Satellites primarily detect submarines in shallow water using non-acoustic methods that spot the submarine itself or the disturbances it creates on and below the surface.
Traditional sonar (sound-based detection) is not effective from space, as sound waves do not travel well from air to water. Instead, satellites rely on sensors that analyze light, radar, heat, and magnetic fields. Detection is easier in shallow water because the submarine is closer to the surface, making its effects more pronounced.
Here are the main methods satellites use:
Direct Visual Detection (Optical Imaging)
In clear, shallow coastal waters, high-resolution satellite cameras can directly photograph a submerged submarine.
• How it works: This is similar to spotting a dark shape in a swimming pool from above. Sensors are optimized for blue-green light (450-550 nm), which penetrates water most effectively. The satellite may detect the submarine's dark hull or its shadow on the seabed.
• Limitations: This method is completely dependent on water clarity (it's ineffective in murky or sediment-filled water) and sunlight. The submarine must be very shallow, typically just tens of feet below the surface.
• LIDAR: Some satellites use a technology called LIDAR (Light Detection and Ranging), which shoots a blue-green laser beam into the water. By measuring the light that reflects off the submarine's hull, it can determine its depth and shape, but this also requires clear water.
Detecting Surface Wakes (Synthetic Aperture Radar)
The most reliable method is detecting the tiny disturbances a submerged submarine creates on the ocean's surface. Synthetic Aperture Radar (SAR) satellites are ideal for this.
• How it works: SAR is a powerful radar system that scans the ocean surface to measure its texture and roughness. It works day or night, regardless of cloud cover. A submerged submarine, even at depth, displaces water as it moves. This creates a complex pattern of subtle waves and currents on the surface, often called a "Kelvin wake."
• What SAR "sees": While invisible to the naked eye, this wake pattern changes the surface's roughness just enough for the sensitive SAR to detect it. The satellite can identify this unnatural, linear pattern against the random chaos of natural ocean waves.
• Shallow Water Advantage: In shallow water, these surface effects are often stronger and more distinct, making them easier for SAR to spot. SAR can also easily detect a periscope or snorkel mast that breaks the surface.
Detecting Thermal Signatures (Infrared)
Submarines leave a "thermal wake" that can be spotted by satellites with infrared (IR) sensors.
• How it works: IR sensors detect differences in temperature. A moving submarine churns the water, mixing cooler water from below with warmer surface water (or vice-versa). This leaves a "scar" of slightly different-temperature water along its path.
• Reactor Heat: A nuclear submarine's reactor constantly releases a large amount of heat into the ocean, creating a persistent warm-water signature that can be detected.
• Shallow Water Advantage: This mixing and heat are more likely to reach the surface quickly and in a more concentrated way when the submarine is in shallower water.
Detecting Magnetic Anomalies
A submarine's massive steel hull is essentially a giant, moving piece of metal that distorts the Earth's natural magnetic field.
• How it works: Highly sensitive satellite-mounted magnetometers (known as Magnetic Anomaly Detection, or MAD sensors) can scan the ocean and detect these tiny, localized distortions, or "anomalies," which reveal the submarine's presence.
• Limitations: The strength of a magnetic field drops off very quickly with distance. This method is more effective when the submarine is closer to the sensor, making it more viable from low-flying aircraft than from orbit. However, as sensor technology improves, satellite-based MAD becomes increasingly feasible, especially for submarines in shallow water (which are closer to the satellite).
