Search And Rescue Transponder (SART) - How Does It Work?

Search And Rescue Transponder (SART) – How Does It Work?

Last Updated on October 11, 2022 by Amit Abhishek

Search And Rescue Transponder or SART in short is an integral part of ships GMDSS ( Global Maritime Distress and Safety System ) requirement under SOLAS after EPIRB.

A search and rescue transponder is a self-contained, waterproof transponder that when actuated during emergency reacts to the emission of a radar to send its current location.

It is both a vital and mandatory ( All GMDSS vessels up to 500 ton must carry one SART ) device on board ship, consists of 3 basic equipment i.e a powerful battery, omni–directional radar receiver and its transmitter.

They are designed to be compact and easy to use, are used / fitted on ships / vessels, life raft , boats and survival crafts. Once activated they can last for at least 96 h.

Normally mounted in a bulkhead bracket of the mother ship they can be carried in one hand to the liferaft when abandoning the ship and mounted on the canopy of the liferaft using the telescopic pole.

Search And Rescue Transponder (SART): Purpose, Requirement & Use

The main purpose of search and rescue transponder (SART), is to receive and respond to the radar signals from aircraft or ship ( equipped with X-band radar ) with a response (homing) signals.

These response or homing signal when seen from ships or aircraft radar will be indicated as a line of 12 dots 0.64 n miles apart with the first dot shows the exact point of the SART device.

These sequence of dots on an X band-radar help the rescue team to easily recognize and locate the survival craft tracking the source of distress signal from the SART device.

Under Global Maritime Distress and Safety System ( GMDSS ), all passenger ships are required to carry at least 2 SART device. Similarly cargo ships up to 500 ton must carry one SART.

While Cargo ships above 500 tons much carry two SART device. Similarly all life rafts much also include / have one SART, further there is also specific battery requirement for these transponders.

For example; the battery should be able to operate under most extreme conditions i.e between -20°C to 55°C. Further, it should be able to operate continuously for 8 hrs and at-least 96 hrs on standby.

Search And Rescue Transponder (SART) - How Does It Work?
Image: SART General Assembly | Credit:

SART – General Features, Location & Functioning

  • Made of waterproof reinforced plastic it can withstand extreme weather condition and prolonged sun exposure.
  • The SART device is generally orange in color ( internationally Accepted standard ), but a few times can also be seen in yellow color.
  • It is made as such it can float freely of the mother ship or survival craft.
  • SART operates in the 9 GHz (3 cm or ‘X-band’) radar frequency band and does nor responds to or show on S-band radar.
  • It is mounted / installed on the ship’s bulkhead near bridge using a fixed support or mounting bracket.
  • SART can only be activated manually ( ON Position ) after break the security tab. Thus only respond when actuated under distress; the rotary switch will auto reset to off from test position once released.
  • Regular testing is advised but should be limited to very short period. Further, nearby ships should be informed in advance before testing.
  • The device can either be used as portable device or mounted on survival craft.
  • When activated the device will flash red light every 2 seconds under standby mode and will sound buzzer every 2 seconds with continues red light when actively transponding.
  • When activated it will send a distress signal that will show on the radar as 12 consecutive dots. As you approach the source the dots will start to get wider and form an arch.

How Do You Activate SART On Ship?

Search And Rescue Transponder (SART) is intended for use only in distress or emergency condition. But they are also to be inspected and tested ( 30 seconds ) during annual survey.

To activate / operate the SART you first need to lift and remove it from the bulkhead bracket ( mounting point ). Then break the safety or security tag away from the body.

In some design you need to pull front lanyard to break safety tab while other designs have different methods to do so ( check manual ). Now to activate rotate the switch ring to ON position.

There are no operational differences between TEST and ON modes; it is just that when set to ON mode it will remain activated while you need to keep the rotating switch at TEST mode during the test interval.

As once released when on Test Mode the rotating switch will automatically preset to OFF position. SART devices are tested annually for a period of 30 seconds to insure they are working properly.

We generally check whether it responds to the radar and show as intended on the radar display on ship. Further we look out for intended sounds ( buzzer ) and signals ( red light ) for 30 seconds.

If it does not respond to the radar or do not flash red light every 2 seconds in standby mode / gives audible beeper each 2 seconds with red light when transponding the TEST is considered a failure.

