Global Positioning System Reference
In-Depth Information
They arose in World War II to guide German rockets to their target cities
and have developed markedly since then. By the late 1950s, IN helped
guide a U.S. submarine beneath the polar ice cap. The developments in IN
sophistication have been theoretical as well as material. That is, engineers'
understanding of how best to process the input data of an IN system has
developed in tandem with improvements in accelerometer and gyro ac-
curacy and robustness. Algorithms such as Kalman filters integrate exter-
nal and internal data in a manner that has been shown to minimize track
error. More than that: it has been shown that when such filters are applied
to IN data, the resulting errors are stable—they do not grow without limit
as they did for the old-fashioned dead reckoning methods.
The development of IN systems over the past 60 years—a pointer to the
future—has seen them evolve from large electromechanical instruments
held in low-friction gimbals (so that they are insulated from unwanted
shocks and vibrations) to small electronic or electro-optic devices that
are much more robust and so can be strapped down to a platform—any
platform. The robustness and small size of modern IN components have
greatly increased their applicability. A trend of the past 20 years is likely to
continue: the increasing use of external data and the integration of many
di√erent forms of remote-sensing data (GPS, radar, radio altimeter, infra-
red, sonar, and so on). Inertial navigation devices and other navigation in-
struments are merging into larger automatic navigation units, with an in-
creasing number of sources of sensor data input and therefore an increasing
capability for autonomous operation in many and variable environments.
6
Radio Direction Finding (RDF)
Radio direction finding is almost as old as radio itself. Radio waves were dis-
covered in the last decades of the nineteenth century. They have a much
longer wavelength than other kinds of electromagnetic waves, such as mi-
crowaves, light waves, infrared radiation, X-rays, and gamma rays. Whereas
visible light has wavelengths of about 500 nanometers (nm)—so that 2,000
waves would extend a distance of 1 mm—radio waves have wavelengths
that vary from somewhat less than a meter to hundreds of kilometers. The
utility of radio waves is that they can convey information as a result of
amplitude modulation (AM) or frequency modulation (FM). Radio waves
6. For a good technical (though nonmathematical) review of the first 40 years of IN
development, see King (1998).
