GPS/MET:  A NEW WAY TO MONITOR THE ATMOSPHERE

The effect of the earth's atmosphere on radio propagation may soon find 
invaluable use in tracking weather systems and looking for signs of global 
change.  UNAVCO--the University NAVSTAR Consortium, a UCAR 
program since 1991--is refining two techniques that will convert 
atmosphere-induced changes in signals from Global Positioning System 
(GPS) satellites into measurements of the meteorological quantities that 
cause those very changes.

One technique uses ground-based GPS receivers to measure atmospheric 
water vapor above the antenna.  The other will use a microsatellite, 
scheduled to be launched later this year, to receive transmissions from the 
constellation of 
24 GPS satellites.

The techniques have several appealing traits:

--  Global coverage would be provided by orbiting GPS transmitters, along 
with satellite-borne and ground-based receivers.

--  Many of the measurements can come from existing or proposed 
systems.

--  The observations would be frequent, with possibly 500 or more profiles 
a day of temperature and/or water vapor from the satellite-borne 
receivers and essentially continuous measurements from ground-based 
receivers.


-- Costs would be low, potentially orders of magnitude less than that of 
traditional methods.

--  Because of the satellite measurements' thorough coverage and 
precision, a global change signal such as increased water vapor might be 
detectable sooner than otherwise possible.

Several research centers are working on the satellite-borne receiver project 
(called GPS/MET), which studies the meteorological application (MET) of 
global positioning satellites (GPS).  Supporters are NSF, NOAA, the 
Federal Aviation Administration (FAA), and NASA headquarters.  
Scientific participants include UNAVCO, NCAR, the University of 
Arizona, and NASA's Jet Propulsion Laboratory (JPL).

The desire to map other planet's atmospheres was the genesis of 
GPS/MET.  JPL and Stanford University began using radio occultation 
techniques in the 1970s during the Mariner and, later, the Voyager 
missions to measure the atmospheric properties of Mars, Venus, and 
Neptune.  A transmitter on the far side of a planet would send a radio 
wave through the planet's atmosphere that would bend around the planet  
and travel on to the surface, where a receiver picked up the signal.  
Measurement of the travel time yielded data on how much the ray had 
been bent and/or slowed down by variations in the density of the planet's 
atmosphere.

The bending and slowing is caused by the refractivity of the atmosphere.  
Just as light rays are refracted as they enter or leave a prism, variations in 
air density and moisture content lead to the atmosphere's refractivity.  
This, in turn, affects the path and speed of a radio wave.  Instead of 
following a straight line, a radio signal will twist and bend continuously 
as it traverses a path into and out of the earth's atmosphere (see figure on 
page 1).  The delay induced by these path changes is separated into dry- 
and wet-path delays.

UNAVCO's scientists started using GPS signals in 1984 to measure tiny 
movements in the earth's crust that are results of tectonic or seismic 
activity.  The atmosphere, particularly water vapor, is a source of 
measurement noise in GPS geodesy.  Researchers learned to account for 
this noise by referring to simultaneous or archived meteorological data 
and correcting for the observed or expected conditions.  Recently 
developed software that obtains position information from GPS data 
estimates the dry and wet delays.  Once this had been mastered, the idea 
of working backwards--using the delays to retrieve the atmospheric 
information itself--came to light.  As noted in a 20 August 1993 paper in 
the Journal of Geophysical Research, "This 'noise' to the geodesist is a 
'signal' to the atmospheric scientist."

This fall, most likely in October, a spacecraft will be launched specifically 
to receive signals from presently orbiting satellites, thus providing a 
"proof-of-concept" test for GPS/MET.  The Pegasus-launched, low-earth-
orbit microsatellite is being launched in conjunction with Orbital Sciences 
Corporation.  Allen Osborne Associates has assisted in developing GPS 
receivers for the test.

The concept already has shown merit in preliminary tests.  A research 
team--led by Stanford's Lester Yuan and including UCAR president Rick 
Anthes, UCAR Office of Programs director Bill Bonner, and UNAVCO 
manager of research and development Christian Rocken--recently studied 
the possible usefulness of GPS/MET in climate-change work.  They began 
with output from a normal- and doubled-carbon dioxide simulation 
performed by Warren Washington and Jerry Meehl (Climate and Global 
Dynamics Division) using the NCAR community climate model (CCM).  
The group took those data and modeled what would have been observed 
with the GPS/MET satellite-to-satellite method.

Overall, the technique appears best at estimating water vapor change 
where temperature is fairly constant, and vice versa.  For instance, 
increased water vapor in the tropics could be measured quite easily, 
because tropical temperatures vary little.  By the same token, high-altitude 
or high-latitude temperatures could be measured with precision by 
GPS/MET, since the cold air masses in these locales hold very little 
moisture.

Another approach in looking at the earth's atmosphere might be to use 
refractivity itself, without separating out temperature and moisture.  "The 
gut reaction of many meteorologists using temperature and humidity is 
'We want these products,' " says Stick.  "And we can provide them, within 
certain parameters.  But the refractivity is a powerful product itself."

Bob Gall, Bill Kuo, and others in the Mesoscale and Microscale 
Meteorology Division (MMM) have begun to look at the usefulness of 
GPS/MET information in short-term forecasting.  MMM scientist Xiaolei 
Zou has developed a variational data assimilation designed to feed data 
into the nonhydrostatic Penn State/NCAR mesoscale model, version 5 
(MM5).  In a series of simulations, she found that using refractivity to 
construct vertical profiles of water vapor is significantly more accurate 
than using traditional techniques.  The refractivity provides useful 
temperature information where none is available, and it also yielded some 
minor improvements in the wind fields.

"GPS/MET," says Bill, "could be a very valuable data source for numerical 
weather prediction over the ocean and particularly over the Southern 
Hemisphere, where routine rawinsonde observations are limited."  As 
soon as actual GPS/MET data are available, MMM plans to launch a full 
assessment of its value in short-range numerical forecasting.

Meanwhile, NOAA is proceeding with plans to mount a set of GPS 
receivers alongside each of its 32 profiler/radiometer sites across the 
central United States.  A test last year by UNAVCO and North Carolina 
State University showed an average discrepancy of only several percent 
between the readings from radiometers and GPS signals in measuring 
water vapor in the column of air above each site.

With the potential uses of GPS/MET data so compelling, researchers are 
holding their breaths as they await the results of this fall's proof-of-
concept test.  "In the lower stratosphere," says Stick, "we should be able to 
observe variations in temperature of less than a degree Celsius."  The 
NCAR CCM work hints that the effects of climate change on water vapor 
in the tropics might be measurable with a signal-to-noise ratio of 3,000.  --
BH