[flavius29663]

> before GPS was viable

GPS can be jammed (see Russia-UKraine war), so inertial systems are still very important for rockets, for example some HIMARS rockets start with GPS and then rely only on inertial while getting close to target.

[missedthecue]

Himars relies on inertial navigation the entire flight and uses GPS updates to course correct. If the GPS is blocked for a sufficient amount of flightime, even with the intertial navigation, the accuracy can become unusably low.

This is how the Russians have been throwing double digit percentages of launches off course.

[ethbr1]

Terminal guidance since ~1995 on higher-end weapons has switched to hybrid inertial + scene matching (various sensor types).

F.ex. the 90s Tomahawk used terrain contour matching to orient itself

For more details see https://apps.dtic.mil/sti/tr/pdf/ADA315439.pdf (US translation of a mid-90s Chinese survey of the guidance space, but it covers the material and is publicly available)

Afaik, most modern systems use infrared target matching for final course correction. (Initially developed to allow anti-shipping missiles to autonomously prioritize targets, but now advanced enough to use in land scenarios as well)