What does the compass error acronym "UNOS" refer to?

The acronym "UNOS" stands for "Undershoot North Overshoot South." This term is used in aviation, particularly in relation to magnetic compass errors when navigating.

When a helicopter or aircraft is flying in the northern hemisphere, the compass will tend to indicate a heading that is lower than the actual heading when turning to the east (undershoot), and when turning to the south, the compass may lag and indicate a heading higher than the true heading (overshoot). This is a crucial detail for pilots to be aware of, as it affects navigation accuracy and flight safety. Understanding this behavior helps pilots correct for these errors and maintain accurate course and heading during flight.

The other options provided do not accurately capture the correct interpretation of the acronym. Although they may contain variations of the terms in "UNOS," they fail to represent the standard aviation mnemonic used to remind pilots of the compass error tendencies in relation to the north and south headings.