Navigation Without External Signals
Inertial navigation is the only positioning technology that requires no external signals — no satellites, no radio beacons, no landmarks. By continuously measuring acceleration and rotation from a known starting point, an INS computes position through pure dead-reckoning. This self-contained nature makes INS indispensable for submarines, missiles, spacecraft, and any platform operating in GPS-denied environments.
The Sensor Triad
An inertial measurement unit (IMU) contains three orthogonal accelerometers and three orthogonal gyroscopes. The gyroscopes track the vehicle's orientation, allowing the navigation computer to transform accelerometer measurements from body frame to navigation frame. After subtracting the gravity vector, the remaining specific force is integrated to yield velocity, then integrated again to yield position. This double integration is both the method's power and its Achilles heel.
The Tyranny of Integration
Double integration amplifies every sensor error. A tiny accelerometer bias — just 1 milligravity (0.01 m/s²) — produces a position error that grows quadratically: 18 m after 1 minute, 1.8 km after 10 minutes, 29 km after 1 hour. Gyroscope drift is even more insidious: it misaligns the reference frame, coupling the enormous gravity vector (9.8 m/s²) into the horizontal channels. The simulation lets you watch these errors accumulate in real time.
Sensor Grades and Applications
INS accuracy spans orders of magnitude depending on sensor quality. Consumer MEMS IMUs ($5, 10 mg bias, 100°/hr drift) are usable only with continuous GPS aiding. Tactical-grade IMUs ($1k, 1 mg, 10°/hr) support minutes of GPS outage. Navigation-grade IMUs ($50k, 0.05 mg, 0.01°/hr) enable hours of standalone navigation for aircraft and ships. Strategic-grade IMUs ($500k+) guide ICBMs and submarines for weeks. The simulation reveals why this hierarchy exists.