Display of AIS Targets: Vessels

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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Display of AIS Targets: Vessels

Postby jimh » Fri Jan 19, 2018 12:04 am

When a chartplotter is connected with an AIS receiver so the chartplotter receives data from the receiver about an AIS target, the chartplotter typically draws the target as an overlay of the electronic navigation chart display. The AIS transmission from a vessel will include various data, generally in two forms: static data about the ship, and dynamic data about its navigation.

Static Ship Data is sent in AIS Message 5, which also includes voyage related data; among the data sent are:

--vessel MMSI
--vessel IMO number
--vessel CALLSIGN
--vessel NAME
--vessel type and cargo type
--vessel overall dimensions and reference point for the reported position
--type of electronic position fixing device
--ETA at destination
--maximum present static draft

More details are found in the USCG NavCen website: https://www.navcen.uscg.gov/?pageName=AISMessagesAStatic

This data is only transmitted every SIX MINUTES. In order to receive this data, your AIS receiver has to have been receiving the transmission from the vessel for perhap as long as six minutes. If your receiver misses the transmission, it will take another six minutes for it to be sent again. For vessels whose AIS signals are weak and fading in and out, a receiver may miss this message data several times. (AIS Message 5 is sent only from CLASS-A AIS transponders. For vessels with a CLASS-B transponder, AIS Message 24 Part B Static Data Report includes similar data about ship dimensions and relative position of the GNSS sensor.)

An AIS target transmits more frequently AIS Messages giving position reports. AIS Messages 1, 2, or 3 (all AIS position report messages but sent in different circumstances) are sent as often as every two seconds (when moving rapidly), at 10-second intervals (when moving more slowly), and at three minute intervals when at anchor. The position report includes the following data:

--vessel MMSI
--vessel navigation status
--vessel rate of turn
--vessel speed over ground
--accuracy of position report
--vessel latitude
--vessel longitude
--vessel course over ground
--vessel true heading

It is important to note that when a vessel transmits its position, it is really sending the position of the antenna of its GNSS receiver or other sensor. For large vessels that might be 1,000-feet long and 150-feet wide, the location of the position sensor is significant to know. If the vessel has its navigation house at the stern, the GNSS antenna will typically be there, too. The position reported is thus associated with the location of the GNSS antenna; the bow of a large vessel with its GNSS antenna at the stern could be 900-feet away from the reported position.

The technical details of the AIS system are set out in:

Recommendation ITU-R M.1371-5 (02/2014)
Technical characteristics for an automatic identification system using time division multiple access in the VHF maritime mobile frequency band

That recommendation is silent on the topic of how an AIS vessel should be displayed on a chartplotter. In general, there seems to be two methods in use:

--the target vessel is drawn at its reported position with a generic icon that does not show any ship dimensions, or

--the target vessel is drawn as a ship outline in proportion of its reported length and breadth, relative to the reported position of the GNSS sensor on the ship.

These two general types of display are shown below:

The AIS target is drawn on this chartplotter as a generic icon without any ship dimensions being represented; this screen capture is from an Lowrance HDS chart plotter.

The same AIS data as a above, but drawn on an electronic chartplotter than can show the ship representation on the chart with its proportional dimensions; this screen capture is from PolarView NS, a chartplotter program that runs on a computer, rather than on dedicated marine electronic hardware.

In the above comparison, the same data was available to both chart plotters; the data was played out from a file of recorded AIS messages to each chartplotter. The difference in representation is due to differences in the chartplotter capabilities to display AIS targets.

It is important to remember that static data about the ship sent in AIS Message 5 is only transmitted at six minute intervals, and a chartplotter cannot draw the ship as a proportionally represented icon until it has received this data.

Posts: 6905
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula

Re: Display of AIS Targets: Vessels

Postby jimh » Fri Jan 19, 2018 12:11 am

It would be interesting to conduct in this thread a survey of other brands of chartplotter to demonstrate how they display AIS targets. If readers have screen captures of their chartplotter showing a CLASS-A AIS vessel target, please show us the display, indicate the brand and model of display, and indicate if the target had been in range for more than 15-minutes. (A period of 15-minutes of continuous reception of AIS data from a vessel would be a reasonable amount of time to expect the vessel to have transmitted its static data AIS Message 5.)

A Class-B AIS vessel should also be transmitting vessel static data, so a screen showing vessel dimensions for a Class-B AIS vessel should also be possible to capture.