- Page contents
- Plots of station positions
Continuous
GPS stations in California
What is GPS?
How does it work?
How
do pilots, boaters, and hikers user GPS?
Where do we work?
Plots of station positions
After some computer processing, GPS measurements produce a time
series of the positions of the stations. Plots (and listings) of
these time series can be reached from our GPS
Home Page.
Continuous GPS stations in California
The Bay Area Regional Deformation network operates about
20 continuous Global Positioning System (GPS) receivers in northern
California. A map of station locations, additional information about
BARD, and access to the data can all be obtained at the BARD
site.
The Southern California Integrated GPS Network operates about
40 continuous Global Positioning System (GPS) receivers in southern
California. A map of station locations, additional information about
SCIGN, and access to the data can all be obtained at the SCIGN
site.
What is GPS?
GPS stands for Global Positioning System. The Global Positioning
System is a group of 24 satellites orbiting the Earth twice a day
at an altitude of about 20,000 kilometers (That's about 12,000 miles).
GPS was designed by the military to locate tanks, planes, and
ships. The system has been adopted by the public for many navigation
and scientific applications. With the proper equipment you can
find your latitude, longitude, and elevation at any time and at
any place on Earth. This means no matter where you go, you can
never get lost.
How does it work?
GPS satellites continuously broadcast messages on 2 radio frequencies.
These messages contain a very accurate time signal, a rough estimate
of the satellite's position in space, and a set of coded information
that a GPS receiver can decipher.
We want to know our latitude, longitude, and elevation. The
receiver uses its internal clock and the coded information from
each GPS satellite to determine the time it took the signals to
reach the receiver. Since the signals travel at the speed of light,
the receiver can calculate the distance to each satellite.
Once the receiver knows the distances to at least 4 satellites,
and their positions, it can determine its clock correction and
position on the Earth.How does it work?
How do pilots, boaters, and hikers use GPS?
All you need is a clear view of the sky (this could be a problem
in the woods or city), and a GPS receiver. Simply turn on the receiver
and within minutes the receiver calculates your position.
Even in the worst weather conditions you can know your location
to within 100 meters (about 300 feet). That accuracy is fine for
most navigation purposes. But since the motion across faults,
such as the San Andreas, is usually less than 5 centimeters (2
inches) per year, the USGS has to use special techniques to get
much better accuracy.
How does the USGS use GPS to measure fault motion?
We want to know how stations near active faults move relative to
each other. When we occupy several stations at the same time, and
all stations observe the same satellites, the relative positions
of all the stations can be determined very precisely. Often we are
able to determine the distances between stations, even over distances
up to several 100 miles, to better than 5 millimeters (about a 1/4
of an inch).
Months or years later we occupy the same stations again. By
determining how the stations have moved we calculate how much
strain is accumulating and which faults are slipping.
Where do we work?
The USGS uses GPS to measure crustal deformation all over the United
States. However most of the work is concentrated in the western states
where most earthquakes occur and where rates of crustal deformation
are high. The processing pages contain maps with station locations.
Two location maps can be reached from the GPS
Home Page and from these, you can reach maps for individual "campaigns"
or sets of stations. |
