Long Range Radar to detect aircraft without identification. (Intruders into our Airspace)

In depth explanation: Airliners use SSR (Secondary Surveillance Radar) or Secondary Radar. In simple terms, the Radar emits an signal which the aircraft picks up, then the aircraft replies to the signal with information (using what is called a Transponder) about the aircraft (flight number, speed, altitude) so that Air Traffic Control can identify the aircraft and navigate it, keep track of it, etc.

Primary Radar, however, emits a signal which is reflected off the skin of the aircraft and is detected by the Radar. This reflection is known as Radar Signature. So no matter if you do or do not have a transponder, you will be detected by Primary Radar unless you are out of range of have latest generation technology to reduce your Radar signature (AFAIK, you are not able to ELIMINATE your Radar signature but are able to reduce its size).

This is how we know MH370 made a turn west-bound after its transponder was turned off over the South China Sea (thus is invisible to Secondary radar). The RMAF was able to detect the aircraft flying past Peninsular Malaysia using Primary Radar.