Maritime Monitoring - Mobility Data - page 225

Table 11.1 AIS Shipborne Mobile Equipment Reporting Intervals.

Ship's Dynamic Conditions - AIS Class A Freq.

Ship at anchor or moored and not moving faster than 3 knots 3 m

Ship at anchor or moored and moving faster than 3 knots 10 s

Speed between 0 and 14 knots 10 s

3 3 Speed between 0 and 14 knots and changing course s

Speed between 14 and 23 knots 6 s

Speed between 14 and 23 knots and changing course 2 s

Speed over 23 knots 2 s

Speed over 23 knots and changing course 2 s

Static : MMSI number (MaritimeMobile Service Identity: a unique ID), name,
type, International Maritime Organization code, call sign, dimension.
Dynamic : Position (longitude, latitude), time, speed, heading, course over
ground (COG), rate of turn (ROT), navigational status.
Trajectory-based : Destination, estimated time of arrival (ETA), draught, dan-
gerousness.

Quality of data is variable and depends, first, on the quality of the AIS device
itself and the way it implements algorithms and protocols. Therefore, data like
coordinates and speed can be more or less accurate. Longitude and latitude
are normally given in 1/10,000 minute that should give 0.18 m. However, con-
sidering this quality factor and intrinsic behaviour of GPS, the International
Maritime Organization only considers an accuracy of 10 m. The quality also
depends on people onboard. Indeed, some data, such as MMSI, name, destina-
tion, or navigational status, are manually set and possibly wrong. Contextual
information associated with geographic positions helps to understand ships'
behaviors according to space, time, destination, and ships' types although they
require error-detection and filtering processes.

Space and Time Gaps
Time is not part of position reports, as the AISwas initially designed for real-time
purpose only. Each received message has to be timestamped by the receiver's
clock. While it communicates on a regular basis, the automatic identification
system does not send these position reports continuously. Transponders broad-
cast data to surrounding listeners at different sampling rates according to ships'
behaviors. Table 11.1 presents sampling rates for AIS class A. Class B devices
behave in a similar way but at different sampling rates. This variation of time
intervals is very specific to the maritime domain and can vary from 2 seconds
for a fast-moving ship to several minutes when anchored.