WAYPOINT - Satellites and seamanship

George Shaw from the Royal Institute of Navigation looks at how good seamanship, backed up by excellent technology can change the entire situation when it comes to SAR operations.

Search and rescue operations demand precision navigation, systematic observation and coordination with MRCC and cooperating vessels, raising unfamiliar challenges that seafarers face only on rare occasions. Vessels work together to carry out rigorous search procedures made possible through integrating ECDIS integration with technological aids. Satellite services providing position navigation and timing (PNT) data, along with ship-toship and shore messaging are crucial to success.

SAR in action

The Search Mission Coordinator, based at the MRCC, will determine the point at which the search begins – the Commence Search Position (CSP) and appoint the Master of a nearby vessel as the On-scene Coordinator (OSC). The CSP is based on the casualty’s last reported position, usually taken from GNSS, and adjusted for the effects of wind and tide. The COSPAS-SARSAT satellite communication system, supported by Inmarsat (MEO) and Iridium (LEO), provides target GPSbased locations to within 100 metres. These may be sourced from longrange tracking (LRIT), emergency beacons (EPIRB/PLB 406MHz transmissions) or AIS messages, such as man overboard alerts. GMDSS emergency alerts via digital selective calling also communicate GPS-based target locations.

GNSS signal interference, such as jamming or spoofing, can degrade CSP data by disturbing or denying the position. If reliable GNSS data is not available COSPAS-SARSAT satellite signals’ timing and Doppler can be processed gradually over 45-60 minutes to triangulate the casualty’s approximate location to around 3nm.

This technique was used during the arduous search for aircraft MH370 in the South Indian Ocean in 2014, where there were no GNSS positions available. The aircraft’s track was estimated to within an ocean swathe 200nm wide, following weeks of innovative postprocessing of just eight infrequent glimpses of Inmarsat ‘pings’. The absence of GNSS can greatly expand the search area, but it does not mean search is impossible.

Search types

MRCC determines the search type and area, depending on the levels of uncertainty. Sectorial search patterns, for example an expanding square, move with the water, using only a compass to establish heading and timing for leg lengths – and ignoring speed over ground (SoG) and course over ground (CoG) indications.

An area search such as a parallel sweep can be instigated over a radius accounting for CSP uncertainty and variations in tidal and leeway drift, with legs navigated using GPS (position, CoG and SoG). Coordinated legs of multiple vessels are separated by a sweep width, set by MRCC and OSC, to accommodate wind, sea and visibility conditions and type of casualty.

Good seamanship is essential to adhere strictly to the procedural patterns shown on ECDIS, coordinating turns with other vessels and maintaining careful observation. Once close to the targets, homing signals from rescue beacons can help pinpoint the location. Radar-triggered SART transponders may also provide highly visible radar display indications of the target’s position.

Satellites in synergy

Satellites play a vital role in SAR operations, with GPS also useful in monitoring completeness of coverage. Yet the threat of GPS disruption at sea is becoming increasingly ubiquitous. This once again reinforces the critical role played by mariners’ skills and vigilance, producing the synergy of technology and seamanship essential for successful rescues.