To get this tech upgrade you must meet the silver medal requirements for the first two mission objectives. At that point, the third objective will be unlocked. Mission objectives are 1: Disable fleeing Imperial transports, and 2: Provide cover for the Rebel recovery crews. The Silver Medal requirements are: completion time: 13:20, enemies destroyed: 45, shot accuracy: 3%, friendlies lost: 1, lives lost: 1, targeting computer efficiency: 3%. Once on the surface in the TIE bomber, the advanced proton bombs upgrade will be in the last domed tower once you bomb it. Swoop down and recover it.