Global Positioning System Reference
In-Depth Information
for stars near the horizon). She consults her almanac to find her stars (this
may sound like astrology, but it is solid science). When she finds a star at
the observed altitude, she then knows the time at Greenwich. She repeats
the procedure for another star to confirm the time estimate. Knowing
GMT and the local time at which she made her observations, she can
estimate her longitude. Latitude can be found in the same manner or, of
course, in the old-fashioned manner from Polaris.
Another way of looking at the altitude intercept method is by consider-
ing the geometry. Let us say that our navigator has shot a star and deter-
mined its altitude to be the angle
a
. In figure 7.10a you can see that if the
position of the star is known, then angle
a
limits the navigator's location to
a circle, as shown.
18
This circle is the global-scale version of an LoP. Viewed
locally it would appear as a straight line, as earlier. Our navigator shoots a
second star and determines that her position is limited to a second circle.
As shown in figure 7.10b, she now knows that her location on the surface of
the earth is at one of the two intersection points of these two circles.
Usually these intersection points are widely separated, and so it is easy to
choose which one applies. In this case, let us say, our navigator is on a boat
in the Pacific; one of the intersection points is in the Pacific Ocean whereas
the other is somewhere in western Canada, so deciding which is her true
position is not hard. If the two intersection points are close enough to-
gether to produce doubt as to which is the true position, then the navigator
will shoot a third star and produce a third circle.
Walking the Line: Continental Exploration and Navigation
An English privateer named William Dampier sailed from home to the
Pacific Ocean four times during the early 1700s; he circumnavigated the
world three times (and produced four best-selling chronicles of his jour-
neys). Sailing the world's oceans was not exactly becoming commonplace
or easy, but in Dampier's day it was becoming more common and a little
less arduous. The European maritime nations had made the seas of the
world their highways, and the tra≈c was increasing. Sea lanes brought
increased trade and spread European wars around the world; the war
18. By ''position of the star'' I mean its location at the time of observation, where time is
GMT. Every star has a position, of course, even if it is below the observer's horizon. Star
location in the night sky is specified by two angles:
declination
and
right ascension
, analogous
to latitude and longitude.
