David Lesher wrote:
A friend gave me the Haynes manual for Xmas, and in reading it [&
looking at the pictures...] I've realized I know diddly about the IU,
besides IBM made it.

I know it saved the bacon when lighting hit the launching stack, but
for example, how did it navigate? I see it had its own radar; I'm
guessing it interrogated ground transponders that let it calculate its
position to compare with the INS. Correct?

Not correct. It had an inertial platform (ST-124) and used inertial
navigation. There was no "external" source of position; the LVDC
computed position and velocity by integrating the acceleration from the
ST-124.