MnrAq

202 Third IMO GHG Study 2014
Every LRIT report is labelled “at sea” if the stored speed value is greater than or equal to 3.0 knots. If the
speed value is below 3.0 knots, the LRIT report is labelled “in port”. The same criteria are applied to the
corresponding AIS data. The AIS data are in the format of hourly messages that include a reliability flag, set
to 1 if the AIS data at that time are reliable and to 0 if they rely more heavily on the extrapolation algorithm.
To investigate any bias that may be introduced by accounting for the cases of low AIS coverage, time spent
at sea according to LRIT data is compared to time spent at sea according to AIS data, for ships that are more
or less well observed in the AIS data. To this end, for each ship in each year, the parameter of AIS coverage is
defined as the ratio of AIS messages with reliability of 1 to all AIS messages.
The plots in Figure 16 show the comparisons over each year for the estimates of days at sea. Perfect agreement
would result in a value of 0 for the mean difference in days at sea. For 2010 to 2012, with good AIS coverage,
there is convergence between LRIT and AIS days at sea, with a slightly higher value for AIS. However, in each
year, as AIS coverage deteriorates, AIS underestimates the number of days at sea compared to LRIT for all years.
For the comparison in 2009, it should be noted that the extrapolation algorithm applies a correction factor
to the AIS data in order to attempt to correct for the expectation of bias when shore-based AIS data is used.
For the comparison shown in Figure 16, the correction factor used is derived from the days at sea reported
in the Second IMO GHG Study 2009 for the year 2007. The poor quality observed in that comparison,
showing that AIS consistently overestimates days at sea relative to the LRIT data, reflects the inadequacy of the
assumption that Second IMO GHG Study 2009 data (for 2007) is representative of the activity of shipping in
2009. Whether this is because the Second IMO GHG Study 2009 data is inaccurate, or cannot be assumed
approximately constant over the period 2007–2009, cannot be identified. However, following observation of
the poor quality of the starting assumption, the assumptions were revised, and the extrapolation algorithm
uses the LRIT data to calibrate observed days at sea in 2009, rather than the Second IMO GHG Study
2009 data, and this definition is provided in Annex 1. This assumption is tested in the uncertainty section to
determine the effect on final results.