Search-and-Rescue Transponders

A Search-and-Rescue Transponder (SART) is a radar transponder, which after having been triggered by a received radar signal will respond with the transmission of a burst of radar signals. The radar response will be visible on the screen of the radar that triggered the SART. As part of the GMDSS, SART devices are used to locate a survival craft or distressed vessel by generating a trace of dots on a rescuing ship's radar display where the nearest dot to the center indicates the position of the transponder within 150 m.
The transponder is an aid to home vessels or rafts during search and rescue operations.

Technical Details

A SART operates on the 9 GHz X-band (3 cm wavelength; 9.2-9.5 GHz) and will only respond to an X-band radar. It will not be seen on the S-band (10 cm wavelength) or any other radar frequency. The SART device may be triggered by any X-band radar signal within a range of approximately 8 nautical miles. Each radar pulse received causes it to transmit a response which consists of a number of sweeps across the X-band. When activated, the transponder first sweeps rapidly (0.4 microsecond) through the band and continues with a slower sweep (7.5 microseconds) back to the starting frequency. This process is repeated for a total of twelve complete cycles. At some point in each sweep, the SART frequency will match that of the interrogating radar and be within the pass band of the radar receiver showing a dot on the radar display.

If the SART is within range, the frequency match during each of the 12 slow sweeps will produce a response on the radar display. This will result in a line of 12 dots equally spaced by about 0.64 nautical mile (300000km/s times 7.9us total sweep-time divided by two due to the time-to-distance scaling of the radar). These dots are quite distinctive, and stand out from the normal radar responses - giving the search vessel both the bearing and range to the SART. If the range to the SART is within a couple of nautical miles, the radar display may show also the 12 responses generated during the fast sweeps. These additional dot responses, which also are equally spaced by 0.64 nautical mile, will be interspersed with the original line of 12 dots. They will appear slightly weaker and smaller than the original dots. As the SART is approached, the side lobe sensitivity of the radar antenna may result in showing the SART responses as a series of arcs or even concentric rings. These can be removed by the use of the gain or anti-clutter sea control. However, it may be operationally useful to observe the side lobe signals as they may be easier to detect in conditions and also they will confirm that the SART is near to the own ship.

The diagrams below show the expected responses at approximately 12 miles, 6 miles, and 2 miles.

When looking for a SART on the radar, it is preferable to use either the 6 or 12 nautical mile range scale. This is because the total displayed length of the SART response of 12 (or 24) dots may extend approximately 9.5 nautical miles beyond the position of the SART and it is necessary to see a number of response dots to distinguish the SART from other responses.
If only responses from the 12 low frequency sweeps are visible - when the SART is at a range greater than about 1 nautical mile, the position at which the first dot is displayed may be as much as 0.64 nautical mile beyond the true position of the SART. If the range closes so that the fast sweep responses are seen also, the first of these will be no more than 150 metres beyond the true position.

The detection range between these devices and ships, dependent upon the height of the ship's radar mast and the height of the SART, is normally about eight nautical miles. Note that a marine radar may not detect a SART even within this distance, if the radar settings are not optimized for SART detection. Especially the settings for anti-clutter sea and rain control must be set correctly to obtain optimal detection performance of SART signals.

Typical SART Specifications