Geology Reference
In-Depth Information
is held fixed until the correlation between the
two signals reaches a maximum. The shift
necessary to maximize the correlation is the
time delay in the arrival of the signal from the
radio source, which is directly proportional
to the baseline distance. Today, approxi-
mately 40 VLBI antennas around the world
systematically make such measurements.
Horizontal velocity estimates with an
uncertainty of about 1 mm/yr are available for
about 60 sites, while uncertainties in the
vertical dimension are approximately two to
three times greater.
Prior to the advent of the Global Positioning
System (GPS), VLBI was a critical tool for
defining plate motions and resolving large-scale
strain gradients (Herring
et al
., 1986). Today,
with much cheaper and more widespread use of
GPS, VLBI still plays a critical role in defining
a reliable reference frame within which GPS
measurements can be interpreted.
1 cm! Because of the satellite orbital geometry,
the accuracy of GPS measurements in the
horizontal dimension will typically be several
times greater than their vertical accuracy. The
number of global navigational satellite systems
is growing. Russia, China, and the European
Union are expanding, upgrading, or developing
independent constellations of satellites that
promise to densify global coverage and
provide more geodetic capabilities over the
coming years.
Two strategies are commonly employed for
collecting geodetic data using GPS in order to
define regional strain fields at scales of ten to
several hundred kilometers. Permanent GPS
receivers continuously record positional data,
are commonly installed in broad arrays, and
offer very precise positioning. These arrays can
address problems, such as the character of
deformation in the months before or after an
earthquake, that require high geodetic precision
and a dense time series of measurements.
Permanent arrays have the drawback that each
individual receiver (they are not inexpensive!)
has to be wholly dedicated to a single measure-
ment site. Since the mid-1990s, numerous
permanent GPS arrays have been installed,
including dense networks in Taiwan and Japan,
and hundreds of more scattered permanent GPS
stations around the world. More recently, the
Plate Boundary Observatory (PBO) has installed
new sites, taken over existing arrays in southern
California (SCIGN) and the San Francisco Bay
area (BARD), and now operates about 1000 sites
in the western United States between the
Rockies, the Pacific, and Alaska. All GPS data
from the PBO are freely available on the web
(www.pboweb unavco.org) to researchers
worldwide. Unfortunately, such open-access
policies have yet to be implemented by many
other countries.
A second GPS survey strategy, known as
campaign mode
, involves periodic resurveys of
an array of GPS sites that are placed at carefully
chosen localities across a specific region. With
this strategy, numerous sites can be visited with
a small number of receivers, and the same
receivers can be used for various projects in
succession. Hence, the infrastructural costs for
Global Positioning System (GPS)
Two recent developments have revolutionized
geodetic approaches for the accurate deter-
mination of positions at the scale of hundreds
of meters to hundreds of kilometers. First, more
than 30 US Department of Defense satellites
have been launched that continuously transmit
coded radio messages that specify the time of
transmission and the satellite's position as a
function of time. This constellation of satellites
is referred to as the Global Positioning System
(GPS). Second, highly sensitive receivers
measure the transit time and phase of the radio
signal in order to determine the distance to the
transmitting satellite. Simultaneous solution of
the distance from the receiver to several (usually
four or more) satellites permits the location of
the receiver to be specified with high precision.
Given the orbital geometry and spacing of the
GPS satellites, acquisition of positional data
from four or more satellites is now usually
feasible almost anywhere in the world. Thus,
based on radio transmissions from satellites
more than 20 000 km above the Earth's surface,
this remarkable technology permits calculation
of one's horizontal location to within less than
