Which radar system operates in en route airspace with a range of 100-250 NM?

The Air Route Surveillance Radar (ARSR) is specifically designed for en route airspace monitoring, which operates within a range typically between 100 to 250 nautical miles. This system is crucial for tracking aircraft as they travel between different air traffic control sectors in the en route phase of their flight, where altitude and route management are key to maintaining safe distances among aircraft.

ARSR provides data to air traffic controllers, allowing them to maintain situational awareness of aircraft in transit. It utilizes long-range capabilities to efficiently monitor air traffic, which is essential for the safety and efficiency of the national airspace system.

In contrast, other radar systems like Precision Approach Radar (PAR) and Airport Surveillance Radar (ASR) are designed for use during the terminal phase of flight, focusing on operations closer to airports. Surface Detection Equipment (ASDE) is intended for monitoring ground operations at airports and does not operate in the en route airspace, hence the unique functionality of ARSR makes it the appropriate choice for the given question.