Abstract:
The University NAVSTAR Consortium (UNAVCO) home page on the WWW is
http://www.unavco.org/facility/ The Global Positioning System Meteorology Experiment (GPS/MET) home
page on the WWW is
http://facility.unavco.org/data/data.html#search.
The GPS/MET experiment is a $4 million proof-of-concept program, which
has been jointly sponsored by the National Science Foundation (NSF),
the National Oceanic
... and Atmospheric Administration (NOAA), the
Federal Aviation Administration (FAA), and the National Aeronautical
and Space Administration (NASA). The program is being conducted by
scientists at the University Corporation for Atmospheric Research
(UCAR), the University Navstar Consortium (UNAVCO), the National
Center for Atmospheric Research (NCAR), the Jet Propulsion Laboratory
(JPL), and the University of Arizona.
The private sector is also participating in GPS/MET. Orbital Sciences
Corporation (OSC) provided launch and spacecraft services. In
addition, Allen Osborne Associates, Inc. supported development of the
special receiver needed to meet the high-accuracy science requirements
for GPS sounding. The GPS/MET receiver is one of two payloads that was
carried into a Low Earth Orbit (LEO) on OSC's MicroLab-1
satellite. MicroLab-1 is one of a three satellite payload which was
launched by a Pegasus rocket on April 3, 1995.
The objective of the GPS/MET experiment is to use signals from GPS
satellites occluded by the Earth to demonstrate active limb sounding
of the atmosphere. The experiment is observing more than one hundred
60-to-100-second-long globally distributed occultations of GPS signals
every day. Dual-frequency GPS occultations are extracted from these
signals by the orbiting receiver and sent to a ground station where
they are processed into different levels of data products. These
GPS/MET data products are available to the scientific community on a
timely basis for weather, climate, and other scientific research.
The following data products are available via FTP or the WWW to
registered users in the scientific community:
+ Level 0 Data Products;
1) Science data
2) Immediate payload health data
3) Spacecraft attitude/ephemeris data
These data are in their raw form straight from the satellite
operations control center. Most users find it easier to work with
level 1 data.
+ Level 1 Data Products;
1) 50, 1, and 0.1 Hz GPS observations
2) Corresponding 1 Second and 30 Second GPS Fiducial observations
+ Level 2 Data Products;
1) Precise Orbit Files for the MicroLab and the GPS satellites
2) Excess phase profiles for individual occultations
3) Metadata on these occultations
+ Level 3 Data Products;
1) Refractivity Profiles
+ Level 4 Data Products;
1) Temperature Profiles
Theoretical studies and preliminary results from the GPS/MET
experiment have shown that accurate profiles of refractivity 'N' can
be obtained using the radio occultation method from an altitude of
about 50 km extending downward close to the surface. Refractivity 'N'
is a function of temperature, pressure, and water vapor. With
independent estimates of either temperature or water vapor, it is
straightforward to compute the other variable. 
