Third IMO Greenhouse Gas Study 2014

80 Third IMO GHG Study 2014
during the period of this study. The trend for the smaller ship types is for increased coverage but the average
total in-service coverage remains 40% and lower for the smallest general cargo carriers and bulk carriers.
Figure 59: Total percentage of in-service time for which high-reliability activity estimates
are available from AIS
However, for the purposes of a high-quality inventory, it is more important for the quality of the AIS coverage
for the ship types and sizes with the greatest share of emissions to be high. Since the coverage statistics of the
highest contributing CO 2 emitters (i.e. the largest ship types and sizes) are also the highest, this is generally the
case. Figure 60 displays the CO 2 emissions weighted average of the percentage of total in-service coverage.
This is decomposed into two categories: i) ships classified as international shipping (see Section 1.2) and ii)
ships classified as domestic and fishing. The subject of the inventories in Section 1.3, international shipping,
has significantly higher coverage quality than the domestic and fishing fleet.
In Section 1.4, where the days-at-sea estimate from LRIT is compared with the estimate obtained from AIS,
there is a significant improvement in quality for the AIS-derived activity estimates when reliable coverage
exceeds 50% of the year. When this finding is placed in the context of the coverage statistics described in this
section, it can be seen that in 2011 and 2012 the coverage statistics lead to high-quality activity and therefore
inventory estimates. However, in the earlier years of this study, the comparatively lower coverage statistics,
relative to the later years, increase the uncertainty of the estimated inventories.
Figure 60: Emissions weighted average of the total percentage of in-service time
for which high-reliability activity estimates are available from AIS