What is the lump/ball thing visible below and to the left of the canopy in this image of a PAK FA (which became the Sukhoi Su-57), and what is it for? It's also seen on other fighters.
Radar has two serious limitations, one is that advanced software can jam it, the other is that you give away your position when transmitting radar signals.
An infra-red sensor can be used to find and destroy targets without giving away the position of the aircraft and cannot be jammed. IRST is especially effective at detecting low-flying vehicles like helicopters that would otherwise be lost against a background. IRST can also be used to find, hunt down and kill ground vehicles.
That is an passive Infra Red Search and Track (IRST) sensor. Used to passively identify and track aircraft using their emitted infrared energy. The system pictured above is used solely for an air-to-air role and does not have a use for identifying and tracking ground targets.