SONAR is mostly used by

1. Nature and Propagation of Sound Waves (basic)

To understand how naval vessels like submarines navigate the deep ocean, we must first master the Nature and Propagation of Sound Waves. Sound is a form of mechanical energy that travels through a medium (solid, liquid, or gas) by causing the particles of that medium to vibrate. Unlike light or radio waves, sound cannot travel through a vacuum because it requires these physical particles to transmit its energy.

Sound waves are primarily longitudinal waves (also known as compressional or pressure waves). In a longitudinal wave, the displacement of the medium is parallel to the direction in which the wave travels. As the wave moves forward, it creates alternating regions of high pressure called compressions (where particles are squeezed together) and low pressure called rarefactions (where particles are stretched apart) Physical Geography by PMF IAS, Earths Interior, p.60. This is very similar to how P-waves (Primary waves) behave during an earthquake, moving rapidly and transmitting energy efficiently through the medium Physical Geography by PMF IAS, Earths Interior, p.61.

In the context of the ocean, sound propagation is far more efficient than the transmission of light or radar. While transverse waves (like S-waves or light waves) involve particles moving perpendicular to the wave direction and often struggle to pass through fluids, longitudinal sound waves thrive in water Physical Geography by PMF IAS, Earths Interior, p.62. Because water is much denser than air, it allows sound to travel nearly five times faster and significantly further, making it the perfect tool for underwater communication and detection.

Feature	Longitudinal Waves (Sound)	Transverse Waves (Light/S-waves)
Particle Motion	Parallel to wave direction	Perpendicular to wave direction
Medium Requirement	Requires a medium (Mechanical)	Can travel in a vacuum (Electromagnetic)
Key Characteristics	Compressions and Rarefactions	Crests and Troughs

Sources: Physical Geography by PMF IAS, Earths Interior, p.60; Physical Geography by PMF IAS, Earths Interior, p.61; Physical Geography by PMF IAS, Earths Interior, p.62

2. Ultrasonic and Infrasonic Waves (basic)

To understand how naval vessels 'see' underwater, we must first understand the spectrum of sound. Sound is a mechanical wave created by vibrations that travel through a medium like air or water. The number of these vibrations per second is called frequency, measured in Hertz (Hz) Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.109. While the human ear typically hears sounds between 20 Hz and 20,000 Hz (20 kHz), the world beyond this range is divided into Infrasonic and Ultrasonic waves.

Infrasonic waves have frequencies below 20 Hz. These are low-frequency sounds that carry immense energy over very long distances because they are less likely to be absorbed by obstacles. In nature, large animals like elephants and whales use infrasound to communicate across hundreds of miles. In a naval context, very low-frequency sounds are monitored to detect distant seismic activity or large-scale movements in the ocean. Conversely, Ultrasonic waves have frequencies above 20,000 Hz. These waves have very short wavelengths, allowing them to be focused into narrow, powerful beams that do not spread out easily. This precision makes them ideal for SONAR (Sound Navigation and Ranging), the primary technology used by navigators to map the seafloor and detect underwater hazards.

Naval platforms rely on sound waves rather than light or radar because of how they interact with water. In the atmosphere, we use electromagnetic waves (like radio or microwaves) for communication, but as noted in Physical Geography by PMF IAS, Earths Atmosphere, p.278, certain high-frequency electromagnetic waves are easily absorbed or reflected by the ionosphere. In the ocean, water absorbs light and radio waves almost instantly, rendering them useless for long-range detection. However, sound travels roughly five times faster in water than in air and can travel vast distances without losing its integrity. This is why marine mammals like the Gangetic Dolphin have evolved to rely on sound for navigation in murky waters Environment, Shankar IAS Academy, Conservation Efforts, p.245, a biological precursor to the sonar technology used by modern submarines.

Wave Type	Frequency Range	Key Naval Characteristic
Infrasonic	Below 20 Hz	Extreme long-range travel; used for monitoring seismic/large-scale movement.
Audible	20 Hz – 20,000 Hz	Range of human hearing; used in basic acoustic communication.
Ultrasonic	Above 20,000 Hz	High precision and directional; the backbone of SONAR and imaging.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.109; Physical Geography by PMF IAS, Earths Atmosphere, p.278; Environment, Shankar IAS Academy, Conservation Efforts, p.245

