What to check on a Radar SART.
A search and rescue transponder is a passive device: it carries no GPS, no distress message and no satellite link. It listens for the 9 GHz pulse of a nearby marine radar and, once triggered, retransmits a swept response that the searching set paints as a line of about twelve dots spaced roughly 0.6 nautical miles apart, walking outward from the transponder's true position toward the edge of the display. That pattern is unmistakable against normal sea and rain clutter, which is the whole point. It differs from an AIS-SART…
What sets a radar SART apart
A search and rescue transponder is a passive device: it carries no GPS, no distress message and no satellite link. It listens for the 9 GHz pulse of a nearby marine radar and, once triggered, retransmits a swept response that the searching set paints as a line of about twelve dots spaced roughly 0.6 nautical miles apart, walking outward from the transponder's true position toward the edge of the display. That pattern is unmistakable against normal sea and rain clutter, which is the whole point. It differs from an AIS-SART, which broadcasts its own position over VHF and needs no interrogating radar at all, and from an EPIRB, which alerts a satellite system and rescue coordination centre long before any ship is close enough to search. A radar SART only works in the final stage of a rescue, once a unit with a 9 GHz radar is already within its response range, typically several miles depending on the searching vessel's antenna height and the SART's own mounting height above the water.
Main components
- Transceiver — a combined receiver and low-power transmitter tuned to the marine X-band, switched into standby by a manual or automatic activation.
- Telescopic or fixed antenna — extended for use, raising the effective height and therefore the detection range.
- Battery pack — a sealed primary cell sized for standby plus active operating life, with a marked expiry date.
- Housing — a waterproof, buoyant case designed to float upright with the antenna clear of the water if it goes overboard on its own.
- Bracket — fitted inside a liferaft or on a lifeboat, holding the unit until it is manually released and hoisted on its pole.
Selecting and sizing a SART
There is little variation in operating principle between models, so selection comes down to endurance, mounting and serviceability rather than performance tuning.
| Criterion | What it decides |
|---|---|
| Battery endurance | Minimum 96 hours standby followed by 8 hours of continuous operation is the baseline most approvals are built around. |
| Mounting method | Fixed bracket on a lifeboat versus loose stowage inside a liferaft for hand deployment on the pole supplied with it. |
| Battery replacement type | Sealed unit replaced whole versus a serviceable pack, which affects who can carry out the periodic exchange. |
| Number carried | Set by vessel type and survival craft arrangement rather than by choice. |
Regulations and class requirements
GMDSS carriage requirements under SOLAS set out how many radar transponders (radar SART, AIS-SART, or a mix) a vessel must carry according to its size and trading area — cargo ships of 500 GT and above typically carry two, smaller cargo ships one, positioned so at least one is readily accessible for each side of the vessel. Devices must meet IMO performance standards for radar transponders, and the battery expiry date is checked at each safety equipment survey; a SART with an expired battery is treated as missing equipment, not as present-but-degraded.
Typical faults
| Fault | Consequence |
|---|---|
| Battery past expiry date | Unit may power on for a self-test but fail to sustain the full response duration during an actual search. |
| Corroded antenna contacts | Weak or intermittent response pattern, sometimes only visible at short range. |
| Bracket release mechanism seized | Crew cannot free the unit quickly when abandoning ship under pressure. |
| Stored with antenna collapsed and never extended in drills | Crew unfamiliar with deployment when it matters, wasting time in the water. |
What to look for in a supplier
- Valid type approval referencing current IMO performance standards for radar transponders.
- Battery service life that lines up with the liferaft's own service interval, so both are replaced on the same visit.
- Bracket and case compatible with the liferaft or lifeboat model already fitted, avoiding a retrofit of the mounting.
- Documented self-test procedure that a crew member can run without opening the sealed housing.
Never key a SART's self-test near port radar traffic for longer than the test tone lasts — the response pattern looks exactly like a real distress signal on any radar within range, and pilot stations have chased false alarms started by a well-meaning drill.