Complex sentence: Modern counter-battery fire relies on counter-battery radars, which calculate the source of incoming artillery shells very accurately and quickly--so quickly, in fact, that return fire can sometimes begin before the first enemy shell has landed.
Split into simpler sentences: Modern counter-battery fire relies on counter-battery radars, which calculate the source of incoming artillery shells very accurately and quickly. So quickly, in fact, that return fire can sometimes begin before the first enemy shell has landed.