Title: INDOEX: Maldives KCO Meteorology GPS Column Water Vapor (Gandrud)
Contact:
Bruce Gandrud
NCAR/ATD
P.O. Box 3000
Boulder, CO 80307-3000
Email: gandrud@ucar.edu
Dataset Description:
Water vapor is not directly sensed by GPS. GPS gives the delay in travel
time, converted to distance units caused by the atmosphere. The atmosphere
delay has 2 components the dry or neutral delay - and the "wet delay" which
is additional delay caused by the bending of the GPS ray by water vapor
molecules. The neutral atmosphere accounts for over 2 meters of the atmospheric
delay but can be well modeled from surface pressure. I used the Marini-Murray
model for the neutral delay.
The amount of bending caused by a given amount of water vapor depends on the
temperature of the water vapor. KFAC is determined by the height integral of
partial vapor pressure of water over temperature squared. (ref 1)
Since the height integral of vapor pressure over temp**2 is not know KFAC
must be approximated. For U.S. continental sites an approximation based on
surface temperature is usually good to a few percent. For tropical sites it is
better to use a monthly KFAC from climatology and that is what was done for
KAAS. (ref 3)
Total delay and KFAC are given in these .TXT files which allows use of
different approaches to generating KFAC (i.e., using numerical weather model
assimilations) or modeling dry delay.
Heading descriptions:
Station name is the name assigned to the GPS site for processing.
YEAR MO DY .FRAC are the date in year, month, day, and day fraction.
PWV_MM is GPS PWV in millimeters
TOT_DEL is total zenith delay of the GPS signals.
N_COR is a PWV gradient term for the north direction
E-COR is a PWV gradient term for the east direction
ERR is a numerical sigma from the least squares estimation run, in mm.
KFAC is the scaling between wet delay and water vapor-
wet delay / KFAC = precipitable water vapor
Press is pressure in mbars
Temp is temperature in Celsius
RHUM is relative humidity.
References:
Bevis et. al. "GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using
the Global Positioning System" J Geophys. Res. Vol. 97, #D14, pp15787-15801
Rocken et. al. "GPS/STORM -GPS Sensing of Atmospheric Water Vapor for Meteorology"
Journal of Atmospheric and Oceanic Tech. Vol. 12, #3, June 1995
Ross and Rosenfeld " Estimated Mean Weighted Temperature of the Atmosphere for
GPS applications" J Geophys. Res. Vol 102, #D18, pp21719-30