For a general introduction to AIS, Wikipedia is a good place to start.
The screen-dump below gives you a first feel for how AIS targets are presented and used in OpenCPN.

The Lookahead mode is activated here and our own vessel is proceeding 352 deg at 13.8 kts, leaving an activated track behind, and projecting a dashed line ahead representing the course. This ends in a dot, that represents where we will be in 6 minutes, or 1.38 nautical miles ahead. We are meeting a vessel, steaming in the opposite direction following the traffic separation. This target also has a line and a dot in-front of it representing course and where it will be in 6 minutes(user configurable), and a track behind it.. The two blue dots with a red/yellow line between them, represents where the vessels will be at CPA - closest point of approach, or when the are nearest each other, with present courses and speeds. The meeting vessel is red, as it is a potential danger to us. An AIS target alert dialog has just popped up.

There is a lot of information about the vessels that transmits AIS signals.

The cursor is on top of the the meeting vessel on a SW course. As we have not yet received full information, this can take a while, we don't get the name but an id number, the so called MMSI number, instead. Exactly what is shown in the yellow pop-up is configured in the AIS tabs Rollover heading.

This vessel is identified by name, and has a green color, as it does not represent any danger to our navigation.

AIS Target Query

What is an MMSI number?
Maritime Mobile Service Identifier (MMSI) is a number identifying a ship in communication. MMSI consists of nine digits. All VHF units using Digital Selective Call (DSC), are programmed with the vessels MMSI. The same applies for AIS and EPIRBs.

What is an IMO Number?
International Maritime Organization (IMO) numbers are a unique reference for ships and for registered ship owners and management companies. They were introduced under the SOLAS Convention to improve maritime safety and security and to reduce maritime fraud. For ships, the IMO number remains linked to the hull for its lifetime, regardless of a change in name, flag, or owner. ( from Wikipedia )

If we instead of just hovering the cursor on an object, double click it, the  dialog above pops up. A lot of relevant information is displayed. CPA, "Closest Point of Approach", to ownship is 12.7 miles in 12min 57s.
More information about the target itself, is available on Internet address:"MMSI number here", in this case:

Buttons in the Target Query Dialog

Create Waypoint drops a mark at the targets current position.

Record Track/ Stop Recording
Pressing the "Reckord Track" button brings up a new dialog

The recently recorded track of the "Poseidon Leader" will appear in the Route & Mark managers track tab on a line starting with "AIS Poseidon Leader.....".
If the "Yes" button is pressed, the Target Query Dialog button ""Record Track" changes to "Stop Recording".  All tracks recorded will behave just like any other track.

Right-clicking a target brings up a dialog focused on AIS.


Aids to Navigation AtoN

More and more aids to navigation are equipped with an ais transmitter, and shows up as a target in OpenCPN like this

Note the word "Real", that says that this is an object that exists in reality.

Here we have an AIS transmitter on a floating W Cardinal Mark.
This is of course real and the buoy is on position. If the buoy is off position, for some reason, the Aton icon becomes red.

There are also Virtual AtoNs marked with their own icon .
Virtual AtoNs are used to quickly make mariners aware of things like new wrecks, uncharted hazards and floating debris. They are normally used for a short duration until  replaced by real buoy, charted, removed or dispersed.

Virtual AtoNs  are also used for information. An example...the position of the Italian Maritime Rescue Coordinating Center. Note the word "Virtual" in the dialog.


OpenCPN handles the situation where MMSI is set to 0 (zero). This can occur with an incorrectly configured Aton. Nevertheless, all available information is shown.

AIS update rate.

How often are the AIS information updated from each target?
The required update rates from an AIS target transmitter is shown below.

Update Rates
    Class A Class B
Static Data   6min 6min
Dynamic Data < 3kn moored 3min  
  upto 2kn 10s 3min
  > 2kn   30s
  < 14kn (turning) 10s (3.33s)  
  < 23kn (turning) 6s (2s)  
  > 23kn 2s  


The AIS Target List

Another way to present the AIS information. The AIS target list is available through the right click menu.
Sorting targets
Click on the header in a column to sort according to this criteria. For example, clicking "Range" sorts the targets with the nearest target on top. Click one more time to get the target, furthest away on top. There is an "autosort" checkbox. When checked, auto sort is on. May be unchecked/rechecked any time. The autosort checkbox is automatically unchecked when target count exceeds 1000. However... It may be rechecked directly by the user, even if the target count is exceeded, and will stay checked.
Active Search And Rescue Target (SART) and Digital Selective Calling (DSC) distress targets, will always stay at the top of the list, even when sorting.

