The goal of this project is to measure water vapor in the atmosphere using the Global Positioning System. Real time GPS orbits and RINEX data are combined with meteorological data to produce maps of Precipitable Water Vapor in real time (less than an hour). CSR has installed four antennas in Texas to augment the CORS network.
Water vapor can be calculated by estimating the delay the troposphere causes on the GPS signal. The contents of the ionosphere and troposphere cause significant delay in transmission of the GPS signal. By taking a receiver at a known location, then calculating the distance to a satellite's known location, and evaluating the measured distance to a satellite given by the antenna's measurements, a total atmospheric delay is calculated. The delay caused by the ionosphere is dispersive in nature, meaning the delay depends on the carrier frequency. Because GPS broadcasts on two separate frequencies, this delay can be easily calculated. By removing this easily calculated ionospheric delay from the total delay a GPS receiver sees, we are left with the tropospheric delay.
tropospheric delay is non-dispersive, meaning it does not depend upon the
carrier frequency. However, the tropospheric delay can be divided into a
part that consists of atmospheric conditions not having to do with water
vapor, called hydrostatic delay, and a part that is caused by the highly
varying water vapor found in the atmosphere, or the wet delay. The
hydrostatic delay is dependent on atmospheric conditions, mainly pressure,
and can easily be removed from the total tropospheric delay, leaving only
the wet delay. The delays to the individual satellites are mapped to the
direction using sophisticated mapping functions that give one value of
zenith wet delay, ZWD, for a give antenna location. Then this delay can
be converted to a measure of PWV by
using a conversion factor based mainly upon temperature that is between 6
and 6.5. The Equation is:
The RINEX data from each site is being constantly downloaded and processed. The Continuously Operating Reference Stations (CORS) network in combination with the Crustal Dynamics Data Information System (CDDIS) update hourly RINEX data for the sites used for this experiment. By combining this data that is gathered with the sites installed by the Center for Space Research, a network of 10-15 sites will be used to constantly update PWV information. GPS satellite orbit data is being obtained from the Jet Propulsion laboratory.
Data is being compiled from several different receivers. To calculate PWV, meteorological data is needed, so we are using CORS and FSL sites that have both observation and meteorological RINEX data. These sites are:
Continuously Operating Reference Stations
ARP3 - Aransas Pass, Texas
ENG1 - English Turn, Louisiana
GAL1 - Galveston, Texas
Forecast Systems Laboratory
AZCN - Aztec, New Mexico
DQUA - Dequeen, Arkansas
HKLO - Morris, Oklahoma
JTNT - Jayton, Texas
LMNO - Lamont, Oklahoma
PATT - Palestine, Texas
PRCO - Purcell, Oklahoma
SJT2 - San Angelo, Texas
TCUN - Tucumcari, New Mexico
VCIO - Vici, Oklahoma
WNFL - Winnfield, Louisiana
WSMN - White Sands, New Mexico
International GPS Service
MDO1 - Fort Davis, Texas
Center for Space Research installed sites
BRWD - Brownwood, Texas
CSR1 - Austin, Texas
LRDO - Laredo, Texas
WTFL - Wichita Falls, Texas
Bevis, M., S. Businger, T. Herring, R. Anthes, C. Rocken, R. Ware, and S. Chiswell, GPS Meteorology: Mapping Zenith Wet Delays onto Precipitable Water, J. Appl. Meteorol., 33, 379-386, 1994.
Bevis, M., S. Businger, T. Herring, C. Rocken, R. Anthes, and R. Ware, GPS Meteorology: Remote Sensing of Atmospheric Water Vapor Using the Global Positioning System. J. Geophys. Res., 97, 15,787-15,801, 1992.
Bevis, M., S. Chiswell, S. Businger, T. Herring, Y Bock, Estimating Wet Delays Using Numerical Weather Analyses and Predictions, Radio Science, 31-3, 477-487, 1996.
Brunner, F., W. Welsch, Effect of the Troposphere on GPS Measurements, GPS World, 42-51, January 1993.
Coster, A., A. Niell, F. Solheim, V. Mendes, P. Toor, R. Langley, C. Ruggles, The Westford Water Vapor Experiment: Use of GPS to Determine Total Precipitable Water Vapor.
Gregorius, T., G. Blewitt, The Effect of Weather Fronts on GPS Measurements, GPS World, 52-60, May 1998.
Niell, A., A. Coster, F. Solheim, V. Mendes, P. Toor, R. Langley, C. Ruggles, The Measurement of Water Vapor by GPS, WVR, and Radiosonde, Presented at 11th Working Meeting on European VLBI for Geodesy and Astometry at Onsala Space Observatory, 23-24 August 1996.
Nerem, R. S., Measuring Atmospheric Precipitable Water Vapor in Texas Using the Global Positioning System, Advanced Research Program/Advanced Technology Program, 1997.
Rocken, C., T. Van Hove, R. Ware, Near Real-Time GPS Sensing of Atmospheric Water Vapor, Geophys. Res. Lett. 24-24, 3221-3224, 1997.
Rocken, C., R. Ware, T. Van Hove, F. Solheim, C. Alber, J. Johnson, Sensing Atmospheric Water Vapor with the Global Positioning System, Geophys. Res. Lett., 20, 2631-2634, 1993.
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Ware, R. C. Alber, C. Rocken, F. Solheim, Sensing Integrated Water Vapor Along GPS Ray Paths, 417-419.
Yuan, L., R. Anthes, R. Ware, C. Rocken, W. Bonner, M. Bevis, and S. Businger, Sensing Climate Change Using the Global Positioning System, J. Geophys. Res., 98, 14925-14,937, 1993.
Principal Investigator: Dr. R. Steve Nerem
Research Assistant: David Whitlock
Any questions or comments regarding this page should be e-mailed to David Whitlock (firstname.lastname@example.org).