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A Search and Rescue Transponder (SART) is a self-contained, waterproof radar transponder intended for emergency use at sea. The radar-SART is used to locate a survival craft or distressed vessel by creating a series of dots on a rescuing ship's radar display. According to the GMDSS (Global Maritime Distress and Safety System) Manual produced by IMO (International Maritime Organization), “SARTs are the main means in the GMDSS for locating ships in distress or their survival craft…” There must be at least one SART on GMDSS compliant vessels up to 500 tons and vessels exceeding 500 tons must carry two.

SART TRANSMISSION
A SART will only respond to a 9 GHz X-band (3 cm wavelength) radar. It will not be seen on S-band (10 cm) or other radar. Shipboard Global Maritime Distress Safety System (GMDSS) include one or more search and rescue locating devices. These devices may be either a radar-SART (Search and Rescue Transponder), or a GPS-based AIS-SART (Automatic Identification System Search and Rescue Transmitter). SARTs have omnidirectional antennas, meaning their signal is more likely to propogate in any direction.

RECEPTION OF SART TRANSMISSION
The Radar-SART may be acquired by any X-band radar within a range of approximately 8 nautical miles (15 kilometers). Each radar pulse received causes the SART to transmit a response which is swept repetitively across the complete radar frequency band. When interrogated, it first sweeps rapidly (0.4 microsecond) through the band before beginning a relatively slow sweep (7.5 microseconds) through the band and back to the starting frequency. This process is repeated for a total of twelve complete cycles. At some point in each sweep, the radar-SART frequency will match that of the interrogating radar and be within the pass band of the radar receiver. If the radar-SART is within range, the frequency match during each of the 12 slow sweeps will produce a response on the radar display, thus a line of 12 dots equally spaced by about 0.64 nautical mile (1.2 km) will be shown. If you are close to the SART, say within 3 miles, the 12 dots will make 12 small arcs across your radar screen. If within a mile of the SART, instead of dots you will begin see 12 concentric circles displayed upon the radar screen. As these circles begin to close, they are an obvious indication that you are very close to the SART. When the range to the radar-SART is reduced to about 1 nautical mile (2 km), 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 (1.2 km), will be interspersed with the original line of 12 dots. They will appear slightly weaker and smaller than the original dots.

RANGE OF SART DETECTION
Depending on the location of your SART, your range of detection will vary. Based on studies discussed in GMDSS: Understanding the Global Maritime Distress and Safety System, there is specific data on the difference of these ranges. The studies showed that if your SART was laying flat in a survival craft, the range of detection would be approximately 1.8 miles. However, the small difference of having the SART upright increases that range to 2.5 miles. If the SART is placed 2 meters above the sea level, a radar may be able to pick up the signal from as far as ten miles away. From this study we can see the importance of mounting the SART as high as possible to ensure rapid and furthest possible detection.

REQUIREMENTS OF SARTS
SARTs must have a battery that will last, in stand by mode, for at least 96 hours. They must be able to operate in temperatures from -20 C to 55 C. It is recommended that a SART antenna should be located at minimum 1 m above the sea. This is recommended to comply with the IMO requirement that a SART must be able to be detected within 5 nautical miles. It is required that every GMDSS compliant vessel be fitted with a minimum of one SART, more than one if they exceed 500 tons.

TYPES OF SARTS
SARTs are typically cylindrical, about the size of a person's forearm, and brightly colored. SARTs come in two primary types; float free or manually activated. If float free, the SART will by automatically activated once it has sunk, presumably with the ship. Manually activated SARTs can be turned on whenever the operation chooses, giving more time for rescue before the ship become completely immersed.

Source
IMO Sub-Committee on Safety of Navigation, Operation of Marine Radar for SART Detection, [http://www.navcen.uscg.gov/pdf/imo/circulars/sncirc197.pdf SN/Circ. 197]

GMDSS Manual: Manual on the Global Maritime Distress and Safety System. London: IMO, 2007. Print.

Tetley, Laurence, and David Calcutt. Understanding GMDSS: The Global Maritime Distress and Safety System. London [u.a.: Edward Arnold, 1994. Print.

Campbell, John. GMDSS: Understanding the Global Maritime Distress and Safety System : The New Marine Radio Communication System. Shrewsbury: Waterline, 1998. Print.