Norwegian AIS-satellites see far more ships

The satellites NorSat-1 and NorSat-2 were successfully launched from Kazakhstan on July 14th 2017.

Norway now operates four micro satellites designed for monitoring ship traffic. This includes the AISSat-1 and AISSat-2 launched in 2010 and 2014.

The main payload on all four satellites is an AIS-receiver for tracking maritime traffic from space via signals from the ship anti-collision system AIS.

The NorSat-1 and 2 satellites carry an upgraded, more sensitive receiver which is processing an increased number of AIS-signals. And while the precursors AISSat-1 and 2 received message types 1,2 and 3, the receiver flying on the two NorSats is receiving simultaneously on all four AIS channels and is capable of receiving message 27. 

All receivers are built by Kongsberg Seatex, and the new version is developed with support from the European Space Agency.

- By now we have been operating the improved AIS-receivers for four months and the results are very promising, says Ivar Spydevold, CEO of Statsat operating the satellites.

Detecting 60 % more ships

According to Spydevold the number of messages from the new AIS-receivers on the NorSats typically reach 2.5 million per day. In contrast, the older version on AISSat-1 and 2 receives a total of 900 000 per day in total from both satellites.

This plot shows how many times a ship is observed via satellite based AIS during 6th August 2017, a typical day in the North Sea. The upper plot shows data from AISSat-1 and 2. The lower plot (illustration below) shows data from NorSat-1 and 2. Red = 1 time, black = 30 times. Illustration: Norwegian Defence Reserarch Establishment (FFI).

  

The rate of messages transmitted from each ship can be high and the number of messages therefore has to be linked to the actual number of ships spotted per day.

The data show that the number of unique ships detected every day by the Norwegian AIS satellites has increased from 24 000 to 40 000 with NorSat-1 and 2 in orbit.

- Detecting 60 - 70 % more ships per day is a significant improvement of the service, says Jon Harr, program manager for the satellite projects at the Norwegian Space Centre.

Message type 27

The AIS is primarily a system aimed at preventing collision between ships. Depending on location and speed, the interval between the AIS-messages varies from a few seconds to several minutes.

Changing course, high speed and heavy traffic require high frequency while steady course at low speed reduces the signal rate needed.

In areas with heavy maritime traffic the number of messages can be so high that it jams the satellite’s AIS-receiver. The solution is message type 27.

This message contains core information like position, speed and course. It is transmitted every 30 second. The number of bits is compressed relative to the standard position reports and designed for long range transmission to and from satellite.

The new AIS receivers on NorSat-1 and 2 are capable of receiving message 27, thus enabling a much better coverage of high traffic areas by the Norwegian AIS satellites.

­- Message 27 makes up just 3 % of the total AIS-messages but it allows our satellites to spot 20 % more ships on a global scale, says Ivar Spydevold.

- In critical areas, like the North Sea or the Mediterranean, the increase in the number of ships seen is substantially higher, since other message types are lost due to message overlapping.

More data needed  

The AIS-data are used by the Norwegian Coastal Administration to manage the national coast and waters related to safety, emergency and transport planning.

The satellites deliver high quality data and the preliminary results are promising, but the results have to be tested over time.

The Norwegian Defence Research Establishment has been appointed by the Norwegian Space Centre to lead a scientific study to accurately quantify the AIS performance of the new NorSat satellites, in comparison with other AIS satellites and with results from the land based AIS network. Other participants to the study are the Norwegian Coastal Administration, Kongsberg Seatex, Statsat AS and Space Norway AS.