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Annex 3 201
In summary, the data do not fulfil the assumption of four reports per day exactly. However, for the most part,
the assumption that the data includes one LRIT position report every six hours per ship is met reasonably well.
To quantify the latter point, the fraction of time intervals between consecutive LRIT messages in the range
from five to seven hours is shown for each ship category in Figure 15: the majority of LRIT reports are recorded
at a frequency of about one every six hours, with most consistency in 2012.
Figure 15: Fraction of time intervals between consecutive messages
that fall between five and seven hours for each ship type and size category 1
The key point is that coverage of LRIT position reports is largely independent of coverage of AIS reports.
Therefore, the LRIT data can shed light on the validity or otherwise of the days at sea and speed profiles
estimated from extrapolated AIS data.
The LRIT data contain six parameters: a unique ship identification number (Ref_ID) (generated in the merging
and anonymization process with fleet technical data), a time stamp, speed, draught, region and organization
source identification. In this section, Ref_ID, time and speed are the only variables used. The original, raw LRIT
data contain geographical location. That information has been stripped out of the data used for this report and
replaced with the speed, which is calculated as the great circle distance between the geographical locations
given in the LRIT report and in the consecutive one, divided by the time difference between the reports. For
a ship travelling at constant speed over the open oceans, the resulting speed value is accurate. For a ship that
changes its course within the time interval between consecutive position reports, its speed is underestimated.
In order to compare ship activity estimated from AIS and LRIT data, respectively, the ships appearing in the
LRIT data are matched to the AIS data and to entries in the ship fleet database, from which the ship category
to which they belong is determined. Table 12 shows how many of the ships identified in the LRIT data are also
found in both the AIS data and the ship database.
Table 12 – AIS to LRIT ship mapping (number of ships)
2009 2010 2011 2012
LRIT ships 6,441 8,716 8,127 8,838
LRIT ships matched to AIS 6,402 8,640 7,261 8,776
Three-way matches (LRIT, AIS and IHS ship parameters) 5,283 8,562 7,261 7,322
1 1–6: bulk carrier; 7: combination carrier; 8–11: chemical tanker; 12–19: container; 20–22: general cargo; 23–25: liquefied gas
tanker; 26–33: oil tanker; 34: other liquids tanker; 35–36: ferry – pax only; 37–41: cruise; 42–43: ferry – ro-pax; 44: refrigerated cargo;
45–46: ro-ro; 47–48: vehicle; 49: yacht; 50: service – tug; 51: miscellaneous – fishing; 52: offshore; 53: miscellaneous – other; 54:
service – other.