Page 58 - Laboratory Manual in Physical Geology

ACTIVITY outdoor location, then the receiver immediately acquires
(picks up) the radio channel of the strongest signal it can
2.1 Plate Motion Inquiry Using detect from a GPS satellite. It downloads the navigational
GPS Time-Series information from that satellite channel, followed by a
second, third, and so on. A receiver must acquire and pro-
cess radio transmissions from at least four GPS satellites
THINK | Is the lithosphere beneath your feet to triangulate a determination of its exact position and

About It really moving? elevation—this is known as a fix. But a fix based on more
than four satellites is more accurate. In North America
OBJECTIVE Use NASA GPS (Global Positioning and Hawaii, the accuracy of the GPS constellation is also
System) time series to determine the vector direction augmented by WAAS (Wide Area Augmentation System)
and rate of lithospheric plate motion where you live. satellites operated by the Federal Aviation Administration.
WAAS uses ground-based reference stations to measure
PROCEDURES
small variations in GPS satellites signals and correct them.
1. Before you begin , read about the GPS time series The corrections are transmitted up to geostationary WAAS
below. Also, this is what you will need to do the satellites, which broadcast the corrections back to WAAS-
activity: enabled GPS receivers on Earth.
____ calculator, ruler
____ protractor (optional), cut from GeoTools GPS Accuracy
Sheet 4 at the back of the manual . The more channels a GPS receiver has, the faster and more
____ Activity 2.1 Worksheets (pp. 55 – 56 ) and pencil accurately it can process data from the most satellites. Most
2. Answer every question on the worksheet in a 12-channel GPS receivers are accurate to within 9 meters
way that makes sense to you and be prepared to of your precise location. Comparable WAAS-enabled GPS
compare your work and inferences with others. receivers are accurate to within 3 meters. The best GPS
receivers have millimeter accuracy and can be used to
measure things like the movement of lithospheric plates
over years of time.

Using Gps to Study Lithospheric
GPS—Global Positioning System Plate Motion

The Global Positioning System (GPS) is a technology NASA compiles geodetic information from receivers
used to make precise (exact) and accurate (error free) located at more than 2000 GPS reference stations through-
measurements of the location of points on Earth. It is out the world. At each GPS station, there is a rigid GPS
used for geodesy—the science of measuring changes monument (concrete and steel structure) attached firmly
in Earth’s size and shape, and the position of objects, to bedrock (solid rock that is beneath the soil and a part of
over time. the lithosphere) or a building that is anchored in bedrock.
The GPS technology is based on a constellation A GPS receiver antenna is firmly attached to the monu-
of about 30 satellites that take just 12 hours to orbit ment, so it will not move unless its bedrock anchor moves.
Earth. They are organized among six circular orbits The bedrock may move in response to volcanic activity or
(20,200 km, or 12,625 mi above Earth) so that a mini- earthquakes. It will also move as the lithospheric plate (that
mum of six satellites will be in view to users anywhere it is a part of) moves.
in the world at any time. The GPS constellation is
managed by the United States Air Force for operations GPS Time Series
of the Department of Defense, but it is free for anyone The exact location and elevation of each NASA GPS
to use anywhere in the world. Billions of people rely on station has been monitored over time, as a time series
GPS daily. (a series of observations made over time, FIGURE 2.3 ), by
the California Institute of Technology’s Jet Propulsion
How GPS Works Laboratory (under NASA contract). Because each GPS
Each GPS satellite communicates simultaneously with station is anchored in bedrock of the lithosphere, the time
fixed ground-based Earth stations and other GPS satel- series data provide data on movement (if any) of the litho-
lites, so it knows exactly where it is located relative to the spheric plate to which it is attached. Plate motion is deter-
center of Earth and Universal Time Coordinated (UTC, mined using one graph of time series data for how the GPS
also called Greenwich Mean Time). Each GPS satellite station changed its latitude (position north or south) and
also transmits its own radio signal on a different channel, another graph of time series data for how the GPS station
which can be detected by a fixed or handheld GPS receiver. changed its longitude (position east or west). The average
If you turn on a handheld GPS receiver in an unobstructed rates of motion, in mm/yr or cm/yr, are provided with the

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