A distributed fibre optic sensing apparatus, comprising: an optical source; a local oscillator path configured to generate a local oscillator signal from the optical source; an optical modulator configured to, in use, modulate radiation from the optical source to provide a first interrogation pulse at a first frequency and a second interrogation pulse at a second, different, frequency in each of a plurality of repeated interrogation periods, where each of the first and second frequencies is different to the frequency of the local oscillator signal; a mixer configured to receive a backscatter signal comprising radiation from the first and second interrogation pulses which is backscattered from within a sensing optical fibre and to mix the backscatter signal with the local oscillator signal to provide a first mixed signal; a detector arrangement (108, 109) configured to receive the first mixed signal and provide a corresponding first digital signal; a processor configured to process the first digital signal, wherein the processor is configured to: process the first digital signal in a first processing channel to demodulate a first phase signal based on a first carrier frequency corresponding to the frequency difference between the first frequency and the frequency of the local oscillator signal; process the first digital signal in a second processing channel to demodulate a second phase signal based on a second carrier frequency corresponding to the frequency difference between the second frequency and the frequency of the local oscillator signal; and determine a temporal difference between the first phase signal and the second phase signal for at least one location along the sensing optical fibre. an optical source; a local oscillator path configured to generate a local oscillator signal from the optical source; an optical modulator configured to, in use, modulate radiation from the optical source to provide a first interrogation pulse at a first frequency and a second interrogation pulse at a second, different, frequency in each of a plurality of repeated interrogation periods, where each of the first and second frequencies is different to the frequency of the local oscillator signal; a mixer configured to receive a backscatter signal comprising radiation from the first and second interrogation pulses which is backscattered from within a sensing optical fibre and to mix the backscatter signal with the local oscillator signal to provide a first mixed signal; a detector arrangement (108, 109) configured to receive the first mixed signal and provide a corresponding first digital signal; a processor configured to process the first digital signal, wherein the processor is configured to: process the first digital signal in a first processing channel to demodulate a first phase signal based on a first carrier frequency corresponding to the frequency difference between the first frequency and the frequency of the local oscillator signal; process the first digital signal in a second processing channel to demodulate a second phase signal based on a second carrier frequency corresponding to the frequency difference between the second frequency and the frequency of the local oscillator signal; and determine a temporal difference between the first phase signal and the second phase signal for process the first digital signal in a first processing channel to demodulate a first phase signal based on a first carrier frequency corresponding to the frequency difference between the first frequency and the frequency of the local oscillator signal; process the first digital signal in a second processing channel to demodulate a second phase signal based on a second carrier frequency corresponding to the frequency difference between the second frequency and the frequency of the local oscillator signal; and determine a temporal difference between the first phase signal and the second phase signal for at least one location along the sensing optical fibre.