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How Cargo Ships and Marine Vessels are Tracked

The Technology of Tracking Ships and Vessels

Between 2011 and 2020, about 876 vessels went missing at sea. Statista cites that the majority of these ships were cargo ships—totaling 348 vessels. These vessels had no rescuer in their time of distress, and so they vanished into ocean waters without a trace. Luckily, there was one vessel that escaped the same fate as other vessels – the Hallgrimur fishing vessel. It had a miraculous escape—thanks to the satellite-based AIS technology installed aboard.

On January 25, 2012, Hallgrimur was caught in terrific weather conditions, causing it to overturn on the icy water between Iceland and Norway. According to the distress signals received at the rescue coordination center, Halligrimur was battling winds at 60 – 70 knots per hour, and a wave height of 50 – 60 feet. Norway’s Joint Rescue Coordination Center then began to track the ship’s AIS signal from the International Space Station in an effort to enhance the rescue operations. In this course, they picked up the AIS signal of another vessel that was somewhere close to Hallgrimur. They engaged this vessel in a bid to search for the four-member crew of Halligrimur, aided by helicopters. Out of the four crew members, the rescue party was able to save only Eiríkur Ingi Jóhannsson, who had spent four hours floating on the North Atlantic Sea before aid arrived. 

Incidents such as these broadcast the importance of marine trackers, without which tracking ships and monitoring their course would be impossible. Thankfully, technology has stepped up and ship tracking techniques have advanced from terrestrial ship locators to space-borne international ship finders. Had it not been for the satellite-based AIS on the ISS, Hallgrimur would have been another ship in distress, lost to the perils of the sea.

Why Tracking Ships Is Important

The incident of Halligrimur is proof of one reason why we should track ship – it can save lives. Other benefits include making it easier for authorities to manage vessels in their territory, enhancing security, as well as some others, outlined below:

Vessel management: Ship trackers help to ensure safer ship routing, enabling Vessel Traffic Monitoring and Information Systems (VTMIS), promoting ship reporting, validating ship declarations, offering efficient Vessel Traffic Services (VTS), managing arrivals, and performing effective vessel traffic analysis.

Search and rescue: During rough times at sea, marine trackers are useful for tracking and assisting distressed vessels, predicting paths, and sending relief for natural disasters.

Security and surveillance: Ship locators play a crucial role in maintaining security in the oceans by monitoring piracy, preventing smuggling, mitigating infectious disease, increasing vessel safety in potentially hostile waters, and monitoring navigation operations.

Environmental conservation: Lastly, ship locators can help by ensuring that vessels do not pollute the waters. They aid in environmental preservation, and fishery protection.

Tracking Technologies Used to Find Vessels

A Vessel Tracking System is used to find ships in the open waters. The three predominantly used VTS are:

  • AIS
  • GPS
  • ShipLoc

Automatic Identification System (AIS)

AIS is a standardized transponder that is mounted on vessels following regulations posed by the International Maritime Organization. The AIS automatically sends the position, identification, and other information pertaining to a ship to other ships and coastal authorities. It is very similar to air traffic control, but for the sea.

Global Positioning System (GPS)

GPS works by picking up signals from the satellite. GPS is actually the American version of this technology, but the broader term is a Global Navigation Satellite System (GNSS). As a VTS, the GPS clearly pinpoints the position and the location of the vessel in the open waters. This comes in handy should an untoward incident crop up. By using GPS, the vessel would be informed of what routes to take, what courses to avoid, and as well be kept in the loop of other important navigation guidelines. 

Want to learn more about Navigating the Past, Present, and Future of GNSS?


ShipLoc is a private VTS solution that gives accurate readings pertaining to the location and position of a vessel, the air pressure in the oceanic area it is routed through, the wave pressure, and all other natural and man-made occurrences that are vital to the ship’s navigation.

AIS Prominence in Tracking Vessels

Besides the aforementioned three Vessel Tracking Systems, there are several others in the market. Nonetheless, AIS has gained prominence due to its many strengths, and has been in research and development over the years – resulting in its upgrade to S-AIS (satellite-based AIS).

The International Marine Organization has laid forth particular standards regarding the carriage of AIS. All vessel owners must ensure that ships of 300 gross tons and above going on international voyages, cargo ships of 500 gross tons and above going on domestic voyages, and all passenger ships, irrespective of their sizes, have AIS on board.

The vessels that are fitted with AIS are required to keep it functioning at all times. An exception to this standard can be permitted where international agreements, standards, or rules provide for the protection of that navigational information. While the AIS is in operation it must:

  • Automatically provide information about the vessel’s identity, position, type, course, navigational status, speed, and other such information to other ships, and shore stations.
  • Automatically receive such navigation-bound information from other ships fitted with AIS.
  • Consistently monitor and track vessels, and exchange the data with the corresponding shore-based authorities.

How AIS Functions

AIS operates terrestrially. The system consists of one Very High Frequency (VHF) transmitter, two VHM Time-Division Multiple Access (TDMA) receivers, one VHF Digital Selective Calling (DSC), and a standard marine electronic communication link to sensor systems and the shipboard display.

The vessels’ position and timing information is taken from an external or integral GPS receiver. All the other information broadcast by the AIS is electronically derived from the shipboard equipment via standard marine data connections.

