NavList:

"It seems clear that INS alone, without checks is not capable of the required
accuracy. The question is what checks were available before satellites,
and also what checks are available without surfacing."

I haven't read all of the messages in this thread, so forgive me if I repeat what has already been posted.

Both satellite and Loran navigation checks were possible while submerged at shallow depths (only the antenna needs to get near the surface.

All inertial navigation systems are subject to drift, but part of the ingenuity behind these systems is the sophisticated Kalman filtering used for correction. The entire navigation system, including error sources, is modelled; any position updates, whether from satellite, radio, or sea floor references are used not just to correct the current location, but to better model all of the drift errors. A crude analogy is the use of early chronometers. Comparison to the longitude of a known location not only gave the absolute time error with respect to Greenwich, but was useful for estimating the rate at which the timepiece was gaining or losing.

Back to submarines and INS, there are two very different systems in use. The submarine has a system that is expected to operate for long periods of time at modest velocities and accelerations. The missiles have a different INS, that only has to remain accurate for a few minutes at extremely high accelerations. And the missile navigation system also has a form of celestial navigation, a stellar sensor that sights upon a particular star after the boost stage. The information from the star sight is fed back into the Kalman filter to estimate what the various drift errors are, so that a more accurate position fix can be calculated from the accelerometer and gyro data generated.

Don Seltzer
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