AIS Overview

Class A Service for commercial vessels

The Automatic Identification System (AIS) is a maritime navigation safety communication system that enables ships to share their identity, position, speed, and other relevant data with other vessels and coastal authorities. It enhances collision avoidance and facilitates maritime traffic management. AIS relies on VHF radio frequencies to broadcast this information and thus ship to ship connections normally have an effective range of about 20 nautical miles (37 km)

The International Maritime Organization (IMO) requires any ship weighing 300 gross tonnes or more to be fitted with AIS equipment when on an international voyage. The regulation also applies to cargo ships of 500 gross tonnes or more involved in a local voyage and to all passenger ships, regardless of size.

The transmitters automatically broadcast information at regular intervals. Depending on its activity and status, a ship’s AIS system transmits navigational data every two to 180 seconds and voyage data every six minutes.

Class B Service for pleasure boats

Pleasure boats are not required to transmit their data to the AIS system but if they chose to do so they use the Class B system which is designed for smaller ships.

Collision Avoidance

AIS was designed as a collision avoidance system and has proved to be such a valuable tool that many (if not most) vessels that go to sea will carry an AIS receiver even if they do not transmit their own data to the AIS system.

Over time the AIS transmissions have also come to be used to feed a vessel tracking system with shore stations monitoring vessels AIS transmissions and placing the data onto the internet. Though, given the range limitations of VHF, using AIS in this way is not a complete solution.

navCenter concentrates on gathering and presenting AIS data to enable a navigator to assess and avoid a collision with another vessel or fixed station. Thus navCenter is primarily concerned with AIS targets near our Own Ship (OS) and displays its data using the "AIS Radar" format.

AIS Radar

Plotters commonly present AIS information overlayed onto a chart. This type of presentation can be very helpful when navigation in restricted waters.

However, when navigating in open water it is often much easier to interpret AIS data when it is presented in a format that is referred to as "AIS Radar".

With the AIS Radar presentation the receiving vessel, commonly referred to as OS (Own Ship) in the AIS system, is displayed in the centre of a screen with the AIS targets displayed at their bearing and distance from the OS. Exactly as a Radar displays its targets.

navCenter is able to process raw AIS data from NMEA2000, SeaTalkNG or NMEA0183 formats and presents the data in the "AIS Radar" format.

AIS Radar View

Our aim is to present the information a navigator needs to assess the potential for a collision with another vessel in the clearest possible manner. To this end we use a combination of shape and colour to enable a navigator to quickly focus on what vessels present the greatest risk and therefore need to be assesed first.

The following screenshot was taken from navCenter running on an iPad.

The View above is presented as "Course Up". In the bottom right corner we can see that the circular range ring is drawn at 2NM from our own ship (OS), in the top right hand corner we can see that our own course is 170° and our speed is 4.5 Kn.

There are 20 targets. Each target is placed at its relative bearing and distance from our OS and orientated to show its course relative to OS. A black or (white when in Dark mode) dot drawn inside a target indicates that the last transmission from that target is considered old based on user-defined settings. Data marked as such should be treated with caution as the vessel may now occupy a different position to that it indicated via its last transmission.

The dashed lines show the targets and our own COG (Course Over the Ground). The diamond at the end of a dashed line indicates the vessel's projected position in six minutes time. This presentation gives the navigator a 'rule of thumb' guide to the relative speed of their own boat and that of the targets around them and will be familiar to navigators who made use of the '6 minute rule' to calculate CPA before the advent of AIS.

6 Minute Rule

Prior to the introduction of AIS, 6 minutes was used as a standard time period between target plots because the distance traveled in 6 minutes is 1/10 of the speed. For example, if a vessel traveled 0.7 miles over 6 minute period of time that vessel speed is 7 knots.

The projected position lines may be switched off if you do not find them useful. However they are an immediate indicator as to whether or not a vessel is actually underway despite it transmitting it's status as "achored" or "moored". Unfortunately this is not an uncommon scenario; particularly with Type B transmissions.

