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Lesson 12 Electronic equipment


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 time when the signal was transmitted, a circle can be drawn on the earth on which the ship is located. If at least 3 circles like this are drawn, 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 per radio signal. The antenna is best placed on the ship on deck.


Most GPS devices give the following information:

  • COG: course over ground
  • SOG: speed over ground
  • Waypoint
  • Sailplan: route of waypoints
  • BTW: bearing to waypoint
  • DTW: distance to waypoint
  • TTG: time to go to the next waypoint
  • ETA: estimated time of arrival
  • DTG: distance to go
  • MOB: marks the location if a person falls overboard.
  • XTE: cross track error, the distance the vessel is to one side of the straight line between two waypoints.
  • VMG: velocity made good.
  • VMG wind: Speed straight into the wind while beating or with the wind while running.
  • VMG waypoint: Speed directly to the next waypoint.

GPS errors

The most important errors of the GPS are:

  • Multipath: the signal doesn't reach the antenna directly but hits another object first.
  • HDOP: Horizontal Dillution of precision: satellites are low above the horizon so the signal needs to travel a long way through the atmosphere.
  • PDOP: Position Dillution of precision: The satellites are not positioned well because they are not devided over the horizon.
  • The GPS uses a model of the earth. This needs to be the same model as is used in the nautical charts. For example WGS-84 or ED 50.
  • UCB: user clock bias. The clock in the GPS is never exactly the same as in the satellite.

Elektronic chartplotters

ECDIS (Electronic Chart Display Information System) meets the IMO requirements and so this system is for professional ships.
An ECS (Electronic Chart Systems) does not meet the IMO requirements, however is used a lot by pleasure crafts.


Electronic charts

RNC's (Raster Navigational Charts) are scanned paper charts that can be viewed in the software program. The disadvantage is that changes in the chart cannot be altered easily.
Vectorcharts exist of layers so certain elements could be left out in case we zoom out. The chart can be updated automatically if you have a paid subscription. Vectorcharts use transversal mercator projection. This also uses a cylinder that is folded around the earth as an imaginary sea chart, so that the map can be projected on the inside. But with transversal mercator projection, this cylinder is horizontal and thus touches 2 meridians that are in line with each other.
ENC's (Electronic Navigational Charts) are the vectorcharts published by the Hydrographic Service of the Royal Netherlands Navy.

Depth sounder

The Depth sounder measures the dept because the seabed reflects the eccho back to the transducer.

dept log

Boat speed log

The boat speed log measures the speed through the water with a transducer. Besides that it measures the distance you have sailed through the water that day (trip), the maximum speed and the average speed.

Below is for Day Skipper only


On the following URL you can download a Grib reader and install it to view gribfiles on your PC: http://www.zygrib.org



AIS stands for (Automatic Identification System). An AIS transmits information about the ship every few seconds, with name, identification number, position, course, speed and destination. The position, course and speed is derived from the GPS. The signal is transmitted via VHF to other ships and shore stations with AIS. Anyone with AIS can view the details of surrounding ships on the on-board computer or the AIS screen or AIS could also be integrated on the electronic chart plotter. The AIS information can also be found on the internet, just click on the map below so that you can see all ships on the North Sea. By clicking on a ship you get more information from the ship.


Static- and dynamic information

An AIS transponder transmits static and dynamic data. The dynamic data of a ship are position, speed over ground (SOG) and course over ground (COG). These are automatically displayed because the AIS is connected to the GPS. Static data are data about the vessel entered in the AIS transceiver, such as the MMSI number, the name of the ship, the call sign of the ship, the type of ship and the dimensions.

Functions AIS

A very practical function of the AIS is that it can also calculate the CPA (Closest Point of Approach). That is the point when two ships approach each other.
AIS can also calculate the TCPA (Time For Closest Point or Approach), which is the time when the two ships approach each other.

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.
T he ROT (Rate of turn) indicates whether the ship is turning or maintaining a fixed course. This is a very practical function, because it is sometimes not clear whether a seagoing vessel has noticed you and whether it turns away from you or keeps a fixed course.

Safety Related Messages (SRMs) are messages that can be sent via AIS to other ships or coast stations.

The Target COG predictor Arrow is an arrow at the front of the target, indicating its COG (Course Over Ground). The length corresponds to its speed and the direction indicates its COG.
A Target track displays a "track" at the back of the target, making it easier to predict its intentions.

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

ais radar overlay

Virtual marks

On the AIS screen you can also find virtual buoys. This offers the possibility to place tons in places where it is normally would be very difficult. For example, in places where the bottom is steep or where is a stong tidal rate, it will normally be difficult to place real buoys.

AIS on electronic chartplotter

The easyest is when the AIS is integrated with your electronic chartplotter or board computer. In this way you will see the targets on the digital nautical chart. OpenCpn has this option, see https://opencpn.org/



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Questions & explanations

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Question 1: COG is ...: 


a: Compass course

b: Ground course

c: True course

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


a: the same as the deviation of the steering compass

b: always 0

c: unknown, so we assume it is 0.

Question 3: When we make a bearing on the steering compass, we lookup the deviation at the:


a: compass course

b: compass bearing

c: magnetic bearing

Question 4: The speed log gives the speed ....:


a: in the chart

b: over the ground

c: through the water

Question 5: If we are beating against the wind the apparent wind, compared to the true wind is....:


a: stronger

b: less

c: equal

Question 6: If we are running with the wind the apparent wind is ... compared to the true wind. 


a: stronger

b: less

c: equal

Question 7: The meaning of BTW is:


a: distance to the next waypoint

b: a bearing to the next waypoint

c: the time we will arrive at the next waypoint

Question 8: A high HDOP value means:


a: the fix is accurate

b: the fix is not accurate

c: good signal

Question 9: We sail into the direction of the tidal current. The SOG compared to the speed log is:


a: higher

b: less

c: equal

Question 10: The GPS antenna is best placed on:


a: the top of the mast

b: the deck

c: the cabin

About Elmar Teeuwen

Elmar is a CWO sailing instructor and RYA Yachtmaster® Offshore, Commercial Endorsement with STCW95 and certified coastal navigation teacher.
He has more than a decade of experience with teaching sailors on the North Sea, Atlantic, Med and Caribbean from beginners to instructor level, as well as regatta teams.
In addition to his native language Dutch, Elmar speaks English, German and Italian.
His knowledge and teaching skills could be off great value to anyone who wants to realise his sailing dreams.