A distributed fibre optic sensor apparatus comprising: an optical source configured to generate at least first and second optical pulse pairs for launching into an optical fibre, the first and second pulse pairs having the same frequency configuration as one another, each pulse pair comprising a first pulse having a first frequency and a second pulse having a second frequency, and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair in that: when the first and second frequencies are different, the first and second pulse pairs are generated so as to be launched into the optical fibre at different times, such that the time between launch of the first and second pulse pairs corresponds to the time for said predetermined relative phase change in a signal at a frequency equal to the frequency difference between the pulses (201, 202) in a pair; and when the first and second frequencies are the same, the phase difference between the pulses of first pulse pair differs from the phase difference between the pulses of the second pulse pair by the predetermined relative phase difference, wherein the phase relationship between the pulses in the first and second pulse pair is varying, the distributed fibre optic sensor apparatus further comprising: at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs. an optical source configured to generate at least first and second optical pulse pairs for launching into an optical fibre, the first and second pulse pairs having the same frequency configuration as one another, each pulse pair comprising a first pulse having a first frequency and a second pulse having a second frequency, and being generated such that the phase relationship of the pulses of the first pulse pair has a predetermined relative phase difference to the phase relationship of the pulses of the second pulse pair in that: when the first and second frequencies are different, the first and second pulse pairs are generated so as to be launched into the optical fibre at different times, such that the time between launch of the first and second pulse pairs corresponds to the time for said predetermined relative phase change in a signal at a frequency equal to the frequency difference between the pulses (201, 202) in a pair; and when the first and second frequencies are the same, the phase difference between the pulses of first pulse pair differs from the phase difference between the pulses of the second pulse pair by the predetermined relative phase difference, wherein the phase relationship between the pulses in the first and second pulse pair is varying, when the first and second frequencies are different, the first and second pulse pairs are generated so as to be launched into the optical fibre at different times, such that the time between launch of the first and second pulse pairs corresponds to the time for said predetermined relative phase change in a signal at a frequency equal to the frequency difference between the pulses (201, 202) in a pair; and when the first and second frequencies are the same, the phase difference between the pulses of first pulse pair differs from the phase difference between the pulses of the second pulse pair by the predetermined relative phase difference, the distributed fibre optic sensor apparatus further comprising: at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs. at least one detector configured to detect any radiation backscattered from said first and second pulse pairs; and processing circuitry coupled to said at least one detector, wherein the processing circuitry is configured to determine a phase value for at least one given section of optical fibre based on the detected backscatter radiation from said first and second pulse pairs.