Effective SART Range ( Maximum Detection range )
Image: Effective SART Range ( Maximum Detection range )

Effective Range

The Search and Rescue Transponder ( SART ) is considered to be a line of sight device much like the VHF radio. Which means it won’t provide greater coverage beyond visible range.

On activation the SART will provide visible indication on radar screen of the searching ship or aircraft. When interrogated by a X-band radar placed at 15 m on ship, it should respond when interrogated up to 8 N-miles.

When mounted at the height of 1 meter from sea level its effective range is just 2.1 N-M TO 2.3 N-M ( here N-M representing nautical mile ) when searched from sea level.

Similarly, when mounted at a height of 1.5 meters it has an range of a little more than 3.1 miles.

Luckily most vessels radars are mounted at more than 10 m or 15 m to be exact for merchant ships. This in fact increase the effective maximum detection range of these SART device.

So when interrogated by a X-band radar placed on an aircraft it has and effective range of more than 30 N-m.

Maintenance and Service Requirements

  • Ensure all crew members knows how it works.
  • Crew should be aware where to mount, how to mount and test the device.
  • Battery should be replaced every 2 to 5 years.
  • Do a visual inspection of the device each week.
  • Before long passage or once a month activate and test the SART for audio visual signals as well as intended results on radar transponder.
  • Make sure it is placed in a way / operated that it is not accidentally activated.
  • Ensure the safety lock is in place and the seal is not broken.
  • When activated accidentally your first response should be to switch off SART immediately and send DSC Safety Alert on VHF CH 70.
  • Also transmit a safety broadcast by RT on VHF Channel 16 to all stations indicating a mistake and you wish to cancel the false alert with your ID, ships info and position.
  • In case the SART fails in inspection or testing ( or is damaged ) send it back to the manufacturer or concerned authority ( most of the time authorized service agent ).

Location Errors ( Things To Consider )

There are inherent delay in SART responses. When interrogated by a X-band radar the SART sweeps through the entire X-band range for radar signal before locking onto the frequency.

You can understand it in a similar way your old car radio search the entire F.M frequency band before locking onto the station frequency.

Such sweeping of frequencies is required by default in SART operation because, all marine radars operate at a different frequencies in the X-band radar range.

This thus results in a delay when at larger distance of 6 to 8 nautical mile, such delay will show its location 150m off its actual position on radar screen.

Difference Between EPIRB and SART?

While most mariners know exactly what are the difference between EPIRB and SART is but many people do not. After all both are safety equipment used in distress assisting search and rescue.

EPIRB or Emergency Position Indicating Radio Beacons is a safety equipment that sends distress signal to the search and rescue coordinators via Cospas-Sarsat satellite network.

They basically sends distress beacon / signal containing encrypted identification number ( which holds information such as imo no, GPS data / ships location, ships name, date of event and mmsi no ) to the nearby shore stations with the help of satellite in the form of hexadecimal code.

On the other hand SART or Search And Rescue Transponder beams back radar signals when interrogated by a X-band radar, thus showing its location / position on the radar screen.

While SART only works in visual range ( the higher the source of radar longer the detection range ); EPIRB can operate in beyond visual range sending distress signal under any condition.

EPIRB data received on shore station is used in the initial rescue initiative while SART provide quick identification for nearby passing vessels or can be used at later stage of rescue operation.


The AIS-SART or Automatic Identification System Search And Rescue Transponder is a self contained radio device that transmit AIS messages containing location, static and safety information of the distressed vessel.

The AIS-SART system derive its data on ships position and time from its built in GNSS receiver ( GPS ). AIS stations on receiving the AIS-SART signal results in a alert on the system screen.

Global Maritime Distress Safety System (GMDSS) require one or more search and resue device in the form of an traditional SART device or Automatic Identification System Search And Rescue Transponder ( AIS-SART).

Unlike traditional SART device that works on radar frequency and can be seen on radar screen of any vessel or installation in range with X-Band radar, AIS-SART can only be detected by AIS installations.

An AIS-SART is designed to be used / deployed in a similar way as any traditional SART device. They need to be mounted at a height of 1 meter on the survival craft.

Also Read:
Do You Know We Write Post On Your Request?

Request your own Topic !

Leave a Comment

Your email address will not be published. Required fields are marked *