An optical fibre sensor system for measuring at a plurality of locations an operating condition, the sensor comprising: a sensor optical fibre; a light source for inputting light into the sensor optical fibre; a light detector for receiving light from the sensor optical fibre; and a processor for outputting measurements of the operating condition corresponding to the plurality of locations based on light received at the light detector; wherein the operating conditions at the plurality of locations are coupled such that an increase in one operating condition results in an increase in the other operating conditions, and vice versa; wherein the sensor optical fibre comprises a plurality of optical gratings (FBG 1-1), each grating in use being disposed at a respective location and arranged to operate over a respective range of wavelengths depending on variations in the operating condition; characterised in that: a first of the plurality of gratings is a master grating (FBG E) and is arranged to operate over a first wavelength range, the first wavelength range being distinct from a second range of wavelengths over which the other gratings are arranged to operate, and wherein the other gratings (FBG A-D and FBG F-I) are arranged such that the respective wavelength ranges over which they operate are spaced apart from the first wavelength range by a respective predetermined interval and such that they overlap with the respective wavelength range of at least one adjacent other grating; wherein the amount by which the respective ranges of wavelengths of the other gratings overlap is such that for each of the other gratings there is a range of unambiguous wavelengths that are unique for that grating and a range of ambiguous wavelengths that overlap with the wavelengths of the adjacent grating; wherein the processor is operable to determine the value of the operating condition at the location of the master grating from the wavelength value received in the first wavelength range, and to determine the value of an operating condition at the location of one of the other gratings based on the wavelength value received in the first wavelength range, a wavelength value received in the second wavelength range, and the predetermined intervals by which the overlapping ranges are spaced apart from one another. a sensor optical fibre; a light source for inputting light into the sensor optical fibre; a light detector for receiving light from the sensor optical fibre; and a processor for outputting measurements of the operating condition corresponding to the plurality of locations based on light received at the light detector; wherein the operating conditions at the plurality of locations are coupled such that an increase in one operating condition results in an increase in the other operating conditions, and vice versa; wherein the sensor optical fibre comprises a plurality of optical gratings (FBG 1-1), each grating in use being disposed at a respective location and arranged to operate over a respective range of wavelengths depending on variations in the operating condition; characterised in that: a first of the plurality of gratings is a master grating (FBG E) and is arranged to operate over a first wavelength range, the first wavelength range being distinct from a second range of wavelengths over which the other gratings are arranged to operate, and wherein the other gratings (FBG A-D and FBG F-I) are arranged such that the respective wavelength ranges over which they operate are spaced apart from the first wavelength range by a respective predetermined interval and such that they overlap with the respective wavelength range of at least one adjacent other grating; a first of the plurality of gratings is a master grating (FBG E) and is arranged to operate over a first wavelength range, the first wavelength range being distinct from a second range of wavelengths over which the other gratings are arranged to operate, and wherein the other gratings (FBG A-D and FBG F-I) are arranged such that the respective wavelength ranges over which they operate are spaced apart from the first wavelength range by a respective predetermined interval and such that they overlap with the respective wavelength range of at least one adjacent other grating; wherein the amount by which the respective ranges of wavelengths of the other gratings overlap is such that for each of the other gratings there is a range of unambiguous wavelengths that are unique for that grating and a range of ambiguous wavelengths that overlap with the wavelengths of the adjacent grating; wherein the processor is operable to determine the value of the operating condition at the location of the master grating from the wavelength value received in the first wavelength range, and to determine the value of an operating condition at the location of one of the other gratings based on the wavelength value received in the first wavelength range, a wavelength value received in the second wavelength range, and the predetermined intervals by which the overlapping ranges are spaced apart from one another.