SSR Garbling RNAV
Garbling occurs when two aircraft are too close to each other and their SSRSSR —Secondary Surveillance Radar returns effectively overlap. For this to be the case, the two aircraft must be in roughly the same place - so in the same direction from the SSRSSR —Secondary Surveillance Radar interrogator, and with a total (so slant range, not plan) distance of less than 1.7nm between them. This last point is just an exam fact you need to learn. Finally, garbling is not a problem with Mode S.