Global Positioning System Reference
In-Depth Information
small deviations from the desired direction, as we can see from the figure.
You are navigator for the third ship. There is a fault with your mariner's
compass, or with the way you read it, so that, unknown to you, your
measurements of bearing have a systematic error of a quarter degree. That
is, each day your heading is further south than you think it is by a quarter
degree. You are trying to steer a course due west but end up passing
Bermuda and reaching North America at Florida. Such are the dangers of
accumulated, systematic errors.
Of course, the real world is not as simple as my simulation suggests.
Currents will vary; there will be di√erences in the magnitude of both
unbiased and systematic measurement errors; storms will disorient you
and blow you o√ course. On the other hand, you will also be able to
compare notes with other ships in your fleet for some of the time and will
be able to make better measurements or reset your dead reckoning path
integration when you make landfall, as at the Azores in my illustration.
The chip log was first introduced in the last quarter of the sixteenth
century. It replaced the older chip of wood and made the estimation of ship
speed more systematic. A standard shape evolved over the years: the log
was shaped like a slice of pie, as you can see in figure 6.13, and was weighted
on the curved surface so that this surface would be oriented to the bottom
when thrown in the water. The log was attached to a long knotted rope
which spooled out from a reel as the log receded from the moving ship. The
knots were regularly spaced; the number that were counted out in a given
time interval (measured by a sandglass) provided an estimate of speed—
hence, our term
knot
for speed measured in nautical miles per hour. While
providing better estimates of a ship's speed through the water than the old
wood-chip method, the chip log still su√ered systematic errors from, for
example, sea currents. Speed over water was well measured, but it is speed
relative to the land that matters when estimating ship movement.
Sir Francis Drake
Born a commoner, the eldest of 12 children of a laborer from a small
provincial town in southern England, Francis Drake was an unpopular
parvenu with many of his contemporaries, such as Martin Frobisher. Drake
earned his living as a privateer—a polite word for a state-licensed pirate—
for 10 years, plundering the Spanish Main on behalf of his queen, Eliza-
beth I. Returning to England from the Caribbean in 1577 (the same year
that the chip log was introduced), he was sent to the South Sea—the
