Submarines must navigate through the water virtually blind since light does not penetrate very far into the water. However, submarines are equipped with nautical charts and sophisticated navigational equipment. When on the surface, a sophisticated global positioning system (GPS) accurately determines latitude and longitude, but this system cannot work when the submarine is submerged. So, how does a submarine find its location or navigate underwater?
What is inertial navigation?
Submarines carry an inertial navigation system (INS), which measures the boat’s motion and constantly updates position. Because it does not rely on radio signals or celestial sightings, it allows the submarine to navigate while remaining hidden under the surface.
INS a self-contained navigation technique in which measurements provided by accelerometers and gyroscopes are used to track the position and orientation of an object relative to a known starting point, orientation, and velocity. It is navigational systems capable of calculating position, either close to some reference system/point or absolute coordinates and composed of at least three gyros and three accelerometers, enabling the system to derive a navigation solution. This navigation solution contains at least the position (normally latitude, longitude)—most of the INS’s output heading, pitch, and roll. The heart of an INS system is its inertial measurement unit (IMU). This mechanism is composed of three orthogonal gyros and three orthogonal accelerometers.
Fiber Optic Gyroscopes are the latest inertial sensors that have replaced the traditional mechanical gyroscopes, owing to promising advantages like no moving parts.
The inertial guidance systems are accurate to 150 hours of operation and must be realigned by other surface-dependent navigational systems. To maintain accuracy, the submarine must periodically update its position using outside navigational radio signals. From the 1960s to the 1990s, transit satellites and LORAN shore stations provided those signals. GPS has now replaced both.
The Dead Reckoning Track (DRT) and the SINS (Ships Internal Navigation System)
Submarine gets a GPS posit before it dives and plots that latitude/longitude position on a paper chart, then every course or speed change is added to the chart to plot the DRT. This method requires some attention to detail to be accurate but being off a few yards is no big deal in the open ocean. To verify the chart, grab a printout from the SINS console and update the chart (at least once every watch).
The SINS system is a computer, a very accurate laser gyroscope and a satellite communication system. The radio receiver gets a posit from the GPS that is sent to the computer. The gyro detects any sub’s motion is heading, speed, depth, roll, pitch sent to the computer to update the lat/long and to store until the navigation team requests it or other ship weapon systems need accurate position info. A laser gyro is VERY accurate, to within +/- seconds of error in a day. Over a week without a satellite update, the system will be off by just a few yards.
What is Bottom-Contour Navigation?
“Bottom-Contour Navigation used specific bottom-contour charts and sonar to fix a ship’s position. These were very highly measured areas, and they were run across several times and fixed position with soundings.
Submarine navigation systems are mainly based on accurate low drift inertial navigation systems (INS) designed to sustain submerged operation for long periods of time. Regardless of velocity aiding sensor and INS quality, the position accuracy will eventually decrease necessitating position updates.
Check out Naval Library App to find out the specifications of all types of submarines.