AIS targets  with Lat > +/-85 deg will show brg "unavailable". OpenCPN is not suited for Polar Navigation.
Target Info button: Click on a target to highlight it and then click the button  to display the information dialog.
Center View button: Highlight a target an click this button and the screen will center on the target.
Create WPT. Drop a waypoint at the current position of the target.
Hide All Tracks If tracking is on in Options->Ships->Ais Targets, all tracks will disappear from the screen. Read more furher down this page.
Show All Tracks Will reverse the previous option, and show all available tracks. If Options->Ships->Ais Targets is off, tracking will start for all targets.
Toggle Tracks

Note the first column in the list "Trk". This indicates the current status of tracking for each target.

AutoSort See "Sorting Targets" above, for an explanation of this box.
Limit Range: The list will only show targets with a range less than, or equal to this value. 
Target Count: Total number of AIS targets.

AIS Transponder and Ownship.

OpenCPN does not show the ownship (i.e. AIVDO message) neither as an AIS target, nor in the target list. AIVDO is processed only in the main navigation thread. 
Earlier releases did show ownship, but this was a bug, simply an illusion.
The earlier presence of AIVDO (ownship) information in the data stream, really only indicated  that the transponder was turned on and received GPS signals. The only true way to monitor the output of a transponder, is with a separate AIS receiver.

AIS data port

Select your AIS input port in the Connection Tab. The AIS button in the ToolBar, appears different according to the following conditions.
  AIS Alive. The default AIS button. This icon appears as soon as an AIS port is active in the tool-box. It is also the normal state with an AIS receiver working and with no targets of interest.

AIS Alive and Activity, a small green circle will illuminate in the lower right corner of the Ais buton. If there is at least one AIS message every 4 seconds, the light will remain illuminated.

AIS Suppressed. This icon indicates only that target suppression is activated in the AIS tab in the ToolBox. Referring to the picture above there are two sort of suppression:
* Suppress display of all moored/anchored targets, max speed specified in dialog.
* Suppress Alerts for moored/anchored targets.
This icon will be over-ridden with the AIS alarm button if an alarm is, or becomes, active according to the AIS tab alarms settings in the toolbox.

AIS Alarm. A target exist that raises the alarm as defined in the AIS tab in the ToolBox. It is the three settings "CPA Calculations", that rules if and when an alarm is raised.

AIS Disabled. No targets, if available, are shown on the screen. This button toggles with the other buttons. For example, if the AIS alarm button is visible, click the button and it will change to the "AIS Disabled" button, and all targets, alarms etc, disappears from the display. Click again and the original button and AIS display comes back.


The ToolBox AIS Tab



This tab controls the behavior of many thing in the AIS display. There are no "best" settings. It all depend on where you are, what kind of navigation you are into and what kind of vessel you are on. It is your responsibility to evaluate this.

CPA calculation: Rules for when the Closest Point of Approach- CPA (when two vessels are nearest each other) and the Time For Closest Point of Approach (TCPA) is calculated, and when warnings are activated. 
There are three tick boxes, which can be activated with user set values.

No CPA Calculation if target range is greater than (NMi)
If the vessel is far away it is less interesting to calculate CPA. In a busy waterway this could cut down on clutter and processing speed if a reasonable value is set.
Warn if CPA is less than(NMI)
Similar arguments to No 1. This controls when a warning for minimum CPA distance is given. A warning can be both on the screen and a sound. The CPA is shown on the screen when a warning is active.
..and TCPA is less than (min)
This Box can only be activated if the previous box is active. If the minimum CPA is far in the future, it's not so interesting. If it's hours away, both vessels have probably changed speed and course in the meantime. Set TCPA to a reasonable value value depending on circumstances. A lower value if negotiating a busy waterway. A higher value when offshore. Many factors plays a role here. If you need to get off watch crew up on deck for a manoeuvre, take this into account, for example. Do you find that you get to many useless alarms? Consider lowering this value and/or the CPA value.

If a target is within the distance set in #1 and  without an active warning, theCPA can be shown anyway through right-clicking on the target.

"Show Target CPA" toggles with "Hide Target CPA"

More About Target CPA, a few more tricks are available.

Lost Targets: Rules for how lost targets should be handled on the display.

Mark Target as lost after (min)
Target is regarded as lost if no transmission are received for the number of minutes set here, the target will change on the display and have a black bar across.
Remove lost targets after (min)
The lost target will be removed from the display after the number of minutes set here.

