Search and Rescue Transponders
	Search And Rescue Transponders are electronic units which react to the emissions of X-band radars. Each time a SART detects a pulse from an X-band radar it transmits a signal which is displayed on the screen of the radar which activated it. This can greatly help a would-be rescuer to locate a liferaft. They can be thought of as 'active' radar reflectors as they electronically enhance the echo received by a radar. All compulsory GMDSS vessels up to 500 tons must carry at least one SART. Above 500 tons, they must carry two. Non-compulsory vessels are strongly advised to carry at least one to aid in any possible rescue.
	Operation of SARTs and Test Procedures
	How do they work? A SART has a receiver which scans for UHF signals between 9.2 and 9.5 GHz - the frequencies on which an X-band radar transmits its signal. As soon as the SART detects a signal it immediately transmits its own signal on the same frequency. This signal consists of a series of twelve pulses, and these are displayed on the screen of the radar as a series of twelve echoes with a gap of 0.6 miles between each of them. The first dot is at the position of the SART, with the remainder radiating in a straight line towards the edge of the screen. As the rescue vessel approaches the SART, the twelve dots each become short arcs. These arcs increase in size as the vessel gets closer, until the signal from the SART is permanently activated by the weakest side-lobes from the radar transmission. The signal from the SART becomes twelve concentric circles on the radar screen and this tells the would-be rescuers that they have more or less arrived. When a SART is switched on it will show a light to indicate that it is working. An approved SART should have sufficient power to operate in this stand-by mode for at least 96 hours. When it receives a signal from an X-band radar, and transmits its own signal, it will either flash this indicating light or in some cases a second light or even a buzzer. This will serve to let the distressed persons know that an approaching radar is activating the SART. If the survivors have a handheld VHF with them then this would be a good time to use it to try calling the approaching ship. Since the radar UHF signals can only effectively travel in a straight line, the distance from which a SART can be activated by a radar is dependent on its own height and the height of the interrogating radar scanner. Most SARTs have an extendible handle to help in positioning it as high as possible in the liferaft or lifeboat. The SART must be secured outside the canopy of the liferaft. Operating it from inside the liferaft will greatly reduce its effectiveness. The International Maritime Organisation stipulates that a SART mounted at a height of one metre must be detectable by a ship's radar with a scanner height of 15 metres at ,distance of at least 5 miles. It has been found from tests that a ship's radar will usually detect a SART laying flat on the floor of a liferaft at around 1.8 miles. If the SART is upright on the floor the detection range increases to about 2.5 miles. It should be possible, under most conditions, to mount the SART at least two metres high. A normal detection range for a SART mounted two metres above sea level by an average ship's radar is about seven to ten miles. However, a search aircraft equipped with an X-band radar should be able to detect it from at least 30 miles when flying at an altitude of around 3,000 feet. All SARTs should be checked on a weekly basis for any physical damage and for the expiration date of the battery. This is normally indicated on the manufacturer's plate affixed to the SART. It is permitted to check the operation of a SART by briefly turning it on and exposing it to the transmissions of the ship's radar. If this is done on board, then the radar screen will be flooded with the concentric circles, showing the proximity of the SART. Such tests should be conducted on a monthly basis and should be kept short so as not to shorten the life of the battery too much and to reduce the risk of other vessels seeing it, resulting in a false distress alert. Ideally, such tests should be conducted when there are no other vessels within radar range so as not to cause interference or false alarms. Vessels which are using their radar to look for a SART should use a range of 6 or 12 miles on the radar for optimum results. If a shorter range is selected, the narrower bandwidth used in the receiver will reduce the brightness of the dots making them harder to see. There is no point in using a longer range, since the maximum distance a SART will be detectable from another vessel is 7 to 10 miles. There are some SARTs which have a so-called anti-collision mode. When operated in this mode they transmit five pulses instead of the normal twelve. Such a unit may well help the radar operator on an approaching ship to see you, but there is a danger that it might be mistaken for the distress signal and the ship may possibly try to rescue you, even if you were not in distress. Such use of a SART is not encouraged. Under distress conditions though, there is no doubt that a SART is a valuable aid for any vessel to carry. It will greatly facilitate any search and rescue operation.
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