...
NaN (Not a Number) is a value of numeric data-type representing an undefined or unrepresentable value, especially in floating point calculations. More here.67/171 for multilateration and 11/171 for trilateration.
NaN w.r.t. CBG
According to the CBG code NaNs represent "bad pairs that lead to no region". This means that landmarks that fail to produce intersection regions, consequently also fail to produce an estimate for the location of the target and instead give out an erroneous value. Author has handled such values with NaN (code snippet below).
...