Mybe this will help with the question of "what exactly is drifting" imagine a single axis "IRS" ie: it's just a single accelerometer on little cart which travels along a straight level rail. It calculates where the cart has moved to along the rail by measuring the cart's acceleration, and integrating that with time to calculate position along the rail. You move the cart from the zero point to the 10 meter point along the track. The accelerometer measures the cart's acceleration as you move it, and the IRS' computer calculates the cart's velocity, and from the velocity, computes the cart's position. If the accelerometer measures acceleration perfectly, and the computer computes perfectly, then the IRS should tell you that the cart's position is 10.00000000 meters, and it's velocity is 0.00000000 m/s. But, nothing is perfect, so there are slight errors in the accelerometer measurements, and slight errors in the computation, so when your cart is stopped at the 10.000 meter mark, the "IRS" says the cart is at the 10.000001 meter position and it's velocity is 0.000001 m/s. That, in a nutshell is "drift" your calculated position and velocity 'drifts" from true position and velocity. It;s the result of errors in the acceleration measurement and errors in the computational methods. In a real IRS there's many, many more sources of errors. You have 3 accelerometers, all with measurement errors, their axes are not perfectly aligned, you have gyros measuring rotation, imperfectly, and all these measurements are used to compute your vleocity and position in 3 dimensions, which inevitability is done imperfectly. so like the cart, only in 3 dimensions, your computed position drifts away from your true position.