5.4 Search and Rescue Transponders (SARTs)

5.4 Search and Rescue Transponders (SARTs)

Quick Answer: A SART works in the 9 GHz band and is received by a vessel's X-band navigational radar. Switched on, it responds only when interrogated by a 9 GHz radar signal. Its reply paints 12 equally spaced dots on the radar display. Its audible tone alarm tells survivors assistance may be nearby. Range is maximised by holding it as high as possible.

Official sub-topic 1-D-22 (SARTs) is a six-question sub-topic with an unusually consistent answer set. Every item turns on the same physics—9 GHz, transponder, line of sight—so understanding the device once secures all six.

What a SART actually is

A Search and Rescue Transponder is not a beacon. An EPIRB (section 5.2) transmits on its own initiative and keeps transmitting. A SART is a transponder: it stays quiet until something interrogates it, then answers.

That split is the design logic of GMDSS: alerting and locating are different jobs done by different devices.

The 9 GHz choice

In which frequency band does a search and rescue transponder operate? 9 GHz. And: "This is a 9 GHz transponder capable of being received by a vessel's X-band navigational RADAR system."

Why 9 GHz? Because that is where X-band marine navigational radar already lives. Practically every vessel and SAR aircraft carries an X-band radar; a SART needs no new receiver anywhere, only an existing radar to interrogate it. Distractor bands—3 GHz S-band, 406 MHz, 2182 kHz—are all wrong, and 3 GHz is the sharpest trap because S-band radar is also common on larger ships. A SART does not respond to S-band radar.

The twelve dots

How should the signal from a Search and Rescue Radar Transponder appear on a RADAR display? A series of 12 equally spaced dots.

The mechanism is worth knowing because it makes the number memorable. When interrogated, the SART sweeps rapidly across the radar band twelve times. Each sweep passes through the radar's receiver passband once, painting one blip. Because the sweeps are evenly timed, the blips land at even range intervals, radiating outward from the SART's own position along its bearing line.

Two display behaviours follow, and both are useful in practice:

1. At longer range, the twelve dots appear as a clean line of separate blips leading away from the SART.
2. As you close in, the dots widen into arcs and eventually into concentric circles—a strong visual cue that you are nearly on top of it.

Distractors offering "a single solid range ring," "twenty continuous dashes," or "a figure-eight pattern" are all wrong. Twelve equally spaced dots.

The audible alarm

What is the purpose of the SART's audible tone alarm? It informs survivors that assistance may be nearby.

The alarm does not call for help and it does not signal the rescuer. It sounds when the SART is being interrogated, which means a radar is illuminating it, which means a vessel or aircraft with radar is within range. For people in a liferaft in poor visibility, that is enormous—it converts an invisible electronic process into a morale-sustaining, actionable signal: help is close, get your visual signals ready.

When does it transmit?

At what point does a SART begin transmitting? If it has been placed in the "on" position, it will respond when it has been interrogated by a 9-GHz RADAR signal.

Both conditions are required, and the answer states them in order:

1. It must be switched on — a SART in standby does nothing
2. It must be interrogated by a 9 GHz radar

This conserves battery. A SART left responding continuously would flatten itself long before a search unit arrived. Options claiming it "transmits continuously once activated," "begins on immersion in water," or "transmits every 50 seconds" describe EPIRB-like behaviour, not a transponder.

Maximising range

How can a SART's effective range be maximised? The SART should be held as high as possible.

At 9 GHz, propagation is essentially line of sight, so the limiting factor is the radar horizon between the SART's antenna and the searching radar's antenna. Every metre of height buys usable range. In practice:

• Mount the SART on its telescopic pole or lash it to the liferaft's highest point rather than leaving it on the floor
• A SART held roughly a metre above the water is typically detected by a ship's radar at around 5 nautical miles, and by a search aircraft at low altitude much farther, because the aircraft supplies the height on its end
• Nothing else the survivor can do—pointing, rotating, shielding—matters nearly as much as height

Exam trap summary