Global Positioning System Reference
In-Depth Information
have a radio receiver that is sensitive to the transmitted frequency. Given
this simple equipment, accurate LoPs were obtained in the early decades of
the radio age via NDBs for ships and airplanes. If the radio waves were
long, so that they propagated over the horizon, the LoPs traveled along
great circle routes, so a navigator had to allow for this when he drew the
LoPs on his chart. Of course, nowadays computer software can take care of
such details and draw the LoPs on an electronic map on a display screen for
whatever map projection is used. For shorter ranges—say, of a higher-
frequency radio beacon at a harbor entrance transmitting line-of-sight
and received by an approaching ship—such earth curvature e√ects are
unimportant.
Several LoPs drawn from di√erent NDBs, all at known locations marked
on a navigator's chart, permit a position fix. In a military context, the
advantage of the old NDBs was that they did not give away any information
about the location of the navigator because their signal could be received
from any direction relative to the transmitter. All of the direction-finding
information was gleaned locally by the navigator operating the receiver.
These early RDF devices were better than older methods of obtaining a
position fix because they were easy to use, as we are about to see, and
they worked in most weather conditions and during hours of darkness. A
2-kW transmitter could be detected from a distance of 140 km (75 nauti-
cal miles).
Both specially constructed towers and existing lighthouses supported
NDB transmitters to aid inland and coastal navigation. Each would have its
own unique frequency or Morse code identifier. The Cunard Line of British
shipping was the first to use radio beacons for navigation, in 1911.
8
Thir-
teen years later, airborne navigators were employing RDF. In the United
States, commercial radio stations acted as NDBs, each transmitting its
unique radio station identifier once per hour for the benefit of airborne or
marine navigators. Many NDB transmitter networks were constructed
around the world in the 1920s and 1930s, particularly along coastlines; a
few still exist.
One disadvantage of early NDBs is that they were not particularly accu-
rate. I can illustrate why with a simple made-up example of a RDF receiver.
In figure 8.4a we have a radio receiver that consists of three antennas ar-
ranged on a wheel. The average power received from the NDB transmitter
8. The earliest systems placed the direction-finding receivers on shore. An incoming
ship would transmit a signal and its direction to a transmitter—say, at a harbor entrance—
and the coastal receiver station would then radio back to the ship.
