As gimbaled inertial navigation systems evolved, they necessarily became increasingly based on intricate mechanical designs. In recent decades, technology has progressed more in the area of electronics than mechanics. This is reflected in the evolution of the strapdown inertial navigation system. With less moving parts and mechanisms than gimbaled systems, strapdown inertial navigation systems have strongly benefited from the advance of computer technologies, being built upon electronics, optics, and solid state technology. The majority of commercially available accelerometers and gyros take advantage of these modern technologies and are manufactured on micro-machined silicon.
Strapdown inertial navigation systems are rigidly fixed to the moving body (Figure 1.3). Therefore strapdown INUs move with the body, their gyros experiencing and measuring 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 body's fixed axes is a moving frame of reference as opposed to the constant inertial frame of reference. The navigation computer uses the gyros' angular information and the accelerometers' linear information to calculate the body's 3D motion with respect to an inertial frame of reference.
Figure 1.3: A Strapdown System