What is Inertial Navigation System?

An Inertial Navigation System (INS) is a navigational aid that makes use of a clock, accelerometers (sensors for measuring acceleration), gyroscopes (sensors for measuring rotation) and a computer to continuously calculate the position, orientation and velocity of an object on the move without any external references. Inertial navigation is a self-contained navigation technique in which measurements provided by accelerometers and gyroscopes are used to track the position and orientation of an object relative to a known starting point, orientation and velocity. Inertial measuring units (IMUs) typically contain 3 orthogonal rate-gyroscopes and 3 orthogonal accelerometers, measuring angular velocity and linear acceleration respectively. These signals are processed with the help of the on board computer and the position and orientation of the object can be tracked.

All the IMUs fall into 1 of the 2 categories outlined below. The difference between the 2 categories is the frame of reference in which the gyroscopes and accelerometers operate. The 2 categories are stable platform systems or gimballed platform systems and the strap down systems.

In stable platform type of systems, the inertial sensors are mounted on a platform which is isolated from any external rotational motion. This is achieved by using gimbals which allow the platform freedom to move in all the 3 axes. The gyroscopes on the platform detect rotations of the platform if any and the signals are fed back to the motors which rotate the gimbals in order to cancel out such rotations and hence keeping the platform aligned with the global frame. To track the position of the object, the angles between adjacent gimbals is calculated with the help of angle pick-offs and to calculate the position, the signals from the accelerometers mounted on the platform are double integrated. To get accurate results, we need to subtract acceleration due to gravity from the vertical channel before performing integration.

In strap down systems the sensors are mounted rigidly onto the device and hence the output is measured in the body frame and not global frame as in the case of stable platform systems. The orientation of the object can be determined by keeping track of the signals from the gyroscopes and integrating them while the position is determined from the accelerometer signals which are resolved into global co-ordinates and then integrated.

Both these types of systems are based on the same principles. Strap down systems are less complex and are physically smaller compared to stable platform systems. Due to this the strap down systems are the more dominant type of INS.

INS is autonomous and does not rely on external aids or visibility conditions and can operate in tunnels or underwater and anywhere. It is immune to jamming and inherently stealthy. It neither receives nor emits detectable radiation and requires no external antenna.

Errors increase with time as they are mean-squared. Cost of acquisition is high, the maintenance costs are also higher and so are the power requirements when compared to GPS receivers. With integration with GPS, these systems have become cheaper, maintenance costs have also gone down, the sensor errors have dropped to acceptable values and when there’s loss of GPS signals this integrated INS can be used.