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Introduction
Water vapor is a highly variable atmospheric constituent. It is fundamental
to the transfer of energy in the atmosphere and in the formation and propagation
of weather. Yet water vapor remains one of the most poorly characterized
meteorological parameters. Improved knowledge of the water vapor field
is needed for a variety of atmospheric research applications and for improved
weather forecasting. Methods for sensing precipitable
water vapor (PWV) with the Global
Positioning System (GPS) promise improvement in short-term forecasting.
To use PWV in forecasting it must be available close to real-time. Cosmic's Ground Based (COSMIC Ground Based GPS) GPS Research Group is analyzing GPS data from the
SuomiNet Network and the NOAA
Forecast Systems Laboratory GPS network and GPS data from CORS DOT NDGPS and USCG/USACE DGPS sites
that have co-located surface met. sensors.
Processing Description
At present we process the GPS data in 1-hour segments. Once every hour
our process transfers via ftp 1 hour of raw GPS data from the NOAA
Forecast Systems Laboratory data hub. This one hour segment is translated
from raw data to RINEX format using TEQC.
Binex data from SoumiNet is sent to COSMIC via the IDD/LDM system developed by UNIDATA. Hourly RINEX files
from the CORS network are ftp'd from the NGS CORS data centers.
One-hour files are processed, with the Bernese software developed
at the University of Berne
Astronomical Institute. For the estimation of PWV we constrain the
station coordinates tightly to positions obtained from processing daily
solutions for the networks. The normal equations (NEQ) file from
the 1-hour analysis is stored. Then we stack the NEQ files from the last
12 1-hour solutions to obtain the equivalent of a 12-hour GPS solution.
The main reason to apply this stacking technique, rather than re-processing
many hours of data every hour is the significant savings in CPU time. This
results in shorter latency with modest requirements in computing power.
Plots of the latest results are available about 1-hour after real time,
currently the biggest factor in getting some of the GPS data
TEC analysis is done at zero-difference level with 1-hour phase files
from all stations in the NOAA/FSL network. We compute an independent
solution each hour and do not require continuity between the hourly TEC solutions.
The software for real-time processing is operated by the Bernese Processing Engine (BPE),
developed jointly by the University of Bern and the UCAR GPS Research Group. Processing is
currently done with ultra rapid predicted GPS orbits from the IGS. These are formed by combining
2 hour orbit solutions + 24 hour predictions from major IGS orbit centers. The BPE operation for
real-time GPS PWV processing was implemented by Teresa Van Hove, who is also responsible for the
ongoing automated process. We are using ocean loading correction models provided by Chalmers
Centre for Astrophysics and Space Science.
Please go to http://www.oso.chalmers.se/~loading/index.html
for experiments on the effects of ocean tidal loading on estimated PWV.
GPS Data
We are presently, on an experimental basis, processing data from about 60
SuomiNet Network sites, and about 40 Sites operated
by the NOAA Forecast Systems
Laboratory. There are also about 60 USCG/ACE and DOT DGPS sites that are part of the Continuously Operating Reference
System (CORS) managed by NOAA's NGS.
Also 4 Scign sites, 1 site operated by Univ of Utah, and 1 USNO site.
All of these sites are using precision survey quality dual-frequency GPS receivers and antennas.
Most have co-located surface met packs, although a few of the CORS sites are using surface met data from nearby ASOS sites.
The Suomi network and FSL network use TrimbleTM Receivers and Antennae; the USCG/ACE and DoT sites are using AshtechTM Receivers and antennae.
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