APEX, APEX II, ESGN systems

[Timmy C]

A colleague of mine is looking for information as to what the terms in the subject refer. They have something to do with subsurface and/or acoustic use during the 1980s in the Arctic. Does any one know what they were, specifically?

I saw that Friedman's Naval Weapons Systems 2006 edition refers to the ESGN as a navigation system onboard the LA class boats, but not sure if there could be another definition.
The 1997/8 edition of Naval Weapons Systems refers to APEX as a superprocessor that was installed on the BEARTRAP squadron of P-3Cs in order to better integrate computer equipment in 1994. Could earlier versions of the processor also be called APEX?

A paper by the US Naval Oceanographic Office refers to APEX as a drifting buoy sensor.