The Electronic Age - The Science of Navigation - page 213

FIGURE 8.5. The Lorenz system. Bomber aircraft were directed to a target city via two
transmitted signals of narrow beamwidth. Here beam 1 is delineated by solid lines,
and beam 2 by dashed lines. The two transmitters were located close to each other,
and the beams overlapped slightly. The transmitted signals were modulated 180 \ out
of phase so that in the overlap region, and only in this region, the receiver signal was
unmodulated. An airplane with a Lorenz receiver that produced an unmodulated
signal would therefore have been within the overlap region.
VOR systems operated by line of sight (they did not benefit from the
OTH capability of longer-wave systems, and anyway, they were generally
intended for short-range applications). They were more accurate than
NDB systems: a VOR receiver could estimate the direction of a transmitter
to within 1\. Modern versions (VOR has evolved considerably over the
decades) are less susceptible to intentional or natural sources of electronic
interference. VOR is now being phased out (another pun, I fear) in favor of
GPS navigational systems. (We saw in chapter 3 that the FAA was promi-
nent among advocates of civilian use of undegraded GPS transmissions.)
LORAN SYSTEMS
At last—an RDF system with a name that is not a TLA. In fact, the Lo ng
Ra nge N avigation system is also exceptional in another way: it bucked the
gradual trend in RDF of increasing transmitter frequencies by lowering its
frequency over the decades as it matured. By choosing a wavelength of
3 km, LORAN-C transmitters generate ground waves that are capable of
traveling 5,000 km—well over the horizon.
LORAN is an example of a hyperbolic navigation system. A forerunner of
GPS in some ways, LORAN was based on timing di√erences between
widely separated transmitters. The idea is shown in figure 8.6. Transmitters