Skip to content

Electronic navigation equipment


AIS stands for (Automatic Indentification System). An AIS broadcasts information about the ship every few seconds, including name, identification number, position, course, speed and destination. The higher the speed, the more often the information is broadcast. The position, ground course and speed is derived from the GPS. The signal is broadcast to other vessels and shore stations with AIS via VHF frequencies 161.975MHz and 162.025MHz. Anyone with AIS can see the data of surrounding ships on the on-board computer or AIS screen. The data can also be found on the internet, just click on the map below to see all ships on the North Sea. Clicking on a ship will give you more information about it.



On some inland waterways and at sea on vessels over 300 GrossRegisterTon, AIS is mandatory and then consists of a GPS receiver, three VHF receivers and a 1 VHF transmitter. The three receivers are for DSC channel 70, AIS1 (channel 87) and AIS2 (channel 88). Via channel 70, a VTS can switch the AIS to another channel. An AIS class A has a repeater function. The AIS repeater receives AIS message data from vessels within its coverage area and retransmits the received information. Yachts usually have a Class B transponder that does not have a repeater function. The repeater function makes it possible to see vessels on AIS that are actually well outside VHF coverage.

Static and dynamic data

An AIS transponder transmits static and dynamic data. The dynamic data of a vessel are Vessel position, SOG/COG/Vessel speed/Course,ahead course, Status/Destination/ETA. These are displayed automatically because the AIS is linked to the GPS. Or they have to be entered manually such as destination port, ETA and draft. Static information is data about the vessel, entered in the AIS transponder, such as MMSI number, vessel name, vessel call sign, IMO number, vessel type and dimensions. Dynamic information is broadcast depending on speed and course change. Static information is broadcast every 6 minutes, when data has changed or on request.

AIS information

Features AIS

A very practical feature of AIS is that it can also calculate the CPA (Closest Point of Approach). This is the point when two ships are closest to each other. AIS can also calculate the TCPA (Time For Closest Point of Approach), which is the time when the two ships are closest to each other.

CPA with the AIS

Various alarms can also be set, for example if the CPA is less than a certain number of Nm, or if the TCPA is less than a certain number of minutes. These are the CPA/TCPA Alerts. The ROT (Rate of turn) indicates whether the vessel is turning or maintaining a fixed course. This is a very practical function, because in practice it is often not clear whether a seagoing vessel has noticed you or not and whether it is turning for you or maintaining a fixed course.

Safety-related messages (SRMs) are safety-related messages that can be sent via AIS to other ships or shore stations.

The Target COG predictor Arrow is line on the front of the target, indicating his COG (Course Over Ground). Its length corresponds to its speed and its direction indicates its COG. A Target track displays a “trace” at the back of the target, which can make it easier to predict his intentions.

The above functions are only displayed for selected ships, because otherwise too much would be demanded of the AIS’s calculation processor, making it slow to respond. Moreover, it is also not interesting to know what the CPA, TCPA, Target COG predictor Arrow or Track is, of a ship sailing away from you on the horizon.


Virtual buoys

On the AIS screen, you may also encounter virtual buoys. These are not actually in the water, but are still displayed on the screen. This allows you to place barrels in places where it is normally very difficult to place barrels.

Aids to Navigation (AtoN) transceivers

AtoN transceivers are affixed to buoys or other marine hazards to transmit information about their location to surrounding vessels.

AIS on electronic chart

Ideally, of course, you can connect the AIS to the electronic chartplotter or on-board computer. That way you can see the targets on the digital nautical chart and that gives a clear overview.

Electronic chartplotters

An ECDIS (Electronic Chart Display Information System) meets IMO requirements and is mandatory for seagoing vessels.
An ECS Electronic Chart Systems does not meet IMO requirements, but is used by recreational sailors.


Electronic cards

Raster charts or RNCs (Raster Navigational Charts) are scanned paper marine maps that are read into the software. Charts can be downloaded on the internet and no subscription is required.
Vector charts must be bought from a supplier. They consist of layers which allow certain elements to be omitted. BAZ updates are possible if you have a subscription.
ENCs (Electronic Navigational Charts) are vector charts issued by Hydrographic Service.

Vector maps have transverse mercator projection, which also uses a cylinder folded around the earth as an imaginary nautical map, so that on the inside the map can be projected. But in transverse mercator projection, this cylinder is horizontal.


The GPS is an electronic positioning system that works using satellites. The satellites have global coverage. Because the speed of the signal from the satellite to the GPS antenna is known and because the departure time of the signal is included, a circle can be drawn on earth for each satellite on which the ship is located. If at least three satellites are used, the position can be determined because the intersection of the three circles is the position. DGPS is more accurate because fixed beacons on earth transmit a correction by radio signal. The antenna is best placed on the ship at a low point on deck (there the antenna oscillates the least).


Most GPS devices can display the following:

  • COG: course over ground, ground course
  • SOG: speed over ground, ground speed
  • Waypoint a position you can program into the GPS.
  • Sailplan: a set of waypoints located on your trip to be sailed
  • BTW: bearing to waypoint, bearing to waypoint
  • DTW: distance to waypoint
  • TTG: time to go, time to go before you reach the next waypoint.
  • ETA: estimated time of arrival
  • DTG: distance to go, distance to the next waypoint
  • MOB: double press and the GPS gives the course and distance to where the drowning person fell overboard.
  • XTE: cross track error, distance perpendicular to the straight line between the last and the next waypoint.
  • VMG: stands for velocity made good.
  • VMG ground is the ground speed.
  • VMG wind is the speed straight into the wind when you are cruising upwind or straight into the wind when you are cruising downwind.
  • VMW waypoint is the speed straight to the waypoint.