Theoretically, only one channel is needed for the transmitting and receiving of information. However, each station relays and receives information over two radio channels to prevent interference, and avoid signal loss from vessels. In the AIS, 2,250 time slots are established every 60 seconds, and into one slot, a position report from one AIS station is recorded. This way, the various AIS stations continuously synchronize themselves to each other so as to avoid slot transmission overlaps.

The S-AIS (Satellite based AIS)

The S-AIS is an upgraded ship locator technology, based on the AIS. Despite the terrestrial AIS having played a significant role in tracking ships, it has certain limitations that called for an upgrade in the technology. First, as with other VHF digital applications, the AIS system coverage range depends on the height of the antenna. Therefore, the typical range within which vessels could be tracked is 20 nautical miles (NM). This means that ships which go beyond 75 km off the coast become invisible to the coastal authorities.

Satellites revolutionized the operations of the terrestrial based AIS, and helped filled in its limitations. Instead of the AIS coverage being dependent on the height of the antenna, the S-AIS utilizes satellites in space. Ideally, when the signals’ straight propagation is considered, they travel 700 km out into space. The International Space Station is situated 408 km above the earth, so no ships can become invisible when they sail deep into the sea.

The European Space Agency (ESA) contacted the Norwegian Defense Research Establishment and allowed their scientists to mount an AIS receiver onboard the space station. On June 1, 2010, the system was turned on, and that opened the door for mariners to look at the global marine traffic. Indeed, this was the technology that saved Jóhannsson, from the Hallgrimur.

On a typical day, the space station observes around 30 thousand ships in the open ocean. This means that there is now an international ship finder that can help to find ships, engage in search and rescue operations, and cater towards the greater good of the community.

How S-AIS Functions

While the satellite-based AIS is constantly evolving, at present there are two techniques that are used to detect the AIS signals from space.

Onboard Processing (OBP)

The OBP leverages sensitive receivers that are similar to their terrestrial equivalents in the way they operate. OBP is only best for use in areas of low-density shipping. When over 1,000 vessels come into the vicinity of the satellite, the service quality degrades as the signals begin to collide with each other, muddying the waters so to speak.

Spectrum Decollision Processing (SDP)

The SDP makes use of receivers that detect, digitize the incoming signals, and process the raw files to enhance the quality. The flow of data here is faster and of higher accuracy, even for high density shipping areas.

Using Satellite Imagery to Track ships

Besides AIS, satellite imagery alone has also emerged as a potential ship locating solution. It renders a greater range of data and can complement AIS in finding vessels. The technology to track ship with the help of satellite imagery uses either optical imagery, or SAR imagery, as further explained below:

Tracking Ships with Optical Remote Sensing Images

GF-4 and other geostationary optical remote sensing satellites have wide coverage and a high temporal resolution. This makes them suitable for the continuous tracking and observation of vessels over a large range. However, since optical sensors are affected by clouds, rain, the sun’s illumination, and other factors, it makes the process more complicated, and sometimes unreliable.

Recent research has put forth a three-staged methodology for tracking ships using optical imageries.

  1. Image Enhancement: The optical image is first subjected to enhancement by the adaptive nonlinear gray stretch (ANGS) method. This helps to highlight the small and dim ship targets.
  2. Target Delineation: After the image enhancement, a multi-scale dual-neighbor difference contrast measure (MDDCM) is applied. This enables the detection of the candidate ship target’s position.
  3. Vessel Tracking: Lastly, the joint probability data association (JPDA) method is employed for multi-frame data association and target tracking.

The results of the research have proven that ships can be detected and tracked better in this manner, as opposed to the traditional methodologies.

Synthetic Aperture Radar Imagery for Ship Detection

As with optical imagery, SAR imagery is also used for ship detection and tracking. A research paper by Xiangguang Leng et al. (2016), suggests that the use of RADARSAT-1, RADARSAT-2, TerraSAR-X, RS-1, and RS-3 images to detect ship targets works rapidly, and robustly.

Using SAR, is a two-fold approach, including ship candidate detection and ship discrimination.

  1. Ship Candidate Detection: First, to remove the land from the space-borne SAR images, an adaptive land masking method is used based on the ship, and pixel size. The image is then down sampled to delineate between the bigger structures, such as islands, from that of small structures i.e. ships. Next, a prescreening algorithm is used to concentrate only on the ocean bodies to locate ships.
  2. Ship Discrimination: False alarms are difficult to eliminate in the ‘fast but dirty’ phase of preliminary ship candidate detection. Therefore, this second phase is employed to check against constant false alarm rate (CFAR) with the help of discrimination algorithms. They generally operate on a chip that contains a target ship candidate and its immediate surroundings, and removes false alarms by discrimination, based on features and azimuth ambiguities.

The inclusion of the discrimination algorithms while tracking ships using SAR imageries has greatly increased the confidence level in this approach. It can now be reliably used to detect and track ships anywhere in the ocean.

Evolving Marine Tracking Technology

Ship locators play a crucial role in the marine traffic ecosystem. From tracking ships to get their position data, to finding distressed and law-violating vessels, Vessel Tracking Systems have proved to be a great asset to the industry.

AIS and S-AIS are the predominantly used technologies to find ships. Where terrestrial AIS fails, space-borne AIS has evolved to ameliorate global marine traffic. Nonetheless, the distance limitations and satellite transmission difficulties pose core problems that impact the highly beneficial operation of the Vessel Tracking Systems. We look towards further advancements, and evolved scientific findings and developments to overcome these underlying challenges in the hopefully near future.