where is karna?

I'm wondering why karna isn't participating in the battle?

Karna doesn't participate in the battle until Bhishma is defeated. As per the epic, Bhishma refuses to battle if Karna will also fight (as he's always disapproved of Karna's friendship with Duryodhan), so Duryodhan pleads with his friend not to fight while Bhishma is commander-in-chief. Karna fights once Bhishma is defeated and Dronacharya becomes commander-in-chief.