DCS F-16C AGM-88 HARM Modes Explained

Another thing to note, although the most dangerous approach… it is recommended that you wait until the enemy SAM site fires at you, for three reasons:

1- To ensure you're in range (will work for most SAMs, except for SA-10, Patriot, Perry, Ticonderoga and Moskva, which will fire at ~40nm, unless you're using ECM)

2- To ensure your HARM will have a radar signal to lock on to.

3- To use the HARM as countermeasure. (Depending on the mission, SAM radars might turn off upon detecting a HARM launch, forcing SAM missiles to lose lock and self-destruct).