What is a Global Positioning System (GPS) and does it work?
Todd
Todd,
GPS is a method to determine horizontal and vertical position of
locations on Earth. It uses satellites that broadcast a signal and
receivers that pick up and record the signal. Hikers can use a hand-held
GPS to find their position. Farmers here in North Dakota use GPS to
locate specific areas in their fields that need special attention.
Volcanologist use GPS as a powerful tool to detect movement at shallow
depths beneath the surface of volcanoes.
GPS uses the relationship between velocity, distance, and time (velocity
3Ddistance x time). With GPS, the velocity is the speed of light. The
signal from the satellite includes the time it was sent. The receiver
notes the time the signal arrived. The difference is the amount of time
the signal was in transit. Knowing the velocity and time (and the
approximate position of the satellite) allows the distance to be calculated.
Volcanologists use relative position, the vertical and horizontal changes
of the same location over a period of time. For example, as magma moves
up into a volcano the volcano swells at distances between points
increases (think of dots on a balloon) and the elevation at specific
points increases. Such a change is a common precursor to an eruption.
GPS is used on the Hawaiian volcanoes and several volcanoes in the
Cascades. It has also been used on volcanoes in Iceland, Italy, and Japan.
For more information on GPS read John Dvorak article in Earthquakes &
Volcanoes (1992, v. 23).
Steve Mattox, University of North Dakota
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