Ship detection is a vital aspect of maritime surveillance as it allows the monitoring of maritime traffic, illegal fishing and sea border activities. This is typically done through the use of an Automated Identification System (AIS), which uses VHF radio frequencies to wirelessly broadcast the ships location, destination and identity to nearby receiver devices on other ships and land-based systems.
AIS are very effective at monitoring ships which are legally required to install a VHF transponder, but fail to detect those which are not, and those which disconnect their transponder. So how do you detect these ‘uncooperative’ ships?
This is where satellite imagery can help. Synthetic Aperture Radar (SAR) imagery uses radio waves to image the Earth’s surface. Unlike optical imagery, the wavelengths which the instruments use are not affected by the time of day or meteorological conditions, enabling imagery to be obtained day or night, with cloudy, or clear skies. As you can see from the image below, ships are very easy to detect given the distinct radar signature difference between the ship and the background ‘sea clutter’. The below SAR satellite imagery can also provide us some important details such as ship dimensions, orientation and location.
But on its own, SAR satellite imagery has its limitations and it is acknowledged that when combined with AIS data, they form a powerful tool for maritime surveillance. AIS data can identify the ships which SAR imagery detects, whilst SAR imagery helps detect ships which may not cooperate with an AIS.
Using simulated AIS data from SPIRE (a company planning to provide the most frequently refreshed global ship tracking data in the industry using satellite AIS), we can show the importance of supplementing AIS data with SAR satellite imagery. EarthImages
was used to gain access to SAR data from the KOMPSAT-5 mission
, which was analysed to detect and locate ships. We tried to match each ship location with a corresponding AIS signal to try and obtain more information on each ship (see image below).
Above: The vessel on the left would have remained unidentified using SAR data alone, but AIS helped us understand more about this ship.
The vessel on the right would have been undetected if one relied purely upon AIS data. There are two reasons for this: this ship may simply not have an AIS transponder or may have turned it off. The latter could be a cause for alarm as this would normally indicate possible illegal activities. Thus, SAR helps to monitor those ships which may not want to be monitored.