In this context, SART stands for Search and Rescue (Radar) Transponder. Like an AIS SART, the purpose is to allow other vessels in the vicinity to see your distress signal, so they can come and help you. Unlike AIS SARTs, Radar SARTs can show up on any X-band pulse radar set.

How does it work? When activated, a Radar SART goes into listening mode, waiting to be struck by a radar transmission from any vessel within range. When an incoming radar signal is received, it transmits a response signal, which clearly identifies the craft in distress on the radar screen. The illustrations below shows a Radar SART on a radar screen range.

SART response at 5-6nm
SART response at 2-3nm
SART response at < 1nm

Whereas an EPIRB or PLB is a 21st Century alternative to rocket flare, to raise a distress alarm, a Radar SART (or AIS SART) is a 21st Century alternative to a hand-held flare, to pinpoint your precise position. The difference being that it has much greater range, being limited only by line of sight between the rescuing ship's radar antenna and the life raft where the Radar SART has been activated.

Radar SARTs do not work on L band radars, or on FM-CW X band radars. Whilst still popular in the commercial world, they are being progressively replaced with AIS SARTs.