The Norwegian Automatic Identification System (NORAIS-2) Receiver replaced its predecessor, NORAIS-1, when it was installed on the International Space Station in February. Four months later, the improvements from NORAIS-1 are significant, with an increase in number of ships tracked daily from roughly 26 000 to 33 000.
As expected, the NORAIS-2 algorithms are more robust against interference, showing even greater improvement in difficult high traffic zones. In the Gulf of Mexico for instance, the NORAIS-2 tracks more than twice as many vessels on a daily basis compared with NORAIS-1.
The result of the performance improvement of the NORAIS-2 Receiver, with more messages detected from more ships, enables operational users of satellite AIS to build a more complete maritime picture, knowing which ships are where doing what.
A more detailed maritime picture enables better organized rescue operations, combat piracy and an ability to uncover illegal activities such as smuggling, oil dumping and illegal fishing.
AIS helps to resolve the difficulty of identifying ships when not in sight (e.g. in fog, at distance, etc.) by providing a means for ships to exchange identification, position, course, speed, and other ship data with all other nearby ships and VTS stations.
It works by integrating a standardized Very High Frequency (VHF) transceiver system with an electronic navigation system, such as a LORAN-C or Global Positioning System receiver, and other navigational sensors on board ship (gyrocompass, rate of turn indicator, etc.).
The International Maritime Organization’s (IMO) International Convention for the Safety of Life at Sea (SOLAS) requires AIS to be installed aboard international voyaging ships of 300 or more gross tonnage, and all passenger ships regardless of size. Due to range limitations of AIS ground based systems, a global surveillance capability via AIS is best implemented via a space borne system.
In 2008, in order to gain early access to experimental AIS data from low Earth orbit, the European Space Agency (ESA) took advantage of on-going AIS receiver developments in Norway for a fast-track procurement of an AIS receiver for deployment to the International Space Station (ISS) named the NORAIS-1 Receiver.
The Norwegian Defence Research Establishment (FFI) was responsible for the overall design, integration, testing and documentation with Kongsberg Seatex being responsible for the design and manufacturing of the AIS payload.
The aim was to collect AIS data from low Earth orbit, through sampling of the AIS signal channels, decoding of AIS messages, and perform measurements of the VHF signal environment in the maritime VHF frequency band. The space-based AIS data was used for evaluation of detection probability models, continuing developing AIS receiver technology, and provide critical input to the development of an operational space-based AIS reception capability.
Due to its flexible software defined radio approach, the NORAIS-1 Receiver supported the development and testing of several improved algorithm uploads, in addition to adding a sampling capability. However, the hardware lacked the capacity to incorporate all the developments into one algorithm for maximum performance.
Additional ideas for higher performance algorithms also surfaced, but could not be implemented for lack of on-board resources. The next generation AIS receiver was developed in parallel with more resources that can demonstrate the capabilities of a more advanced receiver algorithms to the operational users of AIS, while leveraging the existing infrastructure installed for the NORAIS-1 Receiver onboard ISS and data handling on ground. The next generation receiver was named the NORAIS-2 Receiver.
The aggregated knowledge gained by FFI and Kongsberg Seatex during the four and a half years of NORAIS-1 operation was combined by Kongsberg Seatex to form the algorithms implemented in the NORAIS-2 Receiver.
In addition to running more advanced algorithms than NORAIS-1, the NORAIS-2 Receiver operates on all four AIS frequencies, can sample up to 512 MByte of data while simultaneously decoding on all four channels, and still has room for much larger algorithms if required in the future. The four operating frequencies can be set to any frequency within the maritime VHF band for general signal environment measurements.
Basic information about a vessel such as identity, position, speed and heading are transmitted in type 1,2,3 and 27 AIS messages and consequently utilized by space-based AIS services.
A typical day’s distribution of vessels as seen by the NORAIS-2 Receiver are presented in video for type 1-3 (white) and type 27 (red) AIS messages respectively. The type 27 message was designed to increase the detection probability in high traffic zones from space, and by comparison of the North Sea high ship traffic area it is evident that more ships are seen in type 27 data than the type 1-3 data. So far, only a minority of ships has started to use the type 27 message, which is why so many fewer ships are detected globally compared with the type 1-3 messages.
Furthermore, as more and more ships start using the type 27 message on the two new AIS channels, the capability of receiving messages on all four channels simultaneously is a significant improvement to a space-based AIS service.