An apparatus comprising: an optical fiber ; and at least two sites in the fiber comprising a respective fiber grating arranged to have a respective optical response in a wavelength spectrum comprising a distinguishing feature indicative of a value of a respective local parameter at a respective grating site, the two fiber gratings further arranged to form, in combination with a respective portion of the optical fiber which extends between said at least two sites, respective optical backscatter portions that when combined with one another are effective to sense an optical change in the fiber portion between the sites indicative of a value of a distributed parameter, which comprises a parameter modality different from a parameter modality of the respective local parameters at the respective grating sites; wherein the optical responses are simultaneously processed to contemporaneously obtain sensing information; an optical interrogator comprising at least one optical source coupled to one end of the fibre to apply at least one optical pulse, and further comprising a first detection channel configured to monitor the feature indicative of the value of a respective local parameter at one or more respective grating sites; wherein the first detection channel comprises a time division modulator arrange to relate one or more repective reflected pulses to one or more respective grating sites, and further comprises a spectrometer arranged to detect changes in the feature indicative of the value of a respective local parameter at a respective grating site; and wherein the optical interrogator further comprises a second detection channel configured to combine the optical backscatter portions to sense the optical change in the respective fiber portion which extends between said at least two sites indicative of the value of a distributed parameter. an optical fiber ; and at least two sites in the fiber comprising a respective fiber grating arranged to have a respective optical response in a wavelength spectrum comprising a distinguishing feature indicative of a value of a respective local parameter at a respective grating site, the two fiber gratings further arranged to form, in combination with a respective portion of the optical fiber which extends between said at least two sites, respective optical backscatter portions that when combined with one another are effective to sense an optical change in the fiber portion between the sites indicative of a value of a distributed parameter, which comprises a parameter modality different from a parameter modality of the respective local parameters at the respective grating sites; wherein the optical responses are simultaneously processed to contemporaneously obtain sensing information; an optical interrogator comprising at least one optical source coupled to one end of the fibre to apply at least one optical pulse, and further comprising a first detection channel configured to monitor the feature indicative of the value of a respective local parameter at one or more respective grating sites; wherein the first detection channel comprises a time division modulator arrange to relate one or more repective reflected pulses to one or more respective grating sites, and further comprises a spectrometer arranged to detect changes in the feature indicative of the value of a respective local parameter at a respective grating site; and wherein the optical interrogator further comprises a second detection channel configured to combine the optical backscatter portions to sense the optical change in the respective fiber portion which extends between said at least two sites indicative of the value of a distributed parameter. <EOS>