1.3 Search and Rescue Transponders (SART) & AIS-SART

1.3 Search and Rescue Transponders (SART) & AIS-SART

Quick Answer: A Search and Rescue Transponder (SART) is a battery-powered survival craft homing device designed to guide rescue vessels directly to survivors in poor visibility, darkness, or rough seas. Traditional Radar SARTs respond to interrogation sweeps from 9 GHz X-band ship radars by generating a distinctive 12-dot radial line on the radar display. Modern AIS-SARTs utilize Automatic Identification System frequencies (AIS 1 & AIS 2) and built-in GPS to broadcast high-precision position data directly onto navigation displays (ECDIS) as a specialized distress target.

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Technical Principles of the 9 GHz Radar SART

A Radar SART remains in a low-power standby mode until it receives a radar pulse from a searching vessel or aircraft operating on the 9 GHz X-band radar frequency (9.2 to 9.5 GHz / 3 cm wavelength).

CRITICAL EXAM FACT: Radar SARTs respond ONLY to 9 GHz X-band marine radar. They do NOT react to or display on 3 GHz S-band marine radar (10 cm wavelength) used for long-range precipitation penetration.

The Radar Display Response Sequence

When an X-band radar beam sweeps across a active SART, the transponder immediately triggers and sweeps back across the 9.2–9.5 GHz band. This transmission creates a unique graphic display on the searching vessel's radar PPI (Plan Position Indicator) screen:

1. Long Range (>1 Nautical Mile): The SART presents a series of 12 equally spaced dots (blips) aligned along a straight line extending 8 nautical miles outwards from the SART's location away from the center of the radar screen. The first dot closest to the radar center represents the actual position of the survival craft.
2. Close Range (<1 Nautical Mile): As the searching vessel closes to within approximately 1 nautical mile, radar receiver saturation and delay response cause the 12 dots to widen into 12 concentric arcs. When extremely close, these arcs expand into full concentric circles surrounding the searching vessel, confirming immediate proximity to the life raft.

Radar SART Display States:
  Distance > 1 nm:  [Survival Craft] ● ● ● ● ● ● ● ● ● ● ● ●  (12 Dots extending 8 nm outward)
  Distance < 1 nm:  [Survival Craft] ( ( ( ( ( ( ( ( ( ( ( (  (12 Concentric Arcs / Rings)

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Radar SART vs. AIS-SART: Technical Comparison

With the expansion of digital marine electronics, the IMO authorized AIS-SARTs as an acceptable alternative to traditional 9 GHz Radar SARTs under SOLAS regulations.

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The Impact of Antenna Mounting Height on Detection Range

Because both X-band radar and VHF AIS signals travel via line-of-sight, the maximum range at which a searching vessel can detect a SART depends heavily on the mounting height of the SART above the water and the height of the searching radar or VHF antenna.

Mathematical Horizon Range Calculation

[ D = 1.23 \times (\sqrt{h_{radar/vhf}} + \sqrt{h_{sart}}) ]

Where:

• (D) = Maximum detection range in nautical miles
• (h_{radar/vhf}) = Search vessel antenna height in feet
• (h_{sart}) = SART mounting height in feet

Worked Exam Scenario: SART Mounting Height Comparison

Case A (Correct Mounting): A survivor inside a life raft extends the telescopic support pole, securing the SART at a height of 3.3 feet (1 meter) above sea level. The searching vessel has a radar scanner mounted 49 feet high.

[ D = 1.23 \times (\sqrt{49} + \sqrt{3.3}) = 1.23 \times (7 + 1.81) = 1.23 \times 8.81 = 10.83\text{ nm} ]

Case B (Incorrect Mounting): The survivor leaves the SART floating in the water attached by a lanyard, resulting in an effective height of only 0.33 feet (0.1 meter).

[ D = 1.23 \times (\sqrt{49} + \sqrt{0.33}) = 1.23 \times (7 + 0.57) = 1.23 \times 7.57 = 9.31\text{ nm} ]

Furthermore, sea swell and wave attenuation can block a low-floating SART completely, reducing detection range to under 2 nautical miles! This demonstrates why SART poles must be erected immediately upon entering a survival craft.

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Battery Requirements, Operational Testing & Audible Indicators

Battery Performance Specs

GMDSS regulations mandate that all SART batteries must maintain sufficient capacity to operate in:

• Standby Mode: Minimum of 96 hours (monitoring for interrogation pulses).
• Active Transmitting Mode: Minimum of 8 hours continuous interrogation response while being painted by a radar.
• Battery Expiration: Lithium battery packs have a operational shelf life of 5 years and must be replaced prior to the printed expiration date.

Operational Testing Procedure

To perform a routine inspection without causing false distress alarms to nearby ships:

1. Switch the SART selector switch to the TEST position.
2. Observe the visual indicator LED and listen for the built-in audible beeper.
3. Check the vessel's own 9 GHz X-band radar screen (set to 6 nm or 12 nm range scale). The radar screen should display concentric test rings or concentric dots.
4. Release the switch back to OFF or STANDBY immediately upon verification.

Visual and Audible Feedback in Survival Mode

When deployed in a real emergency, a SART provides feedback to survivors inside the life raft. When interrogated by a passing ship's radar, the SART flashes an LED light and emits an audible tone. This alerts survivors that a searching vessel is nearby and painted the transponder, boosting morale and alerting crew to prepare pyrotechnics.