SART stands for Search and Rescue Transponder. A radar SART is an emergency locating device used to find survivors, survival craft or a vessel in distress at sea. It receives pulses from a suitable search radar and answers them with a conspicuous signal pattern on that radar's display. Survivors can use it to mark their location electronically when a suitably equipped vessel or aircraft searches within range. The radar SART serves this on-scene locating function; the initial alert to a rescue coordination centre uses a different transmission path.
How the radar response is generated
Once switched on, a radar SART initially remains on standby, listening for X-band radar pulses in the 9.2–9.5 GHz band. A suitable incoming pulse triggers a sequence of twelve frequency sweeps. The response repeatedly sweeps through the radar frequency band. Whenever its frequency matches the search radar's receiving frequency, that receiver can detect part of the response. These successive responses appear as several more distant targets. The artificial pattern makes the small transponder more conspicuous than an ordinary radar echo from the survival craft.
The search equipment must both trigger the SART and receive and display its response. This requires a compatible X-band radar in a suitable operating mode. An S-band radar works in another frequency band and does not display this response. An AIS receiver alone cannot receive a radar SART either. The presence of equipment labelled “radar” aboard a vessel, or the appearance of an aircraft, therefore does not establish that suitable search equipment is being used. The radar SART itself needs neither entered coordinates nor its own satellite position receiver to produce the response.
What the line of dots means on the search radar
The characteristic display consists of twelve evenly spaced dots along a line on the bearing of the transponder. The line starts approximately at its position and extends outwards, as seen from the searching vessel. These dots come from the repeated responses of a single device. They represent neither twelve people nor a line of survival craft. The bearing helps the search crew approach; the range of the first dot initially gives only an approximate location.
The first visible dot can lie beyond the SART's actual position because time elapses between the triggering radar pulse and the matching part of the response. When only the slow frequency sweeps are received, this offset can reach about 0.64 nautical miles. At closer range, additional responses from the fast sweeps can appear and reduce the offset. An apparently precise display target must therefore not be treated as a survivor already in sight. Some dots may also be missing because of sea conditions, interference or the selected display settings.
On approach, the dots can spread into arcs and eventually concentric rings. The stronger signal is then also received through the radar antenna's sidelobes: the display responds outside the main narrow search direction. Arcs or rings consequently indicate a nearby SART, rather than survivors arranged in a circle. The search crew must make the transition from electronic location to actually identifying and recovering the people.
Activating and positioning the available device
The survivor first needs to identify which device is actually part of the available equipment. Radar SARTs and AIS-SARTs serve a similar locating purpose but use different radio methods. The device name, operating instructions and control markings are decisive. The steps specified for the particular device may include retrieval from a bracket, releasing protected controls and switching it on. For a design with separate manual activation, removal alone is insufficient; the designated control must be placed in the operating position. Any test position must be distinguished from the operating position for an emergency.
After activation, the operating indication is checked and the device is secured against loss at its intended attachment point. In a liferaft, the supplied lanyard connects the SART to the raft. A pole, antenna pocket or suspension point is used as intended for that combination of device and raft. A SART lying loose on the floor or floating horizontally has a less favourable antenna position than an upright mounted device. Buoyancy and watertightness do not themselves hold it in an effective operating position.
For the intended installation, the antenna is at least one metre above the sea surface. This height improves the radio path between the survivors' low position and the search radar. Both the triggering pulse and the transmitted response have to travel that path. The achievable range therefore also depends on the height and characteristics of the search radar antenna, propagation conditions and sea state. Positioning the device high in its intended mounting has a practical benefit; it does not provide a fixed detection range for the actual situation.
The antenna needs separation from bodies and equipment placed close beside it. Occupants, parts of the raft and a nearby battery can attenuate the signal in particular directions. Certain materials in canopy support tubes or reflective tape on the canopy can also produce such reductions. The intended mounting takes operation with the raft canopy into account. A radar reflector deployed on the same liferaft can obscure the SART response; the two locating devices should therefore not be deployed together there. Someone still in the water uses an available holding or attachment arrangement to keep the antenna upright above the surface and as clear of the body as possible. The lanyard keeps the device with the person; it does not itself position the antenna.
Indications on the SART and recognition by the search crew
An operating indication reports that the SART is switched on and on standby. A different visual or audible response indicates that radar pulses have triggered it. How these states are displayed depends on the model; the colour and flash pattern must be correctly interpreted on the available device. A response to a pulse confirms that the SART received that pulse. It does not yet establish that the search crew saw the response or recognised it as an emergency locating signal.
A search radar may be configured for navigation and collision avoidance in a way that makes the SART pattern difficult to see or suppresses it through signal processing. Range scale, gain, clutter controls and tuning affect the display. Settings suitable for finding a SART can simultaneously weaken or remove ordinary radar targets. The radar crew therefore has to decide on that change using its equipment and the traffic situation. The survivor cannot enforce the change from the SART. The immediate task remains to maintain the operating position, tether, upright mounting and unobstructed antenna; the pulse indication alone is no reason to end the locating operation.
The device consumes battery power even on standby; its responses require additional energy. Battery markings, previous use, damage and the operating indication are therefore separate clues to its condition. An expired battery date, cracked housing or unreadable controls need to be noticed when handling issued equipment. Repeated tests are not a way to improve recognition during an emergency. Some radar SARTs transmit the same recognisable response in test mode as during emergency operation; this consumes battery power and can falsely indicate distress to other vessels.
AIS-SARTs and EPIRBs use different transmission paths
An AIS-SART, or AIS Search and Rescue Transmitter, sends data messages through the AIS VHF radio link. An internal position source provides the position for these messages. A compatible AIS receiver with a suitable display can show the device as a distinct emergency locating transmitter. No radar pulse has to interrogate it. The radar SART's line of dots is not part of this method. Older displays may mark it differently; reception and correct interpretation of the AIS-SART message by the search crew are what matter.
An EPIRB instead transmits a coded maritime distress alert on 406 MHz. This passes through the Cospas-Sarsat satellite system, ground stations and alert processing to the responsible rescue coordination service. A radar SART does not establish that alerting path. When both devices are available, their roles complement each other: the EPIRB communicates the emergency, while the SART supplies a conspicuous locating pattern to a suitable radar within range. The radar response does not provide an ordinary voice-radio contact, satellite conversation or confirmation from the rescue coordination service.
Its role during military isolation at sea
A radar SART responds to suitable radar pulses rather than confirmed affiliation with friendly forces. A vessel with no search task can trigger it too. The response marks the location for a suitable receiver and does not authenticate either the survivor or the approaching forces. Under possible hostile threat, this has a direct consequence: the interrogation indication must not be read as proof of friendly contact. Identification and agreed contact procedures have to use the means actually available for that purpose.
The device is what is located first. If the SART and survivors drift apart, its pattern leads to the wrong place for recovering them. Securing the device against loss and keeping it with the people are therefore part of its locating function. When forces actually approach, they still have to identify the people and recover them in the prevailing sea and weather conditions. Another available signalling device or an established radio link can support this final transition; the SART conveys neither the number and condition of the people nor pickup instructions.