity detection system comprises: a first filter for filtering the feedback signal in a first frequency band encompassing at least a portion of the excitation signal frequency spectrum; and a level detector for measuring the filtered feedback signal level; the apparatus for controlling a physical system being characterised in that: the integrity detection system further comprises: a noise level estimator for providing a noise level estimate of the feedback signal in the first frequency band; and a signal ratio computation block for computing a signal ratio, wherein the signal ratio is the ratio of the filtered feedback signal to the noise estimate; wherein said determination is based on the computed signal ratio. a controller configured to output a command signal to control a state of a platform of a physical system and to receive a feedback signal providing information about the state of the platform; and an integrity detection system, wherein the integrity detection system is configured to: output an excitation signal into the closed loop system; receive the feedback signal; determine whether a component associated with the excitation signal is present in the feedback signal; and identify whether or not the closed loop control system is experiencing a failure based on the determination; output an excitation signal into the closed loop system; receive the feedback signal; determine whether a component associated with the excitation signal is present in the feedback signal; and identify whether or not the closed loop control system is experiencing a failure based on the determination; wherein the excitation signal has energy in a frequency spectrum, wherein the integrity detection system comprises: a first filter for filtering the feedback signal in a first frequency band encompassing at least a portion of the excitation signal frequency spectrum; and a level detector for measuring the filtered feedback signal level; a first filter for filtering the feedback signal in a first frequency band encompassing at least a portion of the excitation signal frequency spectrum; and a level detector for measuring the filtered feedback signal level; the apparatus for controlling a physical system being characterised in that: the integrity detection system further comprises: a noise level estimator for providing a noise level estimate of the feedback signal in the first frequency band; and a signal ratio computation block for computing a signal ratio, wherein the signal ratio is the ratio of the filtered feedback signal to the noise estimate; a noise level estimator for providing a noise level estimate of the feedback signal in the first frequency band; and a signal ratio computation block for computing a signal ratio, wherein the signal ratio is the ratio of the filtered feedback signal to the noise estimate; wherein said determination is based on the computed signal ratio.