3. Acoustics in Different Media (intermediate)

In the realm of naval warfare and maritime exploration, the medium dictates the technology. While we rely on Radar (radio waves) and Lidar (light waves) to navigate the air and land, these electromagnetic waves are rapidly absorbed and scattered by seawater. This phenomenon, known as attenuation, makes light practically useless for long-distance underwater detection. In contrast, water is an excellent conductor of mechanical energy. Sound waves, which are longitudinal mechanical waves, can travel much further and faster in water than in air. While sound travels at approximately 340 m/s in air, it speeds up to nearly 1,500 m/s in seawater because water is denser and less compressible than air. Just as light bends (refracts) when it moves between media of different optical densities — such as from air into glass Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148 — sound waves also refract within the ocean. The speed of sound underwater is not constant; it is influenced by three primary variables: temperature, salinity, and pressure. Generally, sound travels faster in warmer, saltier, and deeper (high-pressure) water. This variation creates distinct 'acoustic layers' in the ocean, which submarines use strategically to hide or to detect distant targets. For instance, a phenomenon known as the SOFAR channel (Sound Fixing and Ranging channel) allows low-frequency sounds to travel thousands of kilometers by trapping them in a specific depth layer where sound speed is at a minimum. Naval vessels harness these principles through SONAR (Sound Navigation and Ranging). This technology is divided into two types:

• Active SONAR: The vessel emits a 'ping' (acoustic pulse) and listens for the echo reflecting off an object. This provides precise range and bearing but reveals the vessel's own location.
• Passive SONAR: The vessel simply listens for sounds emitted by other ships, submarines, or marine life. This is the 'silent' mode essential for stealth operations.

While medical professionals use similar high-frequency sound waves for internal imaging, they refer to it as ultrasound. In the context of travel and seafloor mapping, the domain belongs to navigators using SONAR to identify hazards and navigate the lightless depths of the ocean floor.

Sources: Science, class X (NCERT 2025 ed.), Light – Reflection and Refraction, p.148; INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Natural Hazards and Disasters, p.59

4. RADAR: Radio Detection and Ranging (intermediate)

At its core, RADAR (Radio Detection and Ranging) is an object-detection system that uses electromagnetic (EM) waves—specifically radio waves or microwaves—to determine the range, angle, and velocity of objects. Unlike the human eye, which relies on ambient light, RADAR is an active sensing system; it provides its own source of illumination. It works on the Pulse-Echo principle: a transmitter sends out a short pulse of radio waves, which travels at the speed of light. When these waves strike an object (like an incoming vessel or an aircraft), they undergo reflection—rebounding back to the source—where they are picked up by a receiver FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.20.

In the naval and atmospheric context, the choice of frequency is critical. For instance, very high-frequency electromagnetic waves like microwaves are often used because they can be focused into narrow beams for high precision. However, these high-frequency waves are often absorbed or not reflected by the ionosphere, meaning they are primarily used for line-of-sight detection or satellite communication rather than long-range over-the-horizon communication Physical Geography by PMF IAS, Earths Atmosphere, p.278. Beyond simple detection, advanced Weather RADAR is indispensable for naval safety; it monitors atmospheric changes to track tropical cyclones by measuring wind velocity and the movement of the storm, allowing vessels to steer clear of danger Physical Geography by PMF IAS, Tropical Cyclones, p.382.

Feature	RADAR	SONAR
Wave Type	Electromagnetic (Radio/Microwaves)	Acoustic (Sound Waves)
Medium	Air, Vacuum, Space	Water (Liquids)
Speed	Speed of Light (~300,000 km/s)	Speed of Sound (~1.5 km/s in water)
Primary Naval Use	Surface/Air surveillance, Navigation	Submarine detection, Seafloor mapping

One fascinating application of the RADAR principle is the Doppler Effect. If a target is moving toward the RADAR, the frequency of the reflected pulse increases; if moving away, it decreases. This allows naval commanders to instantly know not just where a ship is, but how fast it is approaching. This is similar to how aviators use instruments to detect flow patterns like jet streams to optimize their flight paths Physical Geography by PMF IAS, Jet streams, p.393.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.20; Physical Geography by PMF IAS, Earths Atmosphere, p.278; Physical Geography by PMF IAS, Tropical Cyclones, p.382; Physical Geography by PMF IAS, Jet streams, p.393

5. India's Naval Platforms and Communication (exam-level)

In the vast and often opaque environment of the ocean, naval platforms rely on specialized technologies to "see" and "talk." Because water is nearly impenetrable to light and radar, SONAR (Sound Navigation and Ranging) becomes the primary tool for underwater detection. Unlike radar, which uses electromagnetic waves, SONAR utilizes sound waves, which travel much more efficiently through water. Navigators use two main types: Active SONAR, which emits a pulse (a "ping") and listens for the echo to determine range, and Passive SONAR, which simply listens for the noises emitted by other vessels or marine life. This is why navigators, rather than other professional groups, are the primary masters of this technology for exploration and defense.

