How do submarines use SONAR to navigate?

Answer 1

Contrary to popular belief, submarines don't use their main sonar systems for primary navigation (they don't have windows either), and the Sonar acronym meaning (Sound Navigation and Ranging) is actually a misnomer. However, they do use it for avoiding objects (ships and submarines), so in that sense they do use it for avoidance navigation.

Submarines use 2 types of Sonar modes - Active and Passive. Active Sonar is equivalent to Radar, only it uses sound waves rather than radio waves as the source of the transmission and echo for target range. Passive Sonar, which is used almost 99% of the time, involves simply listening with all systems for signs of ships, aircraft, or submarines (yes, we can hear planes and helos if they're close enough to the water).

Sound travels much faster in water than it does air, and travels much further as well. Active/Passive systems use a transducer array, which is a large array of devices (transducers) that convert electrical signals to sound (for active transmitting) and sound waves to electrical signals (for passive reception of sound or receiving a return echo). Passive-only arrays (towed arrays and secondary forward arrays) use hydrophones rather than transducers, which can only receive sound waves and convert them to electrical signals.

All submarines use chart, satellite, gyro, visual, radar (close to shore), and dead reckoning for primary navigation in most oceans. On rare occasions, uncharted mountains or navigational screwups have led to submarine accidents. My own boat hit an underwater mountain in the Mediterranean in 1977; the USS San Francisco hit an uncharted mountain a few years ago.

The one exception is in the Arctic Ocean, where the ice pack is in constant motion and its configuration is ever-changing. In the Arctic, for fast travel, the boat will submerge to a deeper depth than ice keels can form; for navigating through the ice pack near the surface, forward-looking and top-sounding hi-frequency sonar is used to both detect ice and determine its relative thickness. The periscope can also be raised (the water is extremely clear there) to get a visual confirmation of ice configuration prior to surfacing.

Navigating in the Arctic isn't easy. The aggravating, constant noise from the ice shifting (think about having to listen to popcorn pop for hours while trying to find a target) makes it very difficult to pick out contacts. Fortunately, there aren't any noisy biologics (fish, etc.) or shipping noise to make it worse.

The primary use for a submarine's main Sonar system is to find surface and submerged contacts. Depending on the type of boat (Attack or Missile) and its mission, a boat will either seek out those contacts or seek to avoid them altogether. It is the mission of a Fast-Attack submarine to seek and destroy ships and submarines; a Ballistic Missile submarine's mission is to remain undetected. For those like me who were Sonarmen during the height of the Cold War, being on a Fast-Attack was the only way to go if you wanted to learn your profession.

Not that submarines can't use their main sonar systems to navigate if they wanted to - they could. It can be used for navigation in shallow, close-to-shore waters, but in the deep ocean you can't due to the physical configuration of the Sonar array (more on that below). To use sonar for navigation, a boat needs to use Active Sonar. All boats use Passive Sonar exclusively for normal operations - active is only used to get a range confirmation on a target just prior to torpedo launch, or in other rare circumstances. The reason for this is that a boat's active sonar is extremely powerful, and as such can be heard by other ships and submarines for many, many miles (sound travels faster and farther in water than it does in air). Using active sonar effectively gives away your position, and in submarine warfare, he who hears the other one first, wins.

Sonar depends on a clear echo return from an active pulse for navigation. As such, the echo is dependent on the return angle of the sound back to its source. The problem in deep ocean is that the bottom is far enough away from the Sonar array that the angle at which the sound hits means that the echo will travel away from the source, not necessarily back toward it. This is why fathometer transducers are located on the bottom of the hull and slightly angled toward the bow to compensate for traveling through the water.

Even using Active Sonar for navigation close to shore isn't that easy, nor is it really that accurate. The power output is so great that the resulting echoes make for a pretty messy return image. Sure, you can adjust the power levels, but the reality of today's submarine operations is that the only time you could really use it is when transiting to and from port, and in those cases you'd be using satellite, chart, and visual aids, which are more accurate. Fathometer soundings are used regularly to correlate with chart information, and satellite fixes are checked with other systems.

In the case of using Passive Sonar, how sound travels in water is affected by temperature, salinity, and atmospheric pressure. Sound travels toward colder water, so as such tends to travel toward deep water. However, at deeper depth, pressure becomes the overriding variable and sound is forced back toward the surface. It is Sonar's job to chart the sound profile of the operating area on a regular basis so that the Sonar system can be used to full efficiency. Listening to Passive Sonar is essentially trying to pick out distinct sounds among the thousands of sounds created by biologics and environmental sources (storms, geologic sources, waves, etc.) as well as the thousands of ships and other man-made sources on the ocean or on the shore, trying to determine if the Sonar contact fits into the class of a Merchant ship, Trawler, Warship, or other type (Sailboat, Cruise ship, etc.).