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 Radio Frequency (RF) data can be integrated for maritime domain awareness (MDA) in Starboard.
RF Technical Considerations
Radio frequency satellites measure electronic emissions that are actively emitting from vessels’ onboard equipment. This can include navigation radars, satellite communication devices, and radios. The efficacy of RF as a maritime surveillance tool is determined by the type, frequency, and amplitude of emissions from target vessels.
Common types of RF signals in global oceans. Image source: SpaceNews.
The primary source of vessel emissions collected by RF sensors is navigational radar, which transmits strong pulses in the S- and X- frequency bands, typically sweeping 20 to 60 times per minute. Low-Earth orbit satellites can detect these emissions as they propagate into space. By measuring the angle, frequency, and/or time of arrival, RF satellites can estimate the emitter's location with varying degrees of accuracy.
In contrast to optical sensors, this technology can be employed day or night and in all weather conditions.
Learn more: Watch Operation 18c to see RF used in Starboard
RF is a wide-area, passive-collection source. This means that depending on the frequency band and provider, RF collections can have very low latency and very high coverage of up to 10 million km², but require the vessel to be actively emitting when the satellites pass over. Because of this, RF is more effective when implemented for merchant vessel tracking, as larger vessels will usually operate at least one radar continuously while at sea. In contrast, RF can be difficult to effectively implement in the monitoring of fishing vessels or other small craft due to their less frequent use of detectable electronic emissions.
The following case study explores how RF can form part of a multi-sensor collection plan in the attempted detection of smaller vessels.
Fisheries as the proving ground for open ocean dark vessel detection technologies
Fisheries operations are a uniquely valuable field of maritime intelligence due to their largely unclassified or open-source nature within a globally under-resourced enforcement sector. Limited capacity and chronic under-funding of global fisheries enforcement makes collaboration and information sharing across governments, NGOs, and private-sector partners essential for fostering a culture of mutually-beneficial collaboration. Analysts often work with minimal formal training and resources, yet this environment supports the testing and development of the most complex, multi-sensor detection techniques by integrating several different open-source and unclassified remote sensing technologies–including RF.
Correlating RF detections with aerial surveillance images during Operation Nasse 2022.
When monitoring large areas of open ocean for potential dark fishing vessels, RF scans are an attractive option due to their persistent, wide-area coverage. However, effective interpretation of the findings requires an understanding of the limitations of the technology.
Some key limitations of RF sensors include:
The inability to identify vessel types (unless correlated to an emissions fingerprint archive),
A lack of visual outputs for validation cues,
High false detection rates for vessels that are small, unpredictable, or intentionally avoiding detection, and
Not all electronic devices or hardwares can be detected by RF sensors.
RF detections lack the visual cues that other sensors such as SAR and EO provide for context
One of the reasons that fisheries enforcement must employ complex multi-sensor approaches is due to the small, mobile, and low-emission vessels that are typical of this sector. In addition, smaller vessels such as fishing vessels and yachts will frequently switch off or limit their use of radar equipment for safety or commercial privacy. This can result in very high rates of false negatives when targeting these types of vessels with RF, highlighting the need for comprehensive multi-source collection plans.