Terrain referenced navigation system

A navigation system for an aircraft, comprising:

• a microelectromechanical systems inertial measurement unit (MEMS-IMU);
• a camera arranged to capture, over time, a series of images of the terrain below the aircraft;
• a terrain map comprising terrain elevation data;
• an inertial navigation system (INS) arranged to generate a first position estimate based on one or more signals output by the MEMS-IMU;
• and
• an optical terrain-referenced navigation unit (O-TRN) configured to:
• identify a terrain feature in two or more images captured by the camera;
• calculate a flow rate at which the terrain feature passes below the aircraft based on said two or more images;
• derive a terrain height estimate of the terrain feature based on the calculated flow rate;
• and
• generate a second position estimate based on a comparison between the terrain height estimate and terrain elevation data extracted from the terrain map,
• wherein the navigation system is configured to generate a third position estimate based on the first position estimate and the second position estimate.