GNSS lost: fix by visual bearings

What you do on the bridge

1. Acknowledge the alarm
2. Three bearings, quickly
3. A good cut, and the fix in the ECDIS

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. You are in the approach and the ECDIS loses its GNSS position. What do you do?Often asked

   A good answer covers:

   • Acknowledge the alarm and note the time; the ECDIS is now running on dead reckoning
   • Check the other receiver and the antenna feed before deciding it is a real loss
   • Fix the ship at once by independent means: visual bearings, radar ranges, transits
   • Enter the fix in the ECDIS as a manual fix so the DR restarts from a true position
   • Shorten the fix interval, keep a good lookout, and call the master
   • Warn other users of the position, such as the AIS, DSC and the radar, that their input is now poor

   Show me on the bridgeBoth receivers stop a minute in. Watch the alarm and the position source change to DR.

2. How do you choose the marks for a three-bearing fix, and in what order do you take them?Often asked

   A good answer covers:

   • Charted, positively identified marks, ideally fixed lights or headlands rather than buoys
   • Spread so the position lines cross well: about 60 degrees apart for three, near 90 for two
   • Near marks rather than far ones, since an angular error moves the line less
   • Bearings ahead or astern first, as they change slowly; the one near the beam last, as it changes fastest
   • Take them quickly, note the time, and plot at once

   Show me on the bridgeFrom the starboard wing, say which marks you would use and why.

3. Your three lines make a cocked hat. What does it tell you, and what do you do with it?

   A good answer covers:

   • A small triangle is normal; it comes from reading errors and the time between bearings
   • A large one means a mark wrongly identified, a misread bearing or an unknown compass error
   • Take the ship to be at the corner nearest the danger, not the middle
   • Check the gyro error; a constant error turns all three lines the same way
   • If in doubt, take a fresh set of bearings and fix again
4. What other ways are there to fix the ship near the coast without GNSS?

   A good answer covers:

   • Radar ranges of well defined charted features, which are usually more accurate than bearings
   • Mixed fixes: a bearing and a range of the same or different marks
   • Transits, which need no compass at all
   • Running fix from one mark when only one is in sight
   • Parallel indexing on the radar to monitor the track between fixes
   • Soundings as a check, never as a fix on their own

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're inbound in the Ambrose Channel at six knots on the autopilot. About a minute in, both GNSS receivers will stop, and the ECDIS will fall back on dead reckoning.

   Dead reckoning drifts with every bit of tide and leeway, so we don't trust it for long. We'll acknowledge the alarm, go out to the wing, take three bearings quickly, and put a proper fix into the ECDIS.

2. Acknowledge the alarm

   When the positioning failure alarm sounds on the ECDIS, acknowledge it within a minute. Acknowledging doesn't fix anything: it tells the system, and you, that someone knows the position is now only estimated.

3. Three bearings, quickly

   Go to a wing, key 6 or 7, and stand at the repeater. Pick three different charted marks, aim with the binoculars, and press T for each one. All three need to be inside three minutes, or the ship has moved too far between them.

   Take the marks ahead or astern first, since they change slowly, and the one nearest the beam last.

4. A good cut, and the fix in the ECDIS

   Now look down at the ECDIS, open Sensors, and under Fix from bearings tick your three marks. The lines need to cross at thirty degrees or more, or a small error in one bearing slides the fix a long way.

   The fix should land within about 150 metres of where we really are. If the cocked hat is large, check you named the right marks, and take the set again.

The rules and guidance behind it

• MGN 379 (M+F) Amendment 1: use of electronic navigational aids