A method of operating a hearing aid system comprising the steps of: - providing a first and a second input signal, wherein the first and second input signal represent the output from a first and a second microphone respectively; - transforming the input signals from a time domain representation and into a time-frequency domain representation; - estimating an inter-microphone phase difference between the first and the second microphone using the input signals in the time-frequency domain representation; - determining an unbiased mean phase from a mean of the estimated inter-microphone phase difference or from a mean of a transformed estimated inter-microphone phase difference, wherein the mean is taken over time, characterized by - determining a mapped mean resultant length; - estimating a time difference of arrival using a plurality of unbiased mean phases weighted by a corresponding plurality of reliability measures, wherein each of the reliability measures are derived at least partly from a corresponding mapped mean resultant length; and - using the estimated time difference of arrival for at least one hearing aid system processing stage, and wherein the mapped mean resultant length R̃ ab ( k , l ) is determined, at least partly, using an expression from a group of expressions comprising: - expressions of the form given by: R ̃ ab k l = E f e jθ ab k l p k l wherein indices 1 and k represent respectively the frame used to transform the input signals into the time-frequency domain and the frequency bin; wherein E is an expectation operator; wherein e jθ ab ( k , l ) represents the inter-microphone phase difference between the first and the second microphone; wherein p is a real variable; and wherein f is an arbitrary function. - providing a first and a second input signal, wherein the first and second input signal represent the output from a first and a second microphone respectively; - transforming the input signals from a time domain representation and into a time-frequency domain representation; - estimating an inter-microphone phase difference between the first and the second microphone using the input signals in the time-frequency domain representation; - determining an unbiased mean phase from a mean of the estimated inter-microphone phase difference or from a mean of a transformed estimated inter-microphone phase difference, wherein the mean is taken over time, characterized by - determining a mapped mean resultant length; - estimating a time difference of arrival using a plurality of unbiased mean phases weighted by a corresponding plurality of reliability measures, wherein each of the reliability measures are derived at least partly from a corresponding mapped mean resultant length; and - using the estimated time difference of arrival for at least one hearing aid system processing stage, and wherein the mapped mean resultant length R̃ ab ( k , l ) is determined, at least partly, using an expression from a group of expressions comprising: - expressions of the form given by: R ̃ ab k l = E f e jθ ab k l p k l wherein indices 1 and k represent respectively the frame used to transform the input signals into the time-frequency domain and the frequency bin; wherein E is an expectation operator; wherein e jθ ab ( k , l ) represents the inter-microphone phase difference between the first and the second microphone; wherein p is a real variable; and wherein f is an arbitrary function. wherein indices 1 and k represent respectively the frame used to transform the input signals into the time-frequency domain and the frequency bin; wherein E is an expectation operator; wherein e jθ ab ( k , l ) represents the inter-microphone phase difference between the first and the second microphone; wherein p is a real variable; and wherein f is an arbitrary function. <EOS>