AIS

AIS (Automatic Identification System) is a system of VHF transponders carried by commercial ships and some smaller vessels. It works by sending and receiving vessel speed, heading, position, rate of turn, and identification data over VHF radio. This can then be plotted by Memory-Map on the digital chart. Add in your own GPS information and you'll see all ship movements relative to your position and be alerted of a potential collision or near miss.

• See ships plotted on the chart.

• Audible collision alert.

• Find the ship's name, radio call-sign, size, type of ship, size, rate of turn, destination port and ETA.

• Velocity vector shows ship's course and speed over ground on the chart.

• Table showing all ships sorted by closest point of approach.

• Ships are color coded to show status and possible collision

• Shows class A (commercial ships) and class B (voluntary) AIS transponders.

• Receives AIVDO (Own Ship) sentences, or GPRMC for your own location, speed and direction

• Compatible with Pilot Plug data for commercial ship pilots.

AIS targets displayed on the chart

The AIS targets are shown on the chart as animated icons. Each vessel is shown as a triangle, oriented according to its compass heading.

Each moving target has a velocity vector that shows its course and speed over the ground. All the velocity vectors are drawn on the same time scale, so you can judge the relative speed of different targets. You can determine, for example whether you will pass ahead or astern of a ship by looking at the heads of the velocity vectors. You can change the timescale in the GPS Settings dialog.

The ship colors are as follows:

• Targets with status "Moored" or "Anchored" are shown in grey.

• Targets with a status "Restricted Maneuverability", "Constrained by draft", "Fishing" or "Aground" are shown in purple.

• Targets "Under way using motor" are green.

• Targets "Under way Sailing" are blue.

• Targets on a collision course are red.

• Targets nearly on collision course are yellow.

• If no transmission has been heard from a target in the last 4 minutes, it is white.

Tapping on a target shows its information at the top of the map screen, including the name of the ship. If the signal has only just been acquired, the app will not yet have received the "static information" broadcast by the ship, and it is identified only by its MMSI. The position, speed and direction of the ship is transmitted frequently (depending on the class of transponder and the speed of the ship) but the name of the ship, its size and destination port is only broadcast every few minutes. If you want to be able to call the ship by name on the radio, it is recommended to leave the Android device running continuously. Use the background recording mode if the device will be turned off, but be aware that battery life will be affected.

AIS Target List

Tap Menu, Overlays, and select the AIS tab to view a list of active AIS targets. If the AIS tab is not present, that means no AIS signals have been received. Targets are sorted in two groups, those getting closer to you (taking into account your own motion) are sorted by the predicted closest approach distance. Those moving away from you are sorted by straight distance.

Tap on an item in the list to view it on the map.

Collision Alarm Notifications

The software uses the course and speed of each target and your own course and speed to calculate your closest point of approach. If this is less than a given threshold, a warning is given. When you are on a collision course, a device notification is sent and the collision targets are shown in red.

Targets near the threshold are shown in yellow.

You can adjust the closest point of approach threshold, in terms of time and distance in the AIS Settings menu. (Note this menu item is hidden until the first item of AIS data is received. If you want to explore the settings at home, use a NMEA simulation data file containing AIS data). Targets that will pass closer than the specified distance, and within the specified time, are considered dangerous. These cause the Notification and red target color. The time and distance thresholds are both doubled for the yellow warning targets.

Note that the size of each ship (LOA) is added to the threshold, so you will still get some alarms if you set the distance to zero. If you want to disable the collision detection feature, set the time to zero instead, or just disable the alarm.

You can adjust the alarm threshold for Class B transponders separately from Class A. Typically, you would be comfortable with a closer crossing with a pleasure craft than class A ship.

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