5.4 Navigation Guidance
Like homing guidance, navigation guidance includes
several subcategories. In this section, we will describe inertial, ranging,
celestial, and geophysical navigation techniques.
5.4.1 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.
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.
5.4.2 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.
Global Positioning System used in ranging navigation guidance
5.4.3
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.
6.0 REFERENCES
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