Detecting Wakes from Dark Vessels and Narco Subs with SAR
"There are various wake components that SAR imagery can detect, but all of the components are affected by different parameters."
The biggest narco sub on record was captured and decommissioned in the waters off the coast of Colombia in May 2023, marking another big win for the Colombian Navy in a long string of drug vessel seizures over the past several decades.
The semi-submersible vessel measured approximately 3 meters wide and nearly 30 meters long. Onboard? Three tons of packaged cocaine with an estimated value of just over $100 million en route to Central America - the most common path for cocaine from South America to the United States and Canada. The semi-sub was manned by three Colombian individuals, all of whom claim to have been coerced into manning the vessel on its journey.
In just over 30 years, the Colombian Navy has taken down over 225 semi-submersible vessels, all of which contained drugs, weapons, dead bodies, or all of the above. Of the vessels captured, most were headed to Central and North America, while some were headed through the Atlantic en route to Europe.
One of the vessels, seized in March 2023 in the Pacific Ocean, was carrying two dead bodies and a significant amount of drugs. Just one week after, another semi-sub was captured carrying around 1000 packages of cocaine in the same area.
These types of semi-submersible vessels (narco submarines) have been used by drug organizations for decades, most often for the sole purpose of trafficking cocaine from South America (Colombia and Ecuador specifically) to Central America and the US. The vessels, which are never fully submerged, hang their heads just above the water, making it easier to move undetected and move drugs across large areas and borders.
The problem is staring officials and maritime authorities directly in the face, but mitigating these efforts is more complicated than recognizing and attacking the issue.
The good news is that since the beginning of 2018, more than 1,000 semi-subs have been seized in Colombia. While it is painfully clear that narco submarines are a key component of drug trafficking organizations based in Central and South America, it is also clear to all parties that authorities are aware of it. Of the vessels captured since 2018, on average, they have contained over 280,000 lbs of cocaine, more than 1,500 lbs of marijuana, and, in some cases, weapons and dead bodies.
So, while there is progress being made in the fight against naval drug trafficking, there is plenty of room for improvement in how these drug vessels are identified, tracked, and decommissioned.
Below, we look closer at some of the strategies and technologies used to identify narco subs en route to their destination and explore how space-based technologies can further progress in the war on drugs.
How Do Authorities Detect and Identify Narco Subs?
Semi-submersible narco submarines are designed for the sole purpose of transporting drugs, weapons, and people undetected, so it’s no surprise that it is challenging for authorities to monitor open water areas and detect these vessels without fail.
However, numerous technologies and strategies exist that help maritime authorities manage and mitigate the ongoing (and growing) problem, including:
Aircraft or Drones Equipped with Radar Systems, Cameras, and Sensors
Underwater Sensors and Sonar Systems
Information Sharing Networks
The most common of the above-listed vessel detection strategies are radar and sonar systems.
Still, with the age of technological transformation upon us, specifically considering AI and ML technologies, new opportunities are arising that could help maritime authorities crack down on drug submarines more efficiently than ever before.
What is SAR, and How is it Used to Identify Dark Ships and Drug Vessels?
Synthetic Aperture Radar (SAR) is commonly used alongside other technologies like AIS to identify vessels, validate their position, or locate vessels that go dark at sea. While AIS systems can be easily manipulated or simply turned off, SAR can detect vessels at night, through cloud cover, and almost any other obstacle present in maritime environments.
So, you may wonder, “How can SAR imagery be used to identify narco vessels that are submerged beneath the surface of the ocean?”
While SAR may not be 100% successful in identifying semi-submerged vessels, it can (in some instances) be used to locate the wakes caused by the semi-subs.
Detecting Ship Wakes with SAR
Ships, unsurprisingly, put out wakes behind the vessel as they move through the open ocean. Companies like Spire Global, which specialize in space-based vessel monitoring, often use satellites equipped with SAR sensors to locate the wakes of dark ships (those not transmitting location data via AIS).
While this approach requires a bit of background on a vessel's potential location to be efficient and successful, it has proven useful in combating everything from sanctioned trade to illegal fishing to drug-running operations.
How Does SAR Detect Wakes from Moving Vessels?
There are various wake components that SAR imagery can detect, but all of the components are affected by different parameters.
Those parameters include, but are not limited to:
Environmental conditions such as wind speed, wind direction, wave height, wavelength, and wave direction.
Physical conditions such as incidence angle, polarization, vessel size and length, vessel velocity, and vessel direction.
Of the above parameters, vessel velocity has the highest impact on wake detection via SAR. The faster a vessel is traveling, the bigger and more detectable a ship’s wake will be.
Detectable Wake Components with SAR
There are four primary wake components that make it possible to detect a vessel moving through the water via SAR.
Ship-Generated Internal Waves
(Figure 1) Source: Tings, B. (2021). Non-Linear Modeling of Detectability of Ship Wake Components in Dependency to Influencing Parameters Using Spaceborne X-Band SAR. Remote Sensing, 13(2), 165. https://doi.org/10.3390/rs13020165
The most frequent wake component is the Turbulent Wake. With standard remote sensing, turbulent wake data can be detected for dozens of kilometers behind a moving ship. The wake trails almost directly behind a ship and has an extremely shallow angle (if any).
The turbulent wake consists of two parts: the whitewater and rough ocean surface at the aft of a vessel and a calm ocean surface just beyond the whitewater region.
A Kelvin Wake describes a complex and varying pattern that appears on the ocean surface after an interaction between transverse waves and divergent waves occurs. The interaction results in “cusp waves” - originating at the ship’s hull and forming a trailing v-shaped wake. Kelvin wakes are some of the widest-angled wakes visible on SAR.
When using SAR, Kelvin Wakes create regions of water around the boat with higher backscatter, making them more detectable than the transverse or divergent waves.
A V-Narrow wake is a bright, v-shaped wake with a very narrow angle on both sides of the turbulent wake where the water is calmer. This wake area trails almost directly behind a moving vessel, although not as shallow of an angle as a turbulent wake.
While V-Narrow wakes were previously thought to not be detectable on X-Band SAR, recent studies with manual inspection prove that these types of wakes show up regularly on SAR imagery.
Ship-Generated Internal Waves
Ship-Generated Internal Waves appear on SAR as V-shaped patterns that alternate between bright and dark regions.
The appearance of ship-generated internal waves is rare, as these types of wakes require highly stratified conditions near the ocean’s surface. However, when conditions are right, this type of wake is yet another method of vessel detection via SAR.
How Does This Play Into Tracking Semi-Submersible Vessels?
There is a big difference between the wakes produced by large vessels moving across the water on the surface and wakes produced by semi-submersible narco vessels.
As stated above, the first and primary factor affecting the detectability of wakes with SAR is vessel velocity. Since traffickers are aware of radar detection via wakes, they typically travel at slower speeds during the day to avoid detection. Still, as sensor technology evolves and continues to improve, even the smallest wakes can be detected via SAR, AI, and ML technologies.
One way this can be used is in a “tip-and-cue” manner, where a large area of the ocean is monitored with satellites and automated AI and ML programs. Once an anomaly or wake is detected in the water without the presence of a (legitimate) vessel on AIS, higher-resolution satellites can be tasked to move into the area and observe the event more closely.
The reality is that while SAR technology is not a guarantee or definitive answer in detecting and taking down semi-submersible drug vessels, it is another tool in the toolbox that maritime authorities can use to ensure the highest probability of capture.