Inspecting the AIS targets

There are a lot of targets but almost immediately we can see that:

• Two targets are drawn in red with a black border which indicates that they will cross our path with a Closest Point of Approach (CPA) of under one NM (this is the default distance but can be changed in the AIS Settings view. These targets we need to investigate ASAP.
• Three targets are drawn in orange which indicates that they will will cross our path but at a distance exceeding one nautical mile (NM). A Closest Point of Approach (CPA) of one NM is considered the minimum safe distance of separation. These targets we need to be aware of but are not considered dangerous.
• The remaining 16 targets can be dismissed as presenting no danger of a collision as they are drawn in outline which indicates that the target and our OS are moving away from each other.

The adjacent View is an extract from the earlier AIS Radar View showing the targets that require our immediate attention (those drawn in red with a black border). We can see that:

• The closest target is off our port bow and is heading away from our vessel on a course that will insure our paths do not cross.

  This Target is transmitting that it is underway (as it is not drawn with a 'moored' icon) but is not showing a projected position line. So we can deduce that this target is not moving at all or is moving very slowly.

  This is another benefit we get from drawing the projected position lines.

• The other target, off our port quarter, is heading towards our vessel and so we have tapped the target to display its minimum data screen and enabled "Watch" so that it's CPA is continuously calculated and drawn.

When a target is being watched its CPA is displayed using vectors.

• Our course and the target's courses are represented by the blue lines
• The blue triangles represent our own and the targets projected positions when the CPA is reached
• The orange (warning) or red (Danger) lines connecting these arrows represent the distance and bearing between the vessels at CPA.

From the vector diagram we can see that when the CPA is reached the target vessel will be passing our stern at a distance of less than 1 NM (CPA line is drawn in red). The detail View attached to the target shows that the distance between our vessel and the target at CPA will be 1,778 metres (depending on user-defined settings this distance may also be displayed as 0.96 NM), CPA will occur in 3 minutes 29 seconds when we will have travelled another 482 metres.

A detail View is attached to its target via a dashed line; the View itself is draggable and so can be moved around the screen as required. If a detail View is not shown within the current screen its dashed line will run off the edge of the screen. In this situation a tap on its associated target will bring the detail view back into view.

The icons in the View header perform the following functions:

• Tapping the 'X' will close the View; it Watch is switched on vecors will continue to be drawn for the target's CPA even with the View closed.
• Tapping the downward arrow will Display contact and voyage related data. E.G vessel MMSI, Name, destination etc.
• Tapping the rightward facing arrow will display more position related data for the target; COG, SOG, position etc.

The values shown in the detail view are:

• CPA - Closest Point of Approach between our Own Ship (OS) and the watched target vessel; here 1,778 metres
• TCPA - The time it will take for OS to reach that CPA; here 3 Minutes 29 Seconds
• DCPA - The distance between OS and the target vessel at CPA; here 482 Metres
• Age - The time passed since we last received any data from the target vessel; here 4 Minutes 51 Seconds

Selection From Radar View

The additional values shown on the right and bottom of the detail view are:

• BRG - Bearing of the target vessel from OS
• DST - Distance from OS to the Station
• COG - The Course Over the Ground of the target vessel
• SOG - The Speed Over the Ground of the target vessel
• ROT - The Rate of Turn of the target vessel. This needs further decoding but -VE values indicate turning to port and +VE values turning to starboard
• Lat - The Latitude of the target vessel
• Long - The Longitude of the target vessel

There is no limit on the number of targets that may be "Watched" simultaneously and "Watched" targets will continue to have their CPA vectors drawn even when their detail View is hidden.

AIS Key

AIS Reports

Tapping on the circle in the top right of the screen will display a View containing information on the data used to calculate CPA for targets and give access to several reports.

The first screenshot is showing a report of the time to CPA for each target in descending order; I.e. the targets are displayed in the order that we will encounter them.

The fourth target on the report is being watched. Watching a target can be switched on or off from this report as well as from the main AIS screen.

This target's report entry has been tapped to display much more detail about the target.

