Plain English Meaning
A radar that figures out exactly where an aircraft is from just one reply from its transponder, instead of needing several sweeps. This makes it faster and more accurate, which matters when controllers are watching aircraft fly closely spaced parallel approaches.
Full Definition
A type of secondary surveillance radar that determines an aircraft's bearing using a single pulse return from the aircraft's transponder, rather than relying on multiple sweeps of a rotating beam. By comparing signals received simultaneously across different parts of its antenna, MSSR calculates the aircraft's position more accurately and updates faster than conventional secondary radar, which is why it is used in precision-critical applications such as Precision Runway Monitor (PRM) operations.