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General Description

A standard SART 9 GHz Radar Transponder, produced by Jotron, on board a Norwegian ferry. The unit is 251 mm high.

IMO-approved SART
File:SART at Various Ranges.jpg A SART as scene on radar at various ranges.
Transmitting Frequency
9200 - 9500 MHz swept
Polarization
Is horizontally polarized
Sweep Rate
5μs +/- 0.5μs
Form of Sweep
Sawtooth, fast return <1 μs
Pulse Emission
100μs nominal
Transmitting Antenna
Vertical Beamwidth < 25° Azimuthal Beamwidth Omnidirectional within +/- 2 dB
Effective Isotropic Radiated Power
<400 mW
Effective Receiver Sensitivity
Better than -50 dBm
Recovery Time Following Excitation
within 10 μs
Response Delay
>1.25 μs
Temperature Range
-30°C to +65°C stowage -20°C to +55°C operation

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. 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).

The radar-SART may be triggered 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 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. 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.

IMO Requirements Regarding SARTs

At least one SART shall be carried on every ship of 300 Gross Registered tons and above. At least two SARTs shall be carried on ships of 500 Gross Registered Tons and above. Radar Transponders may be carried in Survival Craft and may be incorporated in a VHF DSC EPIRB. After being turned on the SART remains in 'Standy' mode (for up to 96 hours) until activated by a 9 GHz (9200-9500MHz) radar emission which then causes the SART to transmit a signal appearing as a straight line of 'blips' on the radar display.^[1] As the searching vessel draws closer to the SART position, the blips will increase in size to form arcs and then complete circles, when the vessel is close enough to trigger the SART transmission continually. When transmitting the SART also "rings" thereby indicating to the survivors that searchers are close. Some SARTs have a second facility called ANTI-COLLISION mode. When this mode is used, the SART will transmit 5 pulses ranging over 1 nautical mile. When accompanied with a survival craft, typically the SART will be affixed by a cord.^[2]

A lamp on the SART will flash when the device is activated, giving the survival craft occupants a visual indication that SAR teams are in the immediate vicinity. ^[3]

The Range of a SART is dependent on a number of factors including: the height at which the SART is mounted in a liferaft or lifeboat (as high as possible outside the liferaft canopy - inside the canopy will severely restrict the range); the type of radar; the height of its antenna on the SAR vessel; and the range at which the radar is set. A radar set to short range will reduce brightness of the 'blips' so they may not be seen at extreme range. Radars set to longer ranges, eg. 12 nm or 24 nm , emit longer pulse which are of higher energy than at short ranges. So a SART transmission should be clearly indentifiable on both the 24 nm and 12 nm ranges and all 12 'blips' will be seen. Having identified the SART transmission, the radar range may be progressively shortened as teh SAR vessel approaches but on shorter ranges all 12 'blips' will not be seen. The IMO (International Maritime Organization) IMO performance standard for SARTs recommends a mounted height of 1 metre above sea level to give a range of 5nm, given a ship's interrogating radar antenna mounted at 15 metres above sea level. A well mounted SART may have a detection range of up to 10 nm in good weather, but the detection range is typically 8 nm. SKill of operation of the SAR vessel's radar is also a consideration where the ability to eliminate 'clutter' may affect SART detection. Flat calm weather will contribute to multi-path radar propogation when radar pulses are reflected from the sea surface. High seas/swell will intermittently vary the detection range due to occasional increases in SART height above sea level. Detection of SARTs from aircraft should be about 40nm with an altitude of 3000 metres. ^[4]

Technical Characteristics of the SART

No license or registration is required to carry a SART. ^[5] SARTs are typically cylindrical, about the size of a person's forearm, and brightly coloured. SARTs should be kept clean, so that the manual switch does not become clogged with salt or dirt.^[6] A plate indicating when the battery should be replaced is affixed to the SART by the manufacturer. The should be regularly checked.^[7]

Notes

^ P.C. Smith & J.J. Seaton (1994), p. 17
^ P.C. Smith & J.J. Seaton (1994), p. 41
^ United States Power Squadrons (2006), p.82
^ P.C. Smith & J.J. Seaton (1994), p. 77
^ United States Power Squadrons (2006), p.82
^ P.C. Smith & J.J. Seaton (1994), p. 77
^ P.C. Smith & J.J. Seaton (1994), p. 78

References

IMO Sub-Committee on Safety of Navigation, Operation of Marine Radar for SART Detection, SN/Circ. 197

GMDSS for Navigators, P.C. Smith & J.J. Seaton (ISBN 0-7506-2177X) published 1994 Butterworth-Heinemann Ltd Jordon Hill Oxford Pg.17,41,77-78

The Boatowner's Guide to GMDSS and Marine Radio, United States Power Squadrons (ISBN 0-07-146318-6) published 2006 by International Marine pg. 82

External links

Photo of a SART and radar display
Photo of an active SART as seen on radar
SART Test equipment - Test Box for SART annual survey

Category:Maritime communication Category:Emergency communication Category:Rescue equipment Category:Radar

[1] P.C. Smith & J.J. Seaton (1994), p. 17

[2] P.C. Smith & J.J. Seaton (1994), p. 41

[3] United States Power Squadrons (2006), p.82

[4] P.C. Smith & J.J. Seaton (1994), p. 77

[5] United States Power Squadrons (2006), p.82

[6] P.C. Smith & J.J. Seaton (1994), p. 77

[7] P.C. Smith & J.J. Seaton (1994), p. 78

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