Search and rescue transponder

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A search and radar transponder (SART) is a radar transponder, part of the Global Maritime Distress and Safety System (GMDSS), which assists searchers, using to find the people in distress. Radar for these applications, usually in the X-band, normally depends on the reflections of its energy. When the target is a wood lifeboat, a plastic liferaft, or a person in the water, those targets do not reflect nearly as much energy as would a metal surface. Indeed, since there are random reflections from waves and ripples in the water, it may be impossible to see a small target on radar.

SARTs complement other rescue equipment, such as an emergency position indicating radio beacon (EPIRB). EPIRB signals are received by orbiting satellites, over most of the earth. If the EPIRB does not transmit the GPS coordinates, it may take several direction finding passes of the satellites to localize the beacon. Still, unless GPS is in use, EPIRB will get the rescuers to the general area, but they still need to find the victims in the last few miles.

Localizing with SART

In some cases, lifeboats have passive radar reflectors, made up of light metal at sharp angles, intended to reflect energy efficiently. They are less than ideal, as they can be bulky and not at all useful for rafts or personal floatation gear. SART, however, are much more efficient as radar beacons. They conserve battery life by only operating a receiver, unless they detect an X-band pulse. When they detect such as signal, they transmit an X-band signal in response, which will appear as a bright radar target.[1]

Further, the signal they send is not a single dot, but a pattern that will direct rescuers. A SART semds a sequence of twelve pulses, timed, at first detection, so they will display with the actual SART at the position of the first dot, and the remainder radiating toward the edge of the screen. As the rescuers approach, the dots will become short arcs. The closer the search radar gets to the SART, the larger the arcs, until, when the SART is very near.

SARTs, like any marine rescue signaling device, work best when they are elevated above sea level, distancing them from "clutter" caused by waves. If at all possible, the SART should be mounted about 2 meters above the floor of a liferaft.

Search platform Height of SART Detection range
Ship radar Flat on raft bottom 1.8 miles
Ship radar Vertical on raft bottom 2.5 miles
Ship radar On 2 meter mast 7-10 miles miles
Aircraft at 3,000 feet Variable 30 miles or more

If one considers the geometry between the radar and the SART, the aircraft has a much better angle to separate the SART from sea clutter.

Further help in locating

Ideally, the persons needing rescue also have a waterproof digital selective calling (DSC) VHF marine radio, preferably connected to a GPS receiver or containing one. They also should have day/night flares (smoke/fire) or a flare gun.

It is not at all unlikely that the persons in distress will see the rescuers before the rescuers see them. Visual signals can help, but going onto VHF Channel 16, preferably with the GPS transmitting the exact position, and coaching them close will be extremely helpful.

It may well be that a GPIRB with GPS, or a VHF radio with GPS, will do away with the need for SART. Until there is more experience, a wise mariner has all of them.

Future trends

It has been suggested that SARTs made after 2010 may also respond to interrogation from the automatic identification system, on VHF, in addition to X-band radar

References