Apparatus for determining a time of arrival of a message signal comprising: a quadrature mixer (13, 14) for mixing a received message signal with the signal of a local oscillator, wherein the received message signal comprises a binary sequence modulated by binary frequency shift keying with pulses corresponding to a first value at a first frequency and with pulses corresponding to a second value at a second frequency, wherein the local oscillator has a frequency between the first frequency and the second frequency; a time of arrival detector (18) for determining a time of arrival of the message signal on the basis of the output of the quadrature mixer (13, 14).