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