Introduction - Ubiquitous Positioning - page 7

which is unique in that it preserves direction so that bearings taken from a map
can be used directly to navigate, but this came at the expense of distorting greatly
the size and shape of anything near the poles. We are fortunate today to be able to
use really simple interactive Web-based map tools that allow a much better visual
understanding of geography.
Measuring the angle above the horizon of the sun at noon or a polar star at
night gave an accurate reading for latitude but finding longitude was much more
difficult. Originally, the only method was to measure progress by the technique of
dead reckoning that involved estimating speed and direction. The traditional
method was by timing and counting the passage of knots tied at regular distances
in a line attached to a float thrown overboard. It remained nearly stationary and
thus pulled the line along at the same speed as the vessel. As a voyage progressed,
the accuracy of the estimate was diluted because errors accumulated. The
fortunate navigator would be able to correct the positional estimate when the
lookout spotted a way point that was recognizable. The estimate plotted on the
map would be corrected and the process started afresh. Dead reckoning is still
used as the basis of many navigational systems and has the advantage of not
relying on any external measurements. The technique of map matching to correct
positional estimates is also still in use in systems as cheap as road vehicle
navigational devices and as expensive as cruise missiles.
A big step forward occurred with the invention of the chronometer [1], which
allowed navigators to have constant accurate knowledge of time. The difference
between noon, as measured from the time of the sun reaching its zenith (the
highest point in the sky) and that shown on the chronometer gives the longitude
(by the simple calculation that one hour [positive] difference is equivalent to 15
degrees of longitude [east]. The Greenwich Meridian in East London was
established as the international datum of zero degrees (see Figure 1.1). By the age
of the steamship, nonelectronic navigation had matured to the point where
navigation was reasonably accurate provided the sky was visible often enough.
National mapping agencies were established. For example, the British
Ordnance Survey started its work at the time of the Napoleonic wars at the end of
the 18th century. National agencies used accurate observations, measurements,
mathematics, and cartography to map everywhere of economic or military
significance. Maps also included topography (i.e., were three-dimensional
surfaces) with land heights and sea depths represented in various ways.
Mapmaking is still very important, except it is now highly automated and includes
aerial photography from aircraft, satellite imaging, and both radio and laser
ranging.
The next revolution in navigation came with the invention of wireless radio
communications. It was soon found that radio could be used in two very useful
ways: first, as a method to pass information over very long distances and second,
as a sensing system to detect and guide aircraft and ships. Radio and optical
systems use electromagnetic waves that travel at light speed and are easily
reflected off many surfaces, especially metals. Primary radar was developed in the