2.14 Electronic Navigation: GPS, Chartplotters, Depth Sounder & AIS

Electronic Navigation: GPS, Chartplotters, Depth Sounder and AIS

Modern electronics make navigation easier, but the exam - and good seamanship - treats them as aids that support, never replace, the chart, the compass, and a proper lookout. Every one of them can fail, lose power, or mislead.

GPS

The Global Positioning System uses a constellation of orbiting satellites to fix your position anywhere on earth. A GPS receiver reports:

• Position as latitude and longitude,
• COG (Course Over Ground) - the direction you are actually tracking, and
• SOG (Speed Over Ground) - your actual speed across the bottom.

WAAS (a satellite-based augmentation) sharpens accuracy to a few meters. Two cautions the test likes:

• COG and SOG are over-the-ground values - they already include the effect of current and leeway. COG is therefore not the same as your compass heading; in a cross-current your heading and your COG differ, and confusing the two is a classic error.
• GPS computes positions on the WGS-84 datum. Using a chart drawn on a different datum can offset your plotted position by a boat length or more, so match the chart datum to the receiver.

Accuracy also degrades when satellites are low or clustered (poor geometry, reported as a high HDOP) or when signals bounce off structures (multipath), and the position updates only every second or so, so at speed the displayed dot always lags your true position slightly. A steady, unchanging position on a moving boat is a warning that the receiver has lost its fix and is showing the last one.

Chartplotter

A chartplotter overlays the GPS position on an electronic chart (vector ENC or raster RNC) and lets you build waypoints and routes, watch cross-track error (XTE) and an arrival alarm, and drop a man-overboard mark at the push of a button. Cross-track error is simply how far, and to which side, you have drifted off the straight line between two waypoints - a current setting you sideways shows up as a steadily growing XTE, telling you to steer back toward the line. It commonly integrates AIS and radar on one screen. Its limits: the electronic chart can be out of date, wrongly scaled, or lower resolution than it looks, and a guard-zone or shallow-water alarm is only as good as the underlying data. A plotter that loses GPS or power leaves you blind, so keep a paper chart and know your position independently.

Depth sounder (echo sounder)

The depth sounder measures depth by timing a sound pulse to the bottom and back. You must know where it measures from - the display can read depth below the transducer, below the keel, or below the waterline depending on how the offset is set, and misreading that offset has grounded many boats. It tells you the water beneath you, not ahead of you, so it warns of a shoal only after you are over it. The pulse spreads in a cone, so the reading is really the shallowest point within that cone, and soft mud, weed, or a thermocline can give a false or double return. Used well, comparing sounder readings against charted depths is an excellent independent check on your position - a line of soundings that matches the chart confirms where you are.

AIS

The Automatic Identification System broadcasts a vessel's identity (MMSI/name), position, course and speed over VHF, and displays other vessels as targets, often with a computed CPA (closest point of approach) and TCPA (time to CPA):

• Class A transponders are carried by large commercial and SOLAS vessels - mandatory.
• Class B transponders are the smaller, lower-power sets used voluntarily on recreational and small commercial craft.

AIS is powerful for identifying a ship and calling it by name to arrange a passing, but it has a hard limitation: not every vessel carries AIS - many small craft, sailboats, and some fishing vessels are invisible to it, and a target's data is only as good as what that vessel transmits. AIS is never a substitute for radar, a proper lookout, or the Rules of the Road.

The governing principle

Electronics assist; they do not command. Rule 5 still requires a proper lookout by sight and hearing, and Rule 7 warns against basing action on scanty information - including scanty electronic information. A prudent captain cross-checks the GPS and plotter against the chart, the compass, the depth sounder, and above all his own eyes, and carries a backup: a paper chart, a handheld GPS, and a magnetic compass that needs no batteries.