Display: If and how a vessels course and speed should be displayed and if stationary vessels should be on the screen.

Show target COG predictor arrow, length (min)
An arrow will be shown in front of the vessel representing it's COG (Course Over Ground). The length of this arrow and the dot at the end of it represents the calculated position in the number of minutes set here. Set easily calculated values. If a vessel approaches you doing 15 kts and the time is set to 6 minutes, the dot will be 1.5 nautical miles ahead of the vessel.

The width of the "Target COG predictor arrow" can be adjusted in the opencpn.ini configuration file. Find the line below and adjust the value.
Warning: Don't edit  to the opencpn.ini (opencpn.conf) file when OpenCPN is running.

To track or not to track?

This choice below boils down to two basic scenarios.

  • All targets will be tracked, with the option of turning off tracks for selected targets.
  • No targets will be tracked, with the option of starting to track selected targets
There are more about tracks in the section  "AIS Target List" above.
Note the first column in the Target list -> "Trk". This indicates the current status of tracking for each target.

Show target tracks, length (min)
This is the basic setting that can be refined in the "AIS Target list". With this box ticked targets are tracked even if they are not displayed. Start out with this option ticked, if you hesitate. Come back and fintune the settings later on, if necessary.

A track will be shown behind a target. This helps to judge the target vessels intentions.

When this option is ticked, the context sensitive AIS menu changes to include the toggle option "Hide Target Track" / "Show Target Track".

When this option is not ticked, the context sensitive AIS menu changes to include the toggle option "Show Target Track" / "Hide Target Track". A big difference, in this case, is that OpenCPN has no record of the track. The tracking will start from the moment "Show Target Track"  is clicked.

Hide anchored/moored targets, speed max (kn)
A target   will not be displayed if the speed is less than the value set here. There are two exceptions to this rule, Aids to Navigation (Aton) and "Vessels Not Under Command". The rule is aimed at catching anchored or moored  vessels. The "Navigational Staus" is set by the ships and may not be correct.
Be aware that the COG arrow will not be shown for any target with a COG less than this value, hence it's important to enter a very low value.

Show Area Notices(from AIS binary messages). More about AIS Area Messages here.

Show AIS targets real size. Zooming in on a target will eventually show a rectangle using the real size , if available. The picture below also shows how differences in course and heading are treated.

Show names with AIS targets at scale greater than 1:[select value]
With this box activated.....this is what you get!


Note that this setting also controls if a numerical value will be displayed alongside a tidal current arrow.

Treat WPL sentences as APRS position reports.
APRS (Automatic Packet Reporting System) is utilized by HAM radio operators around the world.The NMEA  WPL sentence is shorthand for for "waypoint location". If this box is ticked, a WPL sentence will be assumed to be an AIS-like APRS target.

Rollover: Tick the boxes for the kind of information shown when hovering with the cursor over a target. This is what you get with all the settings  ticked.

CPA/TCPA Alerts and DSC/SART Emergencies: When an alert is triggered, a jagged circle in red is drawn around the target that set off the alarm. Additional events can be set here, drawing more attention to the alarm. For practical navigation, make sure your alarm sound is set.

Show CPA/TCPA Alert Dialog
Shows an alert dialog on the screen with target information when an alarm is triggered. This setting only applies to AIS CPA/TCPA alerts.  A DSC/SART emergency will always generate a dialog.
Play Sound on CPA/TCPA Alert Dialog and DSC/SART emergencies
Sounds an alarm when an alarm is triggered.
Supress alerts for anchored /moored targets
Don't show alarms for anchored and moored targets.

Select Alert Sound. Select an alarm that suits you, instead of the installed default sound. This alarm will also be used if an anchor watch is set. Due to copyright issues, only a few sound files are included. The default location for these files are  "/usr/share/opencpn/sounds/" on Linux and typically C:\Program Files\OpenCPN\sounds on Windows XP. Users can add their own generic ".wav" sound files. These files can be in any suitable place in the file-system.

Test Alert Sound. Test the selected sound and make sure this important feature is working

Enable Target Alert Acknowledge timeout. Once a target alert is acknowledged, wait the set number of minutes before the alert comes up again (if still applicable).

The Toolbox MMSI Tab


Read all about this tab here Setting Options

When exactly is an alarm activated?

