Civilian GPS is intentionally degraded in accuracy. This is why GPS on smartphones and other devices relies on additional methods for correction.
However, for high-speed objects like missiles, such correction is impossible. Moreover, civilian GPS signals are deliberately designed to be vulnerable to jamming, to disable an enemy’s GPS use in times of conflict.
Therefore, Iran cannot effectively utilize either GPS or GLONASS.
Likely, Iran’s MRBMs achieved an accuracy of tens of meters in a situation where GPS was entirely useless. This is close to the limit of rocket engineering, and unfortunately, further improvement is not expected.
On the other hand, Israel, with access to military-grade GPS signals, can strike Iranian factories, radars, and bases with an accuracy of within one meter.
Naturally, military GPS is also extremely difficult to jam.
Unfortunately, this is a battle that was never meant to be won from the start.