M2 Blog: Defining Radar Accuracy
Defining radar accuracy to help minimize vessel collisions with whales
Posted by M2 Team
March 21, 2025
Why is this important?
Vessel collisions threaten whale species where their habitat overlaps with human activities. To reduce the risk and severity of these collisions, managers use speed limits to slow vessels down when whales are present. An international tracking system called the Automatic Identification System, or AIS, provides data on vessel speeds and is commonly used to monitor and enforce the limits. But only large commercial vessels typically use AIS, and the impacts of smaller vessels on whales are of growing concern. Similar to radars that monitor speed limits on roadways, marine radar can monitor the speeds of smaller vessels in whale habitat.
Marine radar is a common tool used by boaters to understand their surroundings and safely navigate. An important component of radar systems is reporting the speed of other vessels on the water so that boaters can avoid collisions. Radar systems track the location of vessels and use complex algorithms to predict their speed. However, many factors affect tracking accuracy, and there are no specifications that define the accuracy of vessel speeds reported by radar systems in real-world environmental conditions. To use radar for managing vessel speeds, its accuracy should be well understood.
How did we do it?
We took advantage of the dual radar and AIS tracking provided by M2 to estimate the accuracy of vessel speeds reported by the radar systems. AIS data are broadcast by vessels and rely on highly accurate satellite positioning systems onboard (like GPS) to calculate their speed. For vessels that were tracked by both radar and AIS, we calculated radar speed error as the difference between radar- and AIS-reported speeds. We were able to include smaller vessels thanks to Class B AIS, an optional version of AIS used by some boaters but not required by international law.
What did we find?
By looking at more than 3,000 vessel records, we found that vessel speeds reported by radar were usually within 1.8 knots of speeds reported in AIS data. We also looked for differences in accuracy across smaller or larger vessels and did not find any significant trends. Therefore, the accuracy of 1.8 knots can be applied confidently to vessels of all sizes.
How can this be useful?
Radar is a widely available tool that is accessible to a large audience, such as enforcement agencies, marine resource managers, and researchers. The accuracy values can be used to confidently enforce speed limits using radar. For example, in the North Atlantic right whale seasonal management zones, where there is a speed limit of 10 knots for certain vessels, those traveling with radar-reported speeds greater than 11.8 knots could be identified as potential violations with 95% confidence. Knowing the accuracy of radar speed measurements supports more reliable reporting and analysis so that vessel speed limit measures can be effectively designed, monitored, and enforced.