Communications and Electronic Navigation Systems
This paper tests GMDSS carriage and distress procedures plus the electronic aids used for navigation and collision avoidance: ECDIS, radar and ARPA, AIS, GNSS, VDR and LRIT.
GMDSS sea areas
- A1: within VHF coast-station coverage, usually with DSC on VHF channel 70.
- A2: within MF coverage, excluding A1.
- A3: within Inmarsat coverage, roughly 70°N to 70°S, excluding A1 and A2.
- A4: outside all of the above, needing HF. Standard equipment: VHF DSC, MF/HF DSC or Inmarsat, 406 MHz EPIRB, 9 GHz SART, NAVTEX receiver (518 kHz international, 490 kHz national), two-way radiotelephone for survival craft.
Distress procedures
A DSC distress alert carries the MMSI and nature of distress, sent on channel 70, followed by a MAYDAY on channel 16. The EPIRB is activated manually if time allows; the SART is switched on in the liferaft and shows as a line of dots on a rescuing vessel's radar.
Electronic navigation
ECDIS must use official ENC and correct datum and safety settings; ARPA gives CPA and TCPA and requires correct vector and guard-zone setup; AIS supplements radar but does not replace it; VDR and LRIT are mandatory on applicable ships.
Worked example
Radar antenna height 15 m, SART height 1.2 m. Radar horizon = 2.21 × (√15 + √1.2) = 2.21 × (3.87 + 1.10) = 2.21 × 4.97 = about 11 nm. So the SART should appear at roughly 11 nautical miles in good conditions, and any significantly shorter detection range suggests antenna height, shadowing or receiver problems.
Traps
- Sending a distress alert on channel 16 by voice instead of DSC on channel 70 first.
- Assuming AIS targets are all correctly updated; small craft may not carry it.
- Leaving ECDIS on a datum other than WGS 84.
- Forgetting NAVTEX 490 kHz is used for national-language broadcasts.