AIS

For a general introduction to AIS Wikipedia is a good place to start.
The screendump 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 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 1.3 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). 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.

A lot of information about the vessels that transmits AIS signals and are displayed on the chart, is available.
 

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 popup 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.

 

If we instead of just hovering the cursor on an object, right click, the menu item AIS Target Query brings up the dialog above. A lot of relevant information is displayed. TCPA is the time until CPA, how long before we are at the closest point to the other vessel.

 
All the bells and whistles are activated! A new target appears behind an island on our port bow, OpenCPN sounds an alarm and draws a jagged circle around the new target. An AIS alert box with all the information about the new target pops up. We can silence the noise but, has to acknowledge before the box disappears.

The AIS Target List

Another way to present the AIS information. The AIS target list is available through the right click menu.  Click on a 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.
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.
Target Info button: Click on a target to highlight it and then click the button  to display the information dialog.
Jump To button: Highlight a target an click this button and the screen will center on the target.
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.

 

 

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.

AIS data port: Select your AIS input port on your computer. When this is set to anything but "None", the AIS button appears in the ToolBar. This button 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.

CPA calculation: Rules for how and when the Closest Point of Approach- CPA (when two vessels are nearest each other) is calculated. There are three tick boxes, which can be activated with user set values.
1. 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.
2. Similar arguments to No 1. This controls when a warning for minimum CPA distance is given.
3. Similar to No 2 this controls the alarm based on a minimum time to CPA. A target is regarded as lost if regular transmissions is not received, updating the vessels status.

Lost Targets: Rules for how lost targets should be handled on the display.
1. 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.
2. 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.
1. 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.
2. A track will be shown behind a target. This helps to judge the target vessels intentions.



3. An anchored or moored target   will not be displayed if the speed is less than the value set here. This "avigational Staus" is set by the ships and may not be correct.
4. Show Area Notices. More about AIS messages here.

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: 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.
1. Shows an alert dialog on the screen with target information when an alarm is triggered.
2. Sounds an alarm when an alarm is triggered.
3. Don't show alarms for anchored and moored targets.

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

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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. 

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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 SART on this page.

Icon displayed when testing an AIS-SART device.

Potential Danger.

No Danger. Not Identified (Name not received)

No Danger. Identified

Lost Target

Ship which has lost fix - position unavailable not. Displayed at the last known position.

Anchored or moored. Displayed when the transmitted "Navigation status" is "at anchor". 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.

This blue bit is the European, Inland AIS standard, "Blue Flag" signal, commonly seen on inland waters to indicate that the vessel requests a "stbd-stbd" passage or crossing. This Blue Signal is manually switched on/off on 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.


AIS Base Statio

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

DSC Station

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

GpsGate Buddy target.