Aircraft fuel measurement

An aircraft fuel tank system, comprising:

• a fuel tank;
• an array of sensors;
• a light source arranged to illuminate a surface of a liquid fuel in the fuel tank with light so that the light is reflected or scattered by the surface of the fuel onto the array of sensors, the light travelling to and from the surface of the fuel via a measurement path containing a transmission medium, wherein the light source is also arranged to illuminate at least one sensor of the array of sensors with light via a reference path containing a transmission medium with substantially the same refractive index as the transmission medium in the measurement path, the reference path having a known reference distance which does not vary in accordance with the quantity of the fuel;
• and
• a processing system arranged to determine a measured time of flight of the light which travels via the measurement path to each sensor of the array of sensors, measure a reference time of flight of the light which travels via the reference path to said at least one sensor, and estimate the quantity of the fuel in the fuel tank in accordance with the measured times of flight, the reference time of flight and the known reference distance,
• a reflector mounted in fixed relation to the array of sensors, so that the light travelling via the reference path is reflected onto said at least one sensor by the reflector,
• wherein the reflector is a semi-transparent window arranged such that some of the light from the light source is transmitted through the semi-transparent window and illuminates the surface of the fuel to provide the measurement path, and some of the light from the light source is reflected by the semi-transparent window onto said at least one sensor to provide the reference path.