In this post we provide an overview of satellite AIS (Automatic Identification System) receivers – tracking systems that enable efficient space-based maritime vessel detection across the globe – and share details of various products on the global market – if you’re familiar with this technology and would like to skip straight to the product listings, please click here.
In 2004 the International Maritime Organization (IMO) adopted new requirements designed to make it easier for sea-going vessels to be clearly identified.
The requirements stated that all ships of a certain size and undergoing particular kinds of voyage should carry an AIS device that provides real-time information to other vessels and coastal authorities. These requirements refer to:
- All ships of 300 gross tonnage and upwards engaged on international voyages,
- All cargo ships of 500 gross tonnage and upwards not engaged on international voyages, and
- All passenger ships, irrespective of size.
In this article we take a look at how satellite AIS receivers are used to receive and transmit this information from/to such vessels, to enable precise maritime tracking in the most remote areas of the planet, and view commercially-available products from around the world.
About maritime satellite AIS use
The IMO regulation specifies that the AIS on board the vessel needs to provide vital information about it’s characteristics and activity, such as:
- navigational status, and
- additional safety-related information.
These data must be provided automatically to appropriately equipped shore stations, as well as other ships and aircraft.
The AIS must also be capable of fast and accurate data exchange. It should be able to pick up the same type of information it transmits from other ships and exchange data with facilities on the shore.
The systems act as collision-avoidance and navigational aids. Messages sent via AIS can also be picked up by a very high frequency (VHF) receiver in-orbit, a system able to observe maritime activity over a wide area.
Satellite AIS vs terrestrial AIS
The primary advantage of space-based AIS receivers over terrestrial AIS devices is the scale of coverage that can be achieved.
Each satellite can monitor a large area on its own and a network that features enough satellites in suitable orbits can effectively cover the entire surface of the planet for AIS signals.
This means that individual vessels, distributed fleets, or even entire sections of Ocean can be consistently monitored from space – removing the requirement for any terrestrial systems.
How space-based AIS functions
There are currently two methodologies in use for detection of AIS signals from space; on-board processing (OBP) and spectrum de-collision processing (SDP).
OBP involves the use of specialized receivers which, while much more sensitive, basically work in the same manner as terrestrial AIS receivers.
It doesn’t require special processing capabilities and is very effective in low density areas, such as the middle of the Pacific Ocean.
However one of OBP’s shortcomings is that the detection probability is significantly lower in areas of the satellite footprint that have high ship density – particularly at a level of 1,000 ships per footprint.
At this density the signals from individual vessels can start to collide with each other. Statistical analysis has shown that the first pass detection performance of OBP in such high dense areas is quite low and this means that a complete picture of the maritime domain often requires multiple passes.
In addition, if the traffic is very dense (over 2,500 vessels) then slot collisions will occur meaning messages cannot be resolved through OBP, and so gaining a complete maritime picture in such circumstances is impossible.
SDP involves the use of receivers capable of detecting and digitizing the RF spectrum for the AIS channels and then processing the raw spectrum files to control the noise floor and then reconstruct collided messages with highly specialized software algorithms.
First pass detection is very high, even in areas with a high ship density, and full maritime domain awareness can typically be achieved in as little as two passes.
Therefore, if the area of interest contains higher ship densities, SDP methodology is usually recommended in order to achieve detection at a level that will enable operational use of Satellite-AIS (S-AIS) (for further reading please see this exactEarth white paper [PDF]).
The challenge of AIS in space
The deployment of space-borne AIS receivers faces a variety of challenges stemming from the fact that the technology is primarily intended for sea-level reception.
One major concern is related to the self-organization principle of terrestrial AIS communication systems.
All exchanged messages transmitted from ships within the VHF range of 30-40 nm are synchronized, meaning that there are no AIS reports sent at the same time, at the same frequency. This guarantees the proper functioning of the system without any message loss.
An AIS receiver mounted on a satellite however sees several of these self organizing cells within its footprint (>3,000 km diameter).
Due to the fact that there is no synchronization between the cells, only within each cell, it’s likely that the satellite receives AIS messages from multiple vessels sent at the same time with the same frequency.
This can cause message collision resulting in message loss.
Another issue is the saturation of the satellite sensor due to the high volume of messages received, particularly in high density traffic areas such as the Mediterranean or the Baltic Sea.
Message collision and receiver saturation are known to be the main factors that affect the performance of an S-AIS device, measured as the Probability of Detection (PoD) of picking up an AIS position report transmitted by a vessel.
To meet these challenges and improve ship tracking capability from space advanced algorithms, antennas and frequency diversification have been used onboard space-based AIS systems.
AIS Receivers on the global marketplace
In this section, you can find a range of satellite AIS receivers available on the international market. These listings will be updated when new S-AIS receivers are added to the global marketplace for space at satsearch.co – so please check back for more or sign up for our mailing list for all the updates.
Click on any of the links or images below to find out more about the systems. You can also submit a request for a quote, documentation or further information on each of the products listed or send us a more general query to discuss your specific needs, and we will use our global networks of suppliers to find a system to meet your specifications.
The AAC SpaceQuest STS-300 AIS Receiver is a 6-channel onboard processing (OBP) satellite AIS payload capable of receiving more than 1 million AIS messages per day. This high performing payload, derived from the flight proven STS-200 is ideal for CubeSat and microsatellite missions with size, weight and power constraints.
The Alén Space AIS Payload is an Automatic Identification System receiver designed for satellite applications. It is a tracking system for vessels, which is their main method for collision avoidance. The solution is suitable for ship monitoring and maritime traffic control. It operates in the VHF band to receive signals from AIS transponders and monitoring vessels in areas without land network coverage.
The Kongsberg ASR x50 is a 4th generation SAT-AIS receiver designed for space applications. It is a reconfigurable SDR based receiver, designed to support simultaneous on-board AIS decoding and digital sampling. It weighs 1.3 kgs and has a design lifetime of 7+ years in LEO.
The Satlab A/S Polaris 4-channel AIS Receiver is an SDR based second-generation satellite-AIS receiver designed for satellite applications. It is mainly designed for satellite applications in the maritime VHF band. The receiver uses a high-performance, direct-conversion front end and large dynamic range ADC to receive the entire maritime VHF band. It can also be operated in a receive-and-forward mode if an external message store is preferred.
The Satlab A/S QubeAIS Receiver is a flight proven SDR based Automatic Identification System receiver (AIS) designed for LEO satellite missions. The receiver is software configurable for simultaneous reception of either AIS channels 1 & 2 (162 MHz) or the long-range channels 3 & 4 (156.8 MHz). It uses a low-IF frontend in conjunction with a 16-bit fixed point DSP to maximize performance at low power consumption.
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