Nice article on the upgrades the F-117 went through.
Avoiding detection by systems like the P-18 required a combination of careful planning, operational security, and tactics. But Zoltan’s hyperactive battery couldn’t be pinned down. Serb agents were tracking takeoffs from the F-117s’ base in Aviano, Italy, and airspace restrictions indicated that the fighters were following very similar routes, night after night.
According to Dani, when the F-117A was head-on at 26,000 feet and eight miles out, the battery fired two missiles. Each would have closed the distance in seconds. The pilot, Air Force Lieutenant Colonel Darrell Zelko, has never specified how much warning he had of the attack. One still-classified aspect of the F-117 is whether it carries any kind of radar warning receiver. If it does, even the most comprehensive studies of the aircraft have not mentioned it, nor has it been mentioned as part of any upgrade.
Even with the modified radar, it doesn't seem that it's a very practical solution to the stealth problem if they could only engage it at 8 miles. There were also about 850 sorties done by F-117's over Yugoslavia, and this was the only fully confirmed shoot down.