Abstract: An inertial measurement unit includes physically distinct sectors positioned in groups of orthogonally oriented angle rate sensors on a different sector of a base having orthogonally oriented accelerometers positioned thereon. A processor receiving signals from the sensors and accelerometers calculates a change in attitude, position, angular rate, velocity, acceleration of the unit over a plurality of finite time increments, or a combination thereof. The gyros and accelerometers have low-drift measurement accuracy for operation in a GPS-denied environment by preselecting pairs of gyros for physical assignment to achieve low-drift accuracy, determining weights for the gyros to be combined in tiered pairs, preselecting the accelerometers for physical assignment in low-drift pairs, determining weights for accelerometer optimal low-drift pair combining in tiers, or a combination thereof.