INS is a Dead Reckoning System.
It uses two accelerometers.
The principle of measurement is acceleration, if we integrate acceleration/movement with reference to time, we can get Velocity, and if we integrate Velocity with reference to time we can get Distance.
We have 2 integrates aligned with North/South and two aligned with East/West, we can always calculate the present position of the A/c, if we have kept a track of the previous position as is the case in the Dead Reckoning System.
These integrations are aligned on a platform. The platform must be Erect and Stabilized at all the time.
This instrument suffers from the Drift and Topple errors, therefore it should always be kept horizontal.
The system utilizes a Laser gyro unit to achieve this.
The gyro requires to be stabilized every ½ an hour of the flight.
Wander is caused due to the rotation of the Earth and movement of the A/c over the Earth and must be corrected for.
The system requires to be erected before it becomes Operational, this is done by feeding the co-ordinates in the system computer, when the A/c is parked on the Ramp before taxing.
The platform should be aligned properly along the North/South directions.
INS gives great Circle tracks and Great Circle distances.