Discuss Special problems in navigation involving Sub-surface craft.

Of course. This is a fascinating and critical topic. The navigation of sub-surface craft (submarines, Autonomous Underwater Vehicles (AUVs), etc.) is one of the most challenging disciplines in all of navigation.

The core problem can be stated simply: Radio waves do not penetrate water.

This single fact invalidates the primary tool of modern navigation, the Global Navigation Satellite System (GNSS), such as GPS. A submerged craft is fundamentally cut off from the outside world. This creates a unique set of problems that require a complex, multi-layered, and often high-stakes approach to solve.

Here is a discussion of the special problems in navigation involving sub-surface craft.

1. The Fundamental Problem: Positional Uncertainty and Drift

Without access to GPS, a submerged vessel must rely on Dead Reckoning (DR). The primary system for this is the Inertial Navigation System (INS).

• How INS Works: An INS uses a combination of gyroscopes (to measure orientation) and accelerometers (to measure changes in velocity) to calculate the craft’s position relative to a known starting point.
• The “Special Problem” - Drift: No INS is perfect. Tiny, imperceptible errors in the gyros and accelerometers accumulate over time. This error, known as drift, is cumulative. The longer a submarine stays submerged and the faster it travels, the larger its “cone of uncertainty” becomes. The navigator may think the sub is at Point A, when it is actually miles away at Point B.

This ever-growing uncertainty is the central challenge that all other sub-surface navigation techniques are designed to combat.

2. The Challenge of “Fixing” the Position

To correct the INS drift, the crew must obtain a “fix”—an accurate measurement of their position from an external source. The methods for doing this are fraught with their own problems, primarily the risk of detection.

A. High-Risk, High-Accuracy Fixes (Near the Surface):

• GPS/GNSS: The most accurate method. The submarine must come to periscope depth and raise a mast with a GPS antenna.
  • Problem: This is the most dangerous time for a submarine. The mast can be detected by enemy radar (even low-probability-of-intercept radar) and its radio transmissions can be detected by Electronic Support Measures (ESM). This exposes the sub’s position, negating its primary advantage: stealth.
• Celestial Navigation: The traditional method. Using a sextant integrated into the periscope to take sights of the sun, moon, or stars.
  • Problem: Requires coming to periscope depth (risk of detection), clear skies, and a visible horizon. It is also less accurate than GPS and requires significant skill.
• Radar Navigation: Using the submarine’s radar to get a range and bearing from a known coastline or surface feature.
  • Problem: Radar is an active emission. Sending out a radar pulse is like shouting “I AM HERE!” in a dark room. It is easily detected and instantly gives away the sub’s position.

B. Stealthy, Complex Fixes (While Submerged):

Because surfacing is so risky, a suite of advanced techniques has been developed to get a fix while remaining deep and hidden. These are the truly “special” methods of submarine navigation.

• Geophysical Navigation: This involves comparing real-time sensor data with highly detailed, pre-loaded maps of the ocean environment.
  • Terrain Referenced Navigation (TRN) / Bottom Contour Navigation (BCN): The submarine uses a sophisticated echo sounder (a type of sonar) to map the profile of the seabed directly beneath it. A computer then compares this measured profile to a highly accurate 3D bathymetric (seabed) chart stored in its memory. By finding a unique match, it can determine its position with remarkable accuracy.
    • Problem: Requires extremely detailed and accurate undersea maps, which are classified and only exist for certain strategic areas. It is also less effective over flat, featureless abyssal plains.
  • Gravity Anomaly Navigation (Gravimetry): The Earth’s gravitational field is not uniform. It varies slightly depending on the density of the rock beneath the seabed (e.g., a dense underwater mountain creates a stronger gravity field). A submarine can carry a gravimeter to measure these tiny variations. By comparing the measured gravity to a pre-loaded gravity map, it can fix its position.
    • Problem: This is entirely passive and stealthy, but the equipment is incredibly sensitive and complex. It also requires access to highly detailed gravity maps.
  • Magnetic Anomaly Navigation: Similar to gravimetry, this method uses a magnetometer to measure variations in the Earth’s magnetic field caused by the geology of the seafloor. This data is then compared to a magnetic map.
    • Problem: The submarine’s own massive steel hull creates a significant magnetic field that must be carefully compensated for.

3. The Opaque and Dynamic 3D Environment

Unlike an aircraft or ship on the surface, a submarine operates in a full three-dimensional, opaque, and dynamic medium.

• Collision Avoidance (3D): The navigator’s primary safety concern is avoiding collision with seamounts (underwater mountains), wrecks, and other undersea obstacles. This requires accurate charts and reliable forward-looking active sonar, the use of which (pinging) again presents a risk of detection.
• Ocean Currents: The ocean is not static. Unpredictable and powerful underwater currents can push a submarine off course. The INS cannot distinguish between the submarine’s movement through the water and the water’s movement over the seafloor. This is a major source of uncompensated error. A Doppler Velocity Log (DVL), which uses sonar to measure speed relative to the seabed, can help correct this, but it is an active system.
• Acoustic Distortion: The speed of sound in water changes with temperature, pressure, and salinity. Layers like the thermocline can bend sound waves dramatically, affecting the accuracy of all sonar systems used for navigation and collision avoidance.

4. The Overarching Conflict: Precision vs. Stealth

Every decision made by a submarine navigator is governed by a fundamental trade-off:

The need for navigational accuracy is in direct conflict with the need for stealth.

• The most accurate fixing methods (GPS, Radar) are the least stealthy.
• The stealthiest methods (INS, Gravimetry) are subject to error or require extensive, pre-existing data.
• Even using active sonar to avoid a seamount must be weighed against the risk that the “ping” will be heard by an enemy asset.

A submarine commander must constantly ask the navigator, “How sure are you of our position?” The answer to that question—the size of the “cone of uncertainty”—determines what tactical options are available, whether a weapon can be launched, or whether the submarine can safely transit a narrow strait.

Summary

The special problems of sub-surface navigation are:

1. Inability to Use GPS: Forcing reliance on less accurate, self-contained systems.
2. Cumulative INS Drift: Creating an ever-present and growing positional uncertainty.
3. High Risk of Position Fixing: Making every attempt to correct drift a calculated risk that compromises stealth.
4. Reliance on Pre-Surveyed Maps: Advanced techniques like TRN and gravimetry are only as good as the classified maps they depend on.
5. Navigating a 3D Opaque Environment: Dealing with underwater obstacles, unpredictable currents, and acoustic distortions.
6. The Constant Stealth vs. Precision Trade-off: The fundamental dilemma that governs every navigational decision.

Ultimately, submarine navigation is a hybrid art, skillfully blending data from the INS with periodic, carefully chosen “fixes” from a variety of other systems, all while under the immense pressure of remaining undetected.