GPS errors

The main error factors of the GPS are:

  • Multipath: reflection of the signal causing it to arrive at the antenna via a diversion, making the calculated distance to the satellite incorrect.
  • HDOP: Horizontal Dillution of precision: satellites too low on the horizon so the signal has travelled a long way through our atmosphere. This makes the calculated position unreliable.
  • PDOP: Position Dillution of precision: unfavourable mutual positions of satellites making the intersection angle of the circles on Earth too small and thus too unreliable.
  • The GPS works with a mathematical model of the Earth. We call these ellipsoids. Examples are WGS-84 (used since 2003) and ED 50.
  • UCB: user clock bias. The clock built into the GPS is never quite precise, so the calculation of distance to satellites is never quite accurate.

Depth sounder

The depth sounder works with a sound signal that bounces off the bottom and is received back. Inclination can cause a distorted picture, as the distance between boat and bottom appears greater.

depth gauge


The log measures speed through the water by means of a small wheel rotating under the boat. It also measures total log position, maximum speed, average speed and distance travelled in a day (trip).


At the following URL, you can download and install a free Grib reader and use it to view grib files:


On-board computers

Onboard computers are almost impossible to imagine sailors without. They are a good addition to “old-fashioned” navigation as long as the navigator knows what the limitations and risks are and takes them into account when using them. Many sailors do not know the limitations. Sometimes it seems that precisely because of this, more accidents occur because equipment is misinterpreted, than all that modern equipment actually prevents accidents. Nevertheless, I am a great believer in using technology wisely on board.

Limitations of electronic nautical charts

However fast technology develops, it will never be able to 100% replace a good navigator with common sense. Below are some key limitations and risks that will probably always apply:

  1. Electricity and (sea) water will never go well together.
  2. Power on board will always have limited availability.
  3. GPS signals can become disrupted or temporarily lost.
  4. The screens/displays will never be as large as a paper nautical chart, so the overview will always be less.
  5. One of the main risks of chart plotters and electronic nautical charts is that it is possible to zoom in, even if the detail map of the appropriate scale is missing. Sometimes overview maps but no detail maps are installed. As a result, important details are not displayed when zooming in, without being noticed.
  6. The opposite is also very dangerous. Because screens are often small, there is a tendency to zoom out to get a bit more of an overview. But when you zoom out, details fall away, sometimes including depth figures of, say, a small shallow patch of water.
  7. Digital data is often presented so accurately that you would almost think it is reality. The ship’s position constantly projected on the electronic chart, for example, gives a misplaced sense of security. As a result, old-fashioned navigation quickly becomes threadbare which can lead to accidents if the equipment fails. Moreover, greater risks are taken because of this false safety. For example, on a number of occasions I have witnessed sailors relying more on the position on the electronic chart plotter to stay in the channel, rather than simply steering between the buoys (see photo below, where in reality we did sail in the concrete channel). I saw the same principle when I sailed in a non-paved channel between the sandbanks off Zeeland. The GPS chartplotter indicated that we were sailing on a sandbank, but it was clear from the breakers that the sandbanks had moved and that we were sailing safely in deep water. In both cases, it was better to just look around at the buoys and water and use the depth sounder, rather than using the electronic sea chart and relying entirely on it. In these cases, the digital chart was different from reality, even though the charts had been updated recently. Keep in mind that there is always quite some time, between a change in reality and the change in the chart, no matter how often updates (BAZs) come out. Bear in mind that it can take weeks or months before the hydrographic service (or another ship) sees and reports that, for example, a sandbank or buoy has moved and this can be incorporated into an update (BAZ).
  8. Often, the automatically calculated route is not the safest or most efficient. For example, plotters do not sufficiently take into account staying clear of dangerous rocks.
  9. The COG is often less effective than a straight course through the water.
Chartplotter not up to date

Questions & Answers

Question 1: COG: Course Over Ground corresponds to:

a: Compass course
b: Ground course
c: True course

Question 2: The deviation of a hand-bearing compass is:

a: Similar to steering compass
b: To be found in the deviation table
c: Unknown, so assumed as 0

Question 3: A bearing on the steering compass includes the deviation of:

a: Compass course
b: Compass Bearing
c: Magnetic Bearing

Question 4: The log indicates the speed:

a: On the chart
b: Over the ground
c: Through the water

Question 5: Beating against the wind the apparent wind, relative to the true wind is:

a: Stronger
b: Less
c: Equal

Question 6: Sailing with the wind, the apparent wind, relative to the true wind, is:

a: Stronger
b: Less
c: Equal

Question 7: BTW indicates:

a: Distance
b: Bearing
c: Arrival time

Question 8: A high value of the HDOP means:

a: Accurate Position
b: Inaccurate position
c: Good range of satellites

Question 9: We sail straight against the current. The SOG relative to the log speed is:

a: Higher
b: Less
c: Equal

Question 10: The GPS antenna is best placed:

a: In the masthead
b: On deck
c: In the cabin