For long-distance communication above the surface, the physics of the atmosphere dictates the tools used. Radio waves, having the longest wavelengths in the electromagnetic spectrum, are essential for naval fleets. Communication often depends on the Ionosphere, a layer of the atmosphere filled with free electrons. High Frequency (HF) radio waves can be reflected back to Earth by the ionosphere—a process known as skywave propagation—allowing signals to travel far beyond the horizon. However, there are limits: frequencies higher than the "critical frequency" (like microwaves) pass right through the ionosphere into space or are absorbed, making them unsuitable for this specific type of long-range bouncing Physical Geography by PMF IAS, Earths Atmosphere, p.278-279.

To support these platforms, India maintains specialized Naval Ports. Unlike commercial ports that handle cargo, these are strategic hubs designed for warships and repair workshops. Kochi and Karwar (home to Project Seabird) are prime examples of such dedicated naval installations FUNDAMENTALS OF HUMAN GEOGRAPHY, CLASS XII, International Trade, p.76. On a global scale, the Indian Ocean is a theater of high strategic interest, with major powers maintaining naval presences through bases like the French installations at Reunion and Djibouti, or the American presence at Diego Garcia Geography of India, India–Political Aspects, p.72.

Technology	Medium/Mechanism	Primary Use
Active SONAR	Acoustic pulses (Echoes)	Mapping seafloor and locating hazards.
Passive SONAR	Listening only	Stealthy detection of enemy submarines.
Skywave Propagation	Ionospheric reflection	Long-distance naval radio communication.

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.278-279; FUNDAMENTALS OF HUMAN GEOGRAPHY, CLASS XII, International Trade, p.76; Geography of India, India–Political Aspects, p.72

6. SONAR: Mechanics and Maritime Navigation (intermediate)

SONAR (Sound Navigation and Ranging) is the underwater equivalent of Radar. While Radar uses radio waves (electromagnetic) to detect objects in the air, SONAR uses acoustic (sound) waves. This choice is based on physics: electromagnetic waves like light and radio signals are absorbed or scattered very quickly in water. In contrast, sound waves travel significantly further and more efficiently through the ocean, making them the primary tool for navigators to perceive their surroundings in the deep sea.

The technology operates through two primary modes: Active and Passive. Similar to how certain plants use active or passive methods to interact with their environment Environment by Shankar IAS Academy, Plant Diversity of India, p.198, SONAR systems vary in their engagement with the target:

Feature	Active SONAR	Passive SONAR
Mechanism	Emits an acoustic pulse ('ping') and listens for the echo.	Does not emit sound; only listens for external noises.
Primary Use	Determining exact range, bearing, and mapping the seafloor.	Stealth detection of other vessels or marine life.
Advantage	Provides precise distance calculations.	Keeps the host vessel's location hidden.

Navigators utilize SONAR to map the complex topography of the seafloor and identify underwater hazards. However, the heavy use of acoustic technology and maritime traffic contributes to noise pollution, particularly near harbors INDIA PEOPLE AND ECONOMY (NCERT 2025 ed.), Geographical Perspective on Selected Issues and Problems, p.98. While sound is essential for navigation, its intensity can disrupt the marine environment. Additionally, students of geography should distinguish this technology from the Sonar River, which is a main tributary of the Ken River in India Geography of India by Majid Husain, The Drainage System of India, p.16.

Sources: Environment by Shankar IAS Academy, Plant Diversity of India, p.198; INDIA PEOPLE AND ECONOMY (NCERT 2025 ed.), Geographical Perspective on Selected Issues and Problems, p.98; Geography of India by Majid Husain, The Drainage System of India, p.16

7. Solving the Original PYQ (exam-level)

Your understanding of ultrasonic waves and the principle of reflection (echo) is exactly what you need to solve this question. Because sound waves travel much further and more efficiently in water than electromagnetic waves—like light or radar—SONAR (Sound Navigation and Ranging) was developed as the primary tool for underwater detection. By calculating the time delay between sending a pulse and receiving its echo, users can map the seabed and identify submerged objects, directly applying the physics of wave propagation that you have just mastered.

To arrive at the correct answer, think about the specific environment where this technology is essential. While many professionals use specialized equipment, navigators on ships and submarines are the primary group who depend on SONAR for safe travel, obstacle avoidance, and exploration. It essentially acts as their "eyes" in the deep ocean where visibility is zero. This practical application in maritime transport makes (D) navigators the most logical and correct choice.

UPSC often includes lateral distractors to test your precision. For instance, while doctors use similar high-frequency sound waves, the medical application is termed ultrasound or sonography, not SONAR. Similarly, while engineers may use acoustic tools for structural testing, it is not their primary professional identifier. Avoid the trap of choosing an answer just because the technology is technically related; always look for the primary professional application as highlighted in NOAA Ocean Service.