An inertial measurement unit (IMU) is a set of motion sensors that measures how a drone, vehicle or phone is accelerating and turning. From these measurements, the vehicle can work out where it is and which way it is facing, using only its own movement, without GPS. Using an IMU in this way is called inertial navigation.
An IMU usually holds three accelerometers and three gyroscopes, one of each for every direction: forward and back, left and right, up and down. The accelerometers measure changes in speed. The gyroscopes measure turning. Many IMUs also add a magnetometer, a compass sensor. In small drones and phones, these are tiny MEMS sensor chips, mechanical structures carved into silicon. Larger aircraft and missiles use far more accurate, and far more expensive, optical gyroscopes.
How does it find a position? The flight computer reads the IMU hundreds or thousands of times a second. From each reading, it updates the vehicle's speed, and from the speed, its position. The problem is drift. Every reading has a tiny error, and the errors add up. A cheap IMU used alone can be wrong by many metres within a minute. So most vehicles combine the IMU with satellite navigation, such as GPS or India's own system, NavIC. When satellite signals are available, they correct the drift. When they are blocked or jammed, the IMU keeps the estimate going.
Where are satellite signals missing? Under bridges and in tunnels, inside buildings and mines, in narrow city streets, and near borders, where signals can be jammed on purpose. That is why defence forces and drone makers in India buy IMUs, and why soldiers on such missions often carry mesh radios, which pass messages to each other with no tower.
Imagine a survey drone inspecting the underside of a railway bridge near Cuttack. Above the bridge, satellite navigation fixes its position to within about 3 metres. When the drone flies under the deck, the steel and concrete block the satellite signals for 40 seconds. During that time, the IMU measures the drone's movement 1,000 times a second, 40,000 readings in all. The flight computer adds up every movement since the last satellite fix. Over the 40 seconds, its position estimate drifts by up to about 2 metres. The drone's camera also keeps it at a fixed distance from the girders. When the drone comes out from under the bridge, a new satellite fix corrects the drift, and each photo of the girders is tagged with its position. This kind of work is part of drone surveying.
Navigation is only one use. The same unit keeps a drone level in the wind, steadies a phone's camera, and tells a phone screen to rotate. In India, IMUs and navigation units for drones and vehicles are designed in Bengaluru.