An optical time domain reflectometry system comprising: a code generator configured to generate a set of predefined complementary code sequences, said set of predefined complementary code sequences comprising a plurality of first resolution code sequences (200, 202), said code generator being configured to provide said first resolution code sequences as first resolution test signals for transmission on an optical communication system; and a correlator configured to receive reflected portions of said first resolution test signals from said optical communication system as reflected first resolution test signals, said correlator configured to provide first resolution system response data from said first resolution code sequences and said reflected first resolution test signals; wherein said set of predefined complementary code sequences further comprise a plurality of second resolution code sequences (204, 206), said code generator being configured to provide said second resolution code sequences as second resolution test signals for transmission on said optical communication system; said correlator being configured to receive reflected portions of said second resolution test signals from said optical communication system as reflected second resolution test signals, said correlator configured to provide second resolution system response data from said second resolution code sequences and said reflected second resolution test signals; characterised in that said first and second resolution code sequences have first and second frequency spectra, respectively, wherein a Fourier spectrum of said first resolution code sequence has a maximum amplitude at a first frequency and a Fourier spectrum of said second resolution code sequence has a maximum amplitude at a second frequency different from said first frequency. a code generator configured to generate a set of predefined complementary code sequences, said set of predefined complementary code sequences comprising a plurality of first resolution code sequences (200, 202), said code generator being configured to provide said first resolution code sequences as first resolution test signals for transmission on an optical communication system; and a correlator configured to receive reflected portions of said first resolution test signals from said optical communication system as reflected first resolution test signals, said correlator configured to provide first resolution system response data from said first resolution code sequences and said reflected first resolution test signals; wherein said set of predefined complementary code sequences further comprise a plurality of second resolution code sequences (204, 206), said code generator being configured to provide said second resolution code sequences as second resolution test signals for transmission on said optical communication system; said correlator being configured to receive reflected portions of said second resolution test signals from said optical communication system as reflected second resolution test signals, said correlator configured to provide second resolution system response data from said second resolution code sequences and said reflected second resolution test signals; characterised in that said first and second resolution code sequences have first and second frequency spectra, respectively, wherein a Fourier spectrum of said first resolution code sequence has a maximum amplitude at a first frequency and a Fourier spectrum of said second resolution code sequence has a maximum amplitude at a second frequency different from said first frequency.