The Electronic Age - The Science of Navigation - page 223

Charts and maps can be used in conjunction with radars in another way.
A map that is printed on a transparent sheet can be laid on top of a suitably
scaled radar screen which displays, let us say, a group of islands. When the
map and display image coincide, then the pilot or navigator can readily
transfer his location (the center of the radar display) onto the map. This
plot-overlaying positioning technique is known as Fischer plotting .
More sophisticated overlaying of di√erent data goes by the fancy name
(in military circles) of sensor fusion . Data from di√erent electronic sen-
sors can be downloaded directly to a radar display and placed upon it. Thus,
for example, magnetic bearing from a fluxgate compass can be displayed
on a screen, or data from LORAN or GPS can be overlaid. High-end ra-
dars may have a database of cartographic information from a GIS that is
placed on a screen and updated as the coverage area being displayed on the
screen changes. The database may show coastlines, for example, and these
will confirm that certain of the radar signals on the screen correspond to
land clutter, and that others correspond to known electronic beacons or
reflectors. 19
Radar leads the way in terms of electronic remote sensing and ship-
borne (and airborne) navigation equipment. Pilots depend on electronic
navigation equipment as they never have before. They have learned to trust
it because it has become so sophisticated and generally reliable, to the
extent that it has rendered earlier mechanical or optical equipment redun-
dant, except as a backup and for hobbyists. Yet, over the past decade we are
seeing that radar itself is beginning to be overtaken by another form of
electronic navigation—GPS. There are advantages of radar that will pre-
vent its total eclipse, however. In certain circumstances navigation in-
volves more than knowing your own position; for example, in a crowded
harbor entrance in poor visibility, it is useful to also know the positions of
everyone else. 20
19. The use of radar for maritime piloting is well explained in chapter 13 of Bowditch's
The American Practical Navigator , a long-running U.S. government marine navigation guide.
See www.irbs.com/bowditch.
20. An anecdote that I have told elsewhere and often, which I first heard in 1990 (before
GPS), involves an overreliance on newfangled electronic harbor navigation equipment. A
Danish merchant marine captain told me (in a pub on the Orkney Islands o√ northern
Scotland) of a sea trial of new equipment which, it was claimed, enabled a pilot to bring his
boat into harbor without the need for any visual cues. A trial ensued in which the pilot was
below deck, staring at a radar display—his only source of information about his external
surroundings—and attempting to bring his boat to dock. In fact, he crashed the boat into a