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