Assessment of position that happens on Judgement Day? (4,9)
I believe the answer is:
dead reckoning
'assessment of position' is the definition.
'dead reckoning' can be an answer for 'position' (thesaurus). I am not sure about the 'assessment of' bit.
'happens on judgement day?' is the wordplay.
I cannot really see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
'day?' could be 'd' (abbreviation for day) and 'd' is found within the answer.
This explanation may well be incorrect...
'that' is the link.
Can you help me to learn more?
(Other definitions for dead reckoning that I've seen before include "Checking losses in battle" , "Estimation of position at sea by reference to course steered and distance run" , "at sea, it establishes one's position" , "One way to estimate position" , "Navigation without using the stars" .)