The second screenshot is showing a detail view for that same target which contains its MMSI and name along with the basic CPA data.

In order to avoid obscuring AIS stations any stations that are closer to the OS than the target station will float above the detail View. The detail View may be dragged to a different position on the screen if any such targets are obscuring required details.

AIS Menu

Tapping on the Menu icon on the bottom left corner of the View will display the AIS Menu.

Reading from left to right:

• Tapping on the menu icon will close the menu.
• Tapping the gear icon (tinted blue to indicate it is a link) will display the AIS Settings options.
• Tapping the next two buttons will, respectively, reduce or increase the Range for which AIS stations will be displayed.
• Tapping the icon on the far right will toggle the AIS View between Course Up and North Up.

On devices running iOS 16 or later the AIS Range can also be increased or decreased by using a "pinch" gesture. Pinching two fingers togeter will increase the Range while pinching two fingers apart will decrease the range.

AIS Settings

Reading the settings View from Top to bottom:

• CPA Basis. The default value is "Auto" this means if our Own Ship (OS) COG and SOG are available they will be used to calculate CPA for each AIS station. If COG and SOG are not available (or SOG is not above the threshold described below) CPA for each AIS station will be calculated using our Heading and Speed through the water.

  Thus, when we are at anchor, Heading and Speed will be used to calculate the relative position of AIS stations resulting in the stations being placed at the correct bearing to our own heading.

  As we get underway AIS calculations will switch over to using COG and SOG resulting in AIS stations being placed at the correct bearing to our course.

• SOG considered zero at. If an AIS station reports a SOG below this value it will be considered as zero by the AIS system.
• Vessel's MMSI. NEA2000 and SeaTalkNG systems will often carry information about our own boat. By entering our own MMSI here we can ensure that we do not calculate a CPA with ourself. This is not necessary when AIS data is arriving in NMEA0183 format but it will do no harm to enter your MMSI here.
• CPA for Type A. Defines the distance at which the Closest Point of Approach of this target type is to be regarded as potentially dangerous. Type A targets are normally large vessels.
• CPA for Type B. Defines the distance at which the Closest Point of Approach of this target type is to be regarded as potentially dangerous. Type B targets are usually smaller vessels.

• Refresh when Dangerous CPA. The number of seconds beween each refresh of the AIS Radar View when a station is present that is regarded as having a CPA that is potentially dangerous.
• Refresh when Warning CPA. The number of seconds beween each refresh of the AIS Radar View when a station is present that is regarded as a CPA that a navigator needs to be aware of.
• Refresh when no CPA. The number of seconds beween each refresh of the AIS Radar View when there are no stations present that we are approaching. I.e We are moving away from all AIS stations.
• Seconds to age station. If we have not received a transmission from an AIS station within this time period the station will be marked as aged.
• Minutes to age moored ship. If we have not received a transmission from an AIS station that is indicating it is a moored ship within this time period the station will be marked as aged.
• Minutes to delete station. If we have not received a transmission from an AIS station within this time period the station will be deleted from memory.
• Minimum ship speed. AIS stations that report their ship's SOG will be ignored if their SOG does not meet or exceed this value.
• Minimum ship length. AIS stations that report their ships length will be ignored if their length does not meet or exceed this value.
• Hide class B > distance. Class B AIS stations that meet or exceed this value will be ignored. Setting the value to 100 ensures that all Class B stations will be reported on. Setting the value to 0 will cause all Class B stations to be ignored.
• Show 6 minute positions. The position each vessel will occupy in 6 minutes time is calculated and displayed when this option is activated.

Next Steps for AIS

navCenter's AIS processing is under continuing development and we welcome feedback.

If you have suggestions on how we could, say, improve data presentation or if you spot a problem then please get in touch via our Contact us page.

We intend to release the following upgrades to AIS processing:

• Add the ability to obtain AIS data via the Internet
• Create a Zoom feature to utilise a device's entire screen for monitoring the CPA between our OS and an AIS station.
• Enable a navigator to model speed and course changes for OS and view the effect the changes would have on CPA for currently displayed AIS stations.