Application of Synthetic Aperture Radar (SAR) in Starboard

As part of the Using Satellite Data for Analysis in Starboard series, this article uses practical case studies and technical insights to further explore how Synthetic Aperture Radar (SAR) data can be integrated for maritime domain awareness (MDA) in Starboard.

SAR is one of the most commonly used satellite data sources for maritime domain awareness, due to its ability to be used both day and night and in all weather conditions. It is also classed as an active sensor, meaning it does not rely on vessel emissions for positive identification. Despite this, the use of microwave frequency backscatter for vessel detection can result in higher rates of false positives for non-metal structures.

Starboard ingests SAR from numerous sources, including publicly-available data from the Copernicus program.

There are numerous SAR data providers, both public and commercial, which produce imagery suitable for ship detection at resolutions ranging from <1m to over 50 metres. Depending on the imaging mode, individual scans can cover areas from 5 to 200,000+ square kilometres. Notable systems include Sentinel-1 (of the Copernicus program), which is operated by the European Space Agency and provides publicly available low-resolution data with a 250km swath width–but only for coastal areas. Radarsat-2, from the Canadian Space Agency, provides a ship detection mode that captures global oceans up to 225,000 km² at a time with 20-metre resolution and is often used for open-ocean large-vessel surveillance.

Publicly-accessible low-resolution imagers can typically detect only vessels larger than approximately 30 metres in length, but offer reliable global coverage. Detecting smaller vessels generally requires sub-metre resolution from commercial satellites, which comes at higher financial cost and reduced footprint size. There are numerous commercial providers to choose from who are launching new satellites at unprecedented rates, allowing them to scan the same location multiple times per day for persistent monitoring.

Sentinel-1 produces long, narrow, swaths over large areas of coastline

The core technological difference between publicly-accessible sources such as the European Space Agency’s (ESA) Sentinel-1 and at-cost commercial imagers, often lies in their imaging method. Sentinel-1 uses a pushbroom sensor, which collects data line-by-line as the satellite moves along a predetermined orbit. This enables wide-swath coverage (often exceeding 100,000nm²) with consistent revisit rates, but is generally limited to coastal and land area imaging (rather than open ocean).

In contrast, most commercial satellites employ a frame-based sensor, capturing snapshots of smaller areas (~200 nm²) with very high spatial resolution—under 1 metre. This enables finer detail but relies on bespoke tasking requests in highly specific locations, and is subject to satellite availability at the time of collection.

Combining open source and commercial satellite has a wide range of use cases, including the monitoring of military activity, which can be notoriously difficult due to their limited use of electronic emissions and unpredictable routes.

The benefit of using SAR imagery to detect military vessels is that it is an active sensor, and not weather or daylight dependent. RF satellites are passive sensors, relying on active emissions to detect a vessel, and EO relies on clear weather during daylight hours. This means that when military vessels are enforcing strict emissions controls (dark) while active at night, wide-area SAR is the only publicly-available satellite sensor that can still detect them at sea.

The following example illustrates one of the simplest methods of identifying at-sea military activity using SAR.

A CCG vessel operating on AIS appears to be conducting at-sea patrols alone.

With Sentinel-1 overlaid, a similar-looking dark vessel appears nearby.

In this example, overlaying Sentinel-1 imagery to AIS revealed the presence of a larger military task group operating under tight emissions control (dark). While low-resolution, wide-area satellites like Sentinel-1 may not support definitive military vessel identification, they are effective for detecting vessels sharing a similar appearance which can then be flagged for follow-up tasking with higher-resolution imagery (tip-and-cue). Read more about how Sentinel satellites have been used for military tracking in Starboard.

Starboard regularly engages in global MDA operations which seek to advance the use of commercial and open-source technologies for intelligence analysis and collaboration. Many of these operations have resulted in Starboard contributing daily intelligence briefs and analysis on live events of interest such as smuggling and spoofing activities. This is an important source of ground-truthing and control testing for different technologies and capabilities in the field, as well as being able to leverage a broad scope of expertise while working alongside our customers, partners, and industry professionals.

One such operation was Operation TS, which we can use as a case study for the practical implementation of satellite remote sensing in detecting and analysing ship-to-ship (STS) transfers between sanctioned vessels at sea.

The following example from Operation TS highlights a common application of open-source satellite data for investigating AIS spoofing during STS transfers between sanctioned vessels. The Natuna Sea region of overlapping and competing maritime boundaries is a focus area for Op TS, known for its sanctioned oil smuggling operations between vessels who appear on international compliance lists such as Open Sanctions and UANI Ghost Armada.

Sentinel-1 SAR imagery fused with AIS for the rapid identification of STS and spoofing activity.

This can be as simple as ‘spot the difference’, whereby the Sentinel data is used to identify AIS signals which do not have a physical vessel associated in the image. This indicates that a vessel may be spoofing their location while conducting dark STS activity nearby. Watch the full Youtube series on Op TS.

Two different Sentinel images taken 8 hours apart, revealing two pairs of tanker vessels conducting transfer operations without AIS. Nearby, an AIS signal (NEREIDES) has no underlying vessel apparent in imagery.