Lightweight digital computers permit the system to eliminate the gimbals, creating strapdown system, and are called so because their sensors are simply strapped to the vehicle. In a strapdown inertial navigation system the accelerometers are rigidly mounted parallel to the body axes of the vehicle. In this type of system, the gyroscopes do not provide a stable platform. Rather, they are used to sense the turning rates of the craft. The computer determine the craft’s current velocity and position with the help of double numerical integration, combining the measured accelerations and the instantaneous turning rates and guide it along the desired trajectory.

This reduces the cost, eliminates gimbal lock, removes the need for some calibrations, and increases the reliability by eliminating some of the moving parts.

In other way we can understand the function of a strapdown system as :

A strapdown Inertial navigation system (INU) move with the body. Their gyros experiences and measures the same changes in angular rate as the body in motion. The strapdown INU’s accelerometers measure changes in linear rate in terms of the body’s fixed axes. The system body’s fixed axes are a moving frame of reference as it is opposed to the constant inertial frame of reference. Whereas, The navigation computer uses the gyro’s angular information and the accelerometer’s linear information to calculate the body’s motion with respect to an inertial frame of reference.

A strapdown system has a dynamic measurement range, several hundred times than a gimbaled system. That is, it must integrate the vehicle’s attitude changes in pitch, roll and yaw, as well as gross movements. Gimbaled systems could usually update at rate of 50 to 60 updates per second. However, strapdown systems normally update about 2000 times per second. The higher rate is needed to keep the maximum angular measurement within a practical range for real rate gyros i.e. about 4 milli radians.