The GPS receiver at ARM1 -- at the CART site -- was nearly colocated
with an AERI instrument which measures precipitable water from the
bottom up (i.e., the surface). Thus, it is possible to compare
the sounding, GPS and AERI data for that time. A graph showing it is
below.
Note that the GPS is consistently drier than the AERI data. This
relationship is constant at other sites as well.