AIS targets

The essentials

AIS (Automatic Identification System) is the system through which vessels broadcast their identity, position, course and speed over VHF. NavimetriX displays these vessels, the targets, and calculates for each one the CPA (the minimum distance at which it will pass you) and the TCPA (the time remaining before this closest point of approach).

Targets arrive in two ways:

• through your AIS receiver or transponder, over the NMEA link (!AIVDM sentences): Premium subscription, “NMEA” switch on (see NMEA 0183 connection);
• over the Internet, without any equipment: see Internet AIS and sharing.

To see them:

1. In the right-hand “Settings” drawer, “AIS” section, check that “Display AIS targets” is on (it is by default). The “AIS targets” box in the layers menu does the same thing.
2. Tap a target to select it: its name (or its MMSI, the vessel’s identification number) and its average speeds are displayed, along with its track.
3. Long-press a target to open its detailed sheet.
4. Open the “AIS” tab of the left drawer for the list of all targets.

Scaled vessel silhouettes, at high zoom

Going further

Display settings

• Name and speed: a label next to each target. Handy offshore, cluttered near the coast.
• Lost targets: a target that stops sending messages gradually fades, then disappears after the set delay.
• COG vector: a line in the direction of the target’s course over ground, whose length represents the distance covered in that time.
• Track: the recent path of the targets, for all of them or only for those you have selected.
• Class B: class B transponders mostly equip pleasure craft, class A commercial ships. Hiding them declutters the chart in a crowded race start.

CPA and TCPA alarms

The CPA (Closest Point of Approach) is the predicted minimum distance of approach between two vessels. The TCPA (Time to CPA) is the time remaining before that point.

A target goes into alarm when both conditions are met for 15 seconds in a row: its CPA is below the threshold and so is its TCPA (the closest approach is still to come). The target’s icon turns red (a different icon in color-blind mode). “Sound alarm” adds a sound. “Draw CPA line” draws on the chart the point where the target and your boat will be at the moment of closest approach.

Collision alarms do not exist in the browser version, which cannot keep watch in the background. The other navigation alarms are described in Navigation alarms.

On the chart

• Tapping a target selects it (a circle surrounds it) and displays its track; tapping again deselects it.
• Long press: selection and detailed sheet.
• At high zoom, when a vessel’s real length exceeds a few millimeters on the screen, its silhouette is drawn to scale and in its orientation, from the dimensions it broadcasts: you can see where it really is relative to its antenna.

Icons

A vessel that also broadcasts weather appears in light blue; a base station or an aid to navigation that broadcasts weather appears as a cyan square. In-situ weather stations are described in In-situ stations and weather boats.

The detailed sheet

A long press opens the target’s sheet. Depending on what the vessel broadcasts, it contains:

• the MMSI, flag, name, navigation status (underway, at anchor, fishing…), position;
• the instantaneous speed over ground and the speed averaged over 5 and 20 minutes, the course over ground and the heading;
• when your boat’s position is known: the distance and bearing, the CPA and TCPA, and whether you will cross ahead of or astern of the target;
• the age of the last position;
• additional information: call sign, IMO number, vessel type, length, beam, draft, destination;
• for a target that broadcasts weather: wind, gusts, pressure, temperatures, current;
• a link to the vessel’s page on VesselFinder (Internet).

The sheet can be moved, and switches to two columns in landscape.

The AIS tab list

The “AIS” tab of the left drawer lists the targets. At the top, the “Targets” / “Fleet” selector: the fleet compares selected boats with yours (see AIS fleets).

Each row gives the MMSI and name, the instantaneous speed over ground and over 5 and 20 minutes, the course over ground, the distance, the bearing, the CPA and the TCPA. A colored bar on the left summarizes the status:

• “Sort”: “by risk ↓” (default: targets with the lowest CPA and TCPA first), “by MMSI”, “by name”, “by CPA”, “by TCPA”, “by distance”, “by sog ↓”, “by sog5mn ↓”, “by sog20mn ↓”. Sorting by average speed is very useful in racing.
• Filter: the “Filter by name, MMSI, alarm or class (A/B)” field accepts part of a name, digits of the MMSI, “A” or “B” for the class, and “alarm” to keep only the targets in alarm.
• Tapping a row centers the chart on the target and selects it.
• The flag on the right of a row adds the target to the active fleet, or removes it.

See also

• Internet AIS and sharing
• Multiple links and position source (AIS source)
• AIS fleets
• Navigation alarms
• FAQ: How to display AIS targets?
• FAQ: What do the icons displayed on the chart represent?