The errors in GPS vertical measurements are not randomly
distributed, but the availability of continuous data
makes it possible to identify and correct some systematic
errors, including seasonal errors and equipment biases. At
this point in time the largest contribution in the GPS error
budget is likely to be the uncertainty in dening the reference
frame [Argus et al., 1999]. For common time periods,
the dierence between the vertical velocities is determined
to much better than 1 mm/yr after 5 years. A geodetic quality
GPS receiver costs on the order of $15,000 and usually
requires little manual intervention after installation. Data
are downloaded automatically and remotely (in general at
no cost because of prevalent internet accessibility), and the
data from one site can be handled within the infrastructure
developed to handle typical GPS network operations.