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Inertial Guidance

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Navigation guidance includes several subcategories. In this section there are inertial, ranging, celestial, and geophysical navigation techniques.

Inertial Navigation Guidance

Inertial navigation relies on devices onboard the missile that sense its motion and acceleration in different directions. These devices are called gyroscopes and accelerometers.


Mechanical, fiber optic, and ring laser gyroscopes
Mechanical, fiber optic, and ring laser gyroscopes

The purpose of a gyroscope is to measure angular rotation, and a number of different methods to do so have been devised. A classic mechanical gyroscope senses the stability of a mass rotating on gimbals. More recent ring laser gyros and fiber optic gyros are based on the interference between laser beams. Current advances in Micro-Electro-Mechanical Systems (MEMS) offer the potential to develop gyroscopes that are very small and inexpensive.

While gyroscopes measure angular motion, accelerometers measure linear motion. The accelerations from these devices are translated into electrical signals for processing by the missile computer autopilot. When a gyroscope and an accelerometer are combined into a single device along with a control mechanism, it is called an inertial measurement unit (IMU) or inertial navigation system (INS).


Inertial navigation concept






Inertial navigation concept

The INS uses these two devices to sense motion relative to a point of origin. Inertial navigation works by telling the missile where it is at the time of launch and how it should move in terms of both distance and rotation over the course of its flight. The missile computer uses signals from the INS to measure these motions and insure that the missile travels along its proper programmed path. Inertial navigation systems are widely used on all kinds of aerospace vehicles, including weapons, military aircraft, commercial airliners, and spacecraft. Many missiles use inertial methods for midcourse guidance, including AMRAAM, Storm Shadow, Meteor, and Tomahawk.

Ranging Navigation Guidance

Unlike inertial navigation, which is contained entirely onboard the vehicle, ranging navigation depends on external signals for guidance. The earliest form of such navigation was the use of radio beacons developed primarily for commercial air service. These beacons transmit radio signals received by an aircraft in flight. Based on the direction and strength of the signals, the plane can calculate its location relative to the beacons and navigate its way through the signals.


Global Positioning System used in ranging navigation guidance










Global Positioning System used in ranging navigation guidance

The advent of the global positions system (GPS) has largely replaced radio beacons in both military and civilian use. GPS consists of a constellation of 24 satellites in geosynchronous orbit around the Earth. If a GPS receiver on the surface of the Earth can receive signals from at least four of these satellites, it can calculate an exact three-dimensional position with great accuracy. Missiles like JSOW and the JDAM series of guided bombs make use of GPS signals to determine where they are with respect to the locations of their targets. Over the course of its flight, the weapon uses this information to send commands to control surfaces and adjust its trajectory.

Celestial Navigation Guidance

Celestial navigation is one of the earliest forms of navigation devised by humans. and it saw its greatest application in the voyages of the great maritime explorers like Christopher Columbus. Celestial navigation uses the positions of the stars to determine location, especially latitude, on the surface of the Earth. This form of navigation requires good visibility of the stars, so it is only useful at night or at very high altitude. As a result, celestial navigation is seldom applied to missiles, though it has been used on many ballistic missiles like Poseidon. The missile compares the positions of the stars to an image stored in memory to determine its flight path.

Geophysical Navigation Guidance

Perhaps even older than celestial navigation is geophysical navigation, which relies on measurements of the Earth for navigation information. Methods that fall under this category include the use of compasses and magnetometers to measure the Earth's magnetic field as well as gravitometers to measure the Earth's gravitational field.

While these methods have not found much application in missiles, a more useful technique is terrain matching. This method typically requires a radar altimeter that uses radar waves to determine height above the ground. By comparing the contours of the terrain against data stored aboard the missile, the autopilot can navigate its way to a particular location.

A related but more accurate technique is called digital scene matching. In concept, digital scene matching is little different than looking out the window of your car and using landmarks to navigate your way to a specific location. Missiles make use of this technique by comparing the image seen below the weapon to satellite or aerial photos stored in the missile computer. If the scenes do not match, the computer sends commands to control surfaces to adjust the missile's course until the images agree. Digital scene matching is used on the Tomahawk cruise missile.


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Last modified: 10/12/05.