Global Positioning System Reference
In-Depth Information
This level of skill requires a map and a compass and more: a map is
useless if you do not know your position on it. Displaced birds call upon
their built-in GPS system to tell them their latitude and longitude. Latitude
is easy to gauge from the geomagnetic field because, recall, the dip angle
varies with latitude. Birds can estimate longitude in a number of ways,
such as by comparing their internal clock with local sunrise, or by listen-
ing to the earth. Huh? Some birds are sensitive to infrasound—very-low-
frequency sound waves, far below the lowest frequencies that we humans
can perceive. Naturally produced infrasound arises from microseisms , the
rumblings of our oceans. Infrasound waves travel thousands of miles, and
microseisms travel inland long distances. To a bird that can hear infra-
sound, the world is full of acoustic landmarks. The shape of continents
tunes the infrasound, making it di√erent in di√erent places, so that birds
may know their location on the earth just from the infrasound they hear.
Sensor fusion. Migrating birds in particular are good at navigating because
they use many di√erent techniques. Just as we might use GPS and back it
up with a radar reading, a celestial observation, or a sextant measurement,
so birds may use their magnetic sense and back it up with celestial observa-
tions, infrasound, or a sun compass. When I worked in the avionics indus-
try, we referred to the pooling together of navigational data from di√erent
sources as sensor fusion .
The European robin exhibits a particularly nice example of sensor fu-
sion. It probably can see the geomagnetic field, perhaps as a turquoise
shading of the sky getting deeper toward the direction of the North Pole.
Here, optical and geomagnetic information is being blended; in particular,
the geomagnetic data is being displayed in the mind's eye of the bird
(through its right eye only, researcher have discovered). This display con-
stitutes a very sophisticated system that presents complex data in a manner
that can be meaningfully grasped quickly.
There are many other examples of sensor fusion in navigating birds. For
example, pigeons and many other species are sensitive to polarized light
and to the geomagnetic field. Polarized light enables them to sense the
direction of the sun even after it has set or while under clouds. The birds
may prefer the geomagnetic sense in some circumstances and the polar-
ized light information in others. For example, if a bird suspects its mag-
netic reading are false (maybe because of a local magnetic anomaly or
because an ornithologist has placed the bird in an artificial magnetic field),
it will recalibrate the direction it senses magnetically, using the direction
 
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