Passage: The Egyptian and Syrian 2K12's surprised the Israeli military in the 1973 Yom Kippur War, because they were accustomed to having air superiority over the battlefield. The highly mobile 2K12 took a heavy toll on the slower A-4 Skyhawk and even the F-4 Phantom, forming a protective umbrella until they could be removed. The radar warning receivers on the Israeli aircraft did not alert the pilot to the fact that he was being illuminated by the radar. Once the RWRs were reprogrammed and tactics changed, the 2K12 was no longer such a grave threat, but still caused heavy losses to Israeli aircraft.

Question: What was the pilot not alerted to?
he was being illuminated by the radar