Third IMO Greenhouse Gas Study 2014

204 Third IMO GHG Study 2014
Figure 17: Distribution of difference between resampled hourly speeds
and the reported speed within the hour sampled across 10 VLCCs in 2011.
The standard deviation was calculated as 0.75 nm/hr
Similarly, Figure 18 shows the distribution of speed change for a time difference of two hours.
Figure 18: Distribution of difference between reported speeds when the time difference
in reporting is within two hours (sampled as message from 105 minutes to 120 minutes
from the original message) for all VLCCs captured in AIS.
The standard deviation of the sample was 1.85 nm/hr
Figure 19 shows the comparison of LRIT speed and AIS-derived mean speed at sea for each ship category. In
most cases and years, AIS-derived speed is higher than that provided by LRIT. This is not unsurprising as the
LRIT speed is calculated from shortest path between points, which is not necessarily the route the ship will
have taken. Moreover, there will most likely be bias towards reported shore-side speeds, which are typically
lower. The extreme outliers occur when there is a low count of LRIT messages for ships within a type and
size category. Notwithstanding the extreme outliers, there is generally good agreement between the speed
estimates. From 2009 to 2012, the number of categories where the difference in mean category speed is less