Why are velocities and their uncertainties different
depending on the GPS data analist?
Over the past few years, great progress have been made in understanding and
mitigating error sources in GPS data. However, different studies using the
same data may still obtain significantly different estimates of velocities and
their uncertainties because of:
In addition, the NMSZ sites pose a specific problem because most of them are
installed in the Mississippi embayment unconsolidated sediments. For instance,
site MEM2 is installed on a concrete pad with no deep anchoring. However,
the University of Memphis and the University of Arkansas have made a specific
effort to install well-monumented sites using designs made to mitigate
local deformation effects. For instance, University of Memphis uses for
monuments 60' steel H-beams (10" flange, 10" web) driven to refusal into the
ground at the sites located in unconsolidated sediments within the embayment.
Two additional sites (MACC and CJTR), installed on the western edge of the
embayment, use 6-9' x 4"-diameter steel pipes directly cemented into bedrock.
- The length of the time series processed. Uncertainties decrease
with increasing time series length, there is therfore a benefit in including
as much data as possible to increase precision.
- The noise model assumed for calculating uncertainties. We know now
that noise in GPS data is best fit by a model that combines white (random)
and colored noise. The most recent results indicate that the colored noise
correspond to "flicker noise" (spectral index of 1). Colored noise should
be accounted for when estimating uncertainties.
- The geophysical models used in the GPS data processing. Most groups use
similar models. Current developments that will allow the modeling of atmospheric
and hydrological loading should improve precision in the near future.
- The strategy used to define a reference frame and the propagation of
reference frame errors into the final velocities.
- The representation of uncertainties also depends on the level of
confidence chosen -- this choice is an arbitrary decision that depends
of the "risk" that the interpreter is ready to take. The same uncertainties
shown at a 2-standard deviation confidence level (equivalent to a 5% risk
level) will look much larger than at a 1-standard deviation confidence
level (equivalent to a 61% risk level in 2-dimensions).