Abstract: A transmission system having a polarization-sensitive receiver which includes polarization adaptor for changing the state of polarization of a received signal until it corresponds to an optimum polarization. For determining the direction in which the polarization should be adjusted, local signal generators produce polarization variations which influence the output signal of the polarization adaptor and the change in the signal is detected. In order to prevent slow variations in the received signal from affecting the polarization detection, a band-pass filter is included to suppress such variations.