The second part of Mission 13 (fight with Pasternak) is a little tricky for getting an S rank.

You need to shoot down drones during the battle with Pasternak. These are just
rough estimates but I think they are close after repeating the mission over and over again.

C = 0-10
B = 11-20
A = 21-30
S = 30 +

Take note that this is based if you got S ranks on any of the four initial operations. Of course, the plane I used is the CFA-44 and with the assistance of allied ESM fields. It also took me at least one frontline landing (while the drones and Pasternak are looming overhead) to get a fresh load of ADMMs.