Question 1

An examiner asks: where would you find published details of DGPS (Differential GPS) reference station coverage and frequencies?

Accepted Answer

DGPS reference station details — including locations, coverage areas, broadcast frequencies, and identification codes — are published in the Admiralty List of Radio Signals (ALRS), specifically the volume covering radio aids to navigation, which lists DGPS (and other radio beacon) services by region alongside other radio navigation and communication services This is a straightforward "know your publications" question of the type the syllabus frequently tests — the examiner is checking that the candidate knows the Admiralty List of Radio Signals (ALRS) is the standard reference for radio-based navigational aids and communication services, including DGPS reference stations (which transmit correction signals to improve the accuracy of standard GPS positions by compensating for certain systematic errors). The ALRS is published in several volumes covering different categories of radio services (coast radio stations, differential GPS and other radio navigation aids, weather services, etc.), and a competent navigator should know which volume to consult for a given type of information rather than needing to look it up from scratch each time. While DGPS has become less critical for many vessels with the improvement of standard GNSS accuracy and the availability of satellite-based augmentation systems, it remains part of the navigational publications syllabus, and "where would you find information about X" questions of this type are a recurring and easy-to-prepare-for category — the key is being able to confidently name the correct publication (ALRS) rather than guessing at an unrelated one.

Question 2

You're preparing a passage plan for a coastal leg in a yacht drawing 3.2m. At one point the chart shows a least depth of 3.8m at chart datum. What MUST you check before finalising this leg?

Accepted Answer

The tidal height at the time of transit to confirm sufficient under-keel clearance Charted depths are referenced to chart datum (often LAT), so you must add the predicted tidal height for your transit time to get the actual depth, then compare to your draft plus a safety margin to confirm UKC.

Question 3

During your watch the ECDIS gives a "safety contour crossing" alarm. What is the correct response?

Accepted Answer

Assess the situation against the planned route and charted depths, take action to maintain safe water, and inform the Master if there is any doubt A safety contour alarm indicates the vessel may be approaching water shallower than the set safety contour. It must be investigated against the route and charted information, with avoiding action taken and the Master informed as appropriate — never simply silenced.