Very classified so we only know bits and pieces. "Inertial navigation" is part of it - basically very precise dead reckoning. Using specialist equipment like laser ring gyros to get precise measurements.
Then also some other equipment like a magnetometer and magnetographic maps of the world to provide references to counteract the drift in any inertial navigation.
Then also some other equipment like a magnetometer and magnetographic maps of the world to provide references to counteract the drift in any inertial navigation.