An alert is set off if your boat is approaching the CPA, the blue dot on the extension of the course line, and the previously discussed set of conditions are met. Once the distance to the CPA increases, this happens when the involved vessels has passed each other, the alarm stops, even if the other conditions still apply.

One user noticed an alert every few seconds, yet the target was still green.  The explanation is:
Variation between alert status and ship icon color is due to rapid changes in target course/speed. The target color can change from red to green while the alert dialog is showing. This may happen so fast that the red icon is not actually seen. OpenCPN does not automatically cancel the alert dialog in this case, in order to not mask information.


Types of Transponders recognized by OpenCPN
Class A
Class B
APRS                   Automatic Packet Reporting System Targets
ARPA                   Automatic Radar Plotting Aid Tagets
ATON                  Aid to Navigation, both "Virtual" and "Real".
BASE                   Base Station
DSC                    Digital Selective Calling Targets
GPSG_BUDDY      GpsGate Buddy Targets
SAR Aircraft         Aircraft participating in Search and Rescue.
SART                   Search and Rescue Transponder Targets

OpenCPN Remembers the Names of seen and fully identified target. The names are cached internally and helps Identify targets quicker on next start. Targets not yet fuly identified, were the name comes from the cache, are displayed lime green. See example below.
The AIS name caching can be turned off in the opencpn.ini/opencpn.config file by adding a line like this.

Vessel Names & Mmsi File  (New in Version 4.1.x ?)

This file links vessel Names with MMSI numbers so that the vessel Names can be displayed as soon as any data stream is picked up, rather than wait for the vessel name data to be transmitted  (requires a record that you have viewed that vessel at some point in the past).

The name of the file where the list of previous mmsi and ship names are stored is "mmsitoname.csv" in case you would like to clear it. This file is in the same folder as your configuration file (Click the ? icon in the toolbar to see the location, it is platform dependent).


Quick Reference for targets:
Note that AIS targets reporting position unavailable will be rendered in grey colour.

An active AIS SART and is a distress call equal to a "Mayday" transmission.
More about AIS SART.

Icon displayed when testing an AIS-SART device.

   Aircraft participating in Search and Rescue.

Potential Danger.

No Danger. Not Identified (Static voyage data not received)

No Danger. Not Identified (Name from cached data)

No Danger. Identified

Lost Target

Ship which has lost fix - position unavailable. Displayed at the last known position.

Vessel not under command.

Vessel restricted in ability to manoeuvre.

 Vessel constrained by draft.

  Vessel aground.

  Vessel engaged in fishing.

  High Speed- and Wing In Ground- crafts. This includes Hydrofoils, Hovercrafts and low flying crafts utilising the ground effect.

Anchored or moored. Displayed when the transmitted "Navigation status" is "at anchor" or "Moored". There is no guarantee that this status is correct, as it is set manually on the transmitting ship...

...illustrated by this ship. Note the black line on the yellow circle. This indicate that the vessel is turning to port (left), also illustrated by the lag in the display update. ROT - Rate Of Turn is available in the "Ais Target Query" dialog, through the right click menu.

The V-shape stern indicates a Class B target. Tugboats and pilot boats very often carry Class B transponders. These are often designed specifically for small commercial boats, fishing boats and pleasure crafts.

Targets is complying with the Euro AIS Inland specification.

This vessel has the  "Inland Blue Flag" set, it's a part of the European, Inland AIS standard. The "Blue Flag" signal, commonly seen on inland waters, indicates that the vessel requests a "stbd-stbd" passage or crossing. This Blue Signal is manually switched on/off, by the target.

Aton, Aid to Navigation, for example a Lighthouse or a Buoy with an AIS transmitter.  

Aton, Aid to Navigation, that is off it's suposed position. For example a buoy equiped with AIS that has come adrift.

Virtual Aton, Virtual Aid to Navigation, not a real marker. Can be useful for a range of situations. A new wreck, is one example. Further explanations.

Virtual Aton, off position. Actually seen in the wild, but may be a user config error.

AIS Base Station

The following tagets only displays if DSC messages, GpsGate mesages, Radar or APRS messages are mixed in to the incomming AIS stream, by using, for example a multiplexer. More on the following pages.

DSC Station. Only the DSC message received. The position contains only degrees and minutes of Latitude and Longitude.

 DSC Station. DSC and DSE messages received. The DSE message contains the missing decimals of minutes of Latitude and Longitude. The result is a much more accurate position.

DSC Station transmitting a distress signal. Treat this as a "Mayday" call.

GpsGate Buddy target.

ARPA Target

APRS Target

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