Question: How do transport-category jets navigate the North Atlantic?
Gone are the days of navigating with a sextant. Until the early 1980s, my former airline actually utilized a sextant viewing port aboard the Boeing 707 as a secondary means of verifying position. This remained until the airplane was retired from service. Even inertial navigation systems (INSs) have virtually become extinct. Today, inertial reference systems (IRSs) that include laser-ring gyros operate within three separate onboard inertial reference units (IRUs).
Position data from the IRUs is combined with GPS position, VOR position, and DME position. The combined data is processed by two separate flight management computers (FMC) that communicate with each other to provide a more accurate real-time position in addition to numerous other complex functions.
Failures are rare, but the IRS is capable of navigation with only one IRU and one FMC, albeit on a restricted basis that does not allow for low visibility approaches because of accuracy degradation.