The weather conditions made early ground support haphazard, and these difficulties were compounded by North Vietnamese anti-aircraft units, which advanced behind the front line elements. PAVN moved 85 and 100 mm radar-directed batteries south of the DMZ and, on 17 February 81 SA-2 Guideline missiles were launched from the DMZ area, downing three F-4s. This heralded the farthest southern advance of SA-2 units thus far during the conflict. This classic high-low anti-aircraft coverage made aerial attacks extremely hazardous, especially when it was enhanced by the new shoulder-fired Grail.