UCAR > Communications > Staff Notes > May 1995 Search

Early Signals from GPS/MET Are Looking Good

After less than two years from program start, the payload for the global sensing project known as GPS/MET (meteorological applications of the U.S. Air Force's Global Positioning System satellites) is now in orbit. The GPS/MET receiver, no larger than a shoe box, is circling the earth every 100 minutes aboard the MicroLab-1 satellite. As it orbits, the receiver picks up signals transmitted from 24 GPS satellites. Approximately 500 times per day, the ray path between the receiver and one of the GPS satellites passes through the earth's atmosphere. These events--known as radio occultations--provide a way of taking soundings of the atmosphere. The GPS/MET investigators are betting that these signals will provide a new way to obtain vertical profiles of temperature, moisture, and other products with improved resolution and frequency.

While it'll be months before enough data have been received and analyzed to confirm the hypothesis, the experiment seems to be on the right track. The satellite's launch--aboard a Pegasus rocket flown from California's Vandenberg Air Force Base on 3 April--went flawlessly. "We got an excellent ride. It was the best Pegasus launch to date," reports GPS/MET project manager Mike Exner from his UCAR North office. "We achieved almost precisely the orbit we wanted."

Photo of plane bearing GPS/MET 
This Lockheed L1011 jet carried a Pegasus rocket to its launch above Vandenberg Air Force Base on 3 April. In turn, the rocket sent a MicroLab-1 satellite bearing the GPS/MET receiver into orbit. (Photo courtesy Orbital Sciences Corporation.)

Thirteen days after the launch, the GPS/MET payload was turned on for the first time, and within half an hour it was tracking GPS satellites. Since then, the payload has been producing around 25 megabytes of data per day, including hundreds of occultations. "The spacecraft, payload, and the infrastructure we use to talk to it are all healthy," says Mike. "We are now in the process of tuning the flight code and configurable parameters to optimize performance. On 26 April, we completed our first inversion, and the results are excellent." Click here to see results.

Atmospheric soundings are derived from the GPS signals by measuring the excess propagation delay caused by the atmosphere. Using the GPS in the same way geodetic scientists use it to detect minute tectonic plate movements, the precise location of the satellites is determined to very high accuracy. Using the speed of light, the excess propagation delay is converted to excess distance. By subtracting the known distance between the satellites from the apparent distance observed by the receiver when the ray path passes through the atmosphere, the GPS/MET team retrieves an atmospheric signal. The apparent distance is always longer due to bending of the ray path caused by gradients in the density of the atmosphere. The excess distance is then inverted (manipulated mathematically) to obtain pressure, temperature, moisture, and other products.

Although the GPS/MET plan was based on the assumption that only setting occultations would be observed (ones where the transmitting satellite is setting behind the earth with respect to the receiver), early data indicate that the receiver may produce usable data from rising occultations, too. This could double the number of soundings otherwise available.

GPS/MET development has been based at UCAR with $3.3 million in support over three years from the NSF, NOAA, the Federal Aviation Administration, and NASA. Major scientific partners include NCAR; UCAR's University NAVSTAR Consortium (UNAVCO); the University of Arizona, heavily involved in data analysis; and the Jet Propulsion Laboratory (JPL), where signal analysis techniques similar to those used in GPS/MET were developed in the 1970s to investigate the atmospheres of Mars, Venus, and Neptune. Principal investigators, along with Mike, are Randolph "Stick" Ware, UNAVCO director; Christian Rocken, UNAVCO's director of research and development; Ben Hermann, University of Arizona; Bill Kuo, NCAR Mesoscale and Microscale Meteorology Division (MMM); and Thomas Meehan, JPL.

Photo of GPS/MET staff
Key members of the GPS/MET team: (left to right) Stick Ware and Chris Rocken, UNAVCO; Mike Exner and Suze Adams, of the UCAR- based GPS/MET office; Xiaolei Zou, Doug Hunt, and Bill Kuo, all from MMM; and Rick McCloskey and Bill Schreiner, GPS/MET. (Photo by Curt Zukosky.)

Another key player is Orbital Sciences Corporation, which holds commercial rights to the GPS/MET data by virtue of owning the MicroLab-1 satellite and the Pegasus rocket. UCAR's contract with Orbital gives UCAR exclusive rights to the data for scientific uses. UCAR is providing the data to other scientists around the world via the World Wide Web. All data distributed by UCAR will be restricted to noncommercial scientific use for the first 100 days after collection.

The promise of GPS/MET data already has piqued considerable interest among the research community. The project's office now has 18 signed agreements for data exchange covering more than 50 investigators. Their interests, according to Mike, are fairly evenly divided among climate and global change, ionospheric studies, and weather and forecasting. "Once people see the actual data, they'll probably use GPS/MET for a whole lot of things," he adds. Among the parties involved are JPL; NOAA's Forecast Systems Laboratory and Office of Oceanic and Atmospheric Research; NASA's Goddard, Marshall, and Johnson Space Flight Centers; and a number of universities in China, North America, and Europe. Two visitors from the Russian Institute of Atmospheric Physics, Mikhail Gorbunov and Sergei Sokolovskiy, will be at UCAR in May and June to investigate the GPS/MET possibilities.

Scientists at MMM are ready to incorporate refractivity profiles into version 5 of its Penn State/NCAR mesoscale model. Before that can happen, the first data sets must be validated by comparing them with data collected by radiosondes, aircraft, and conventional satellites.

The next few months will be a time of teeth gnashing at GPS/MET as the data stream expands and the concept undergoes its most rigorous testing. The first attempts at creating full-resolution moisture and temperature profiles are expected to take place this month. Watch the Science Briefing column in Staff Notes Monthly for follow-up reports.

Putting the receiver in orbit was a big milestone, says Mike. Getting confirmation that the receiver works well was another big step. And the successful inversion on 26 April was, Mike says, "the icing on the cake." Still, he adds, "as soon as the satellite is launched, you realize you have another immediate crisis on your hands--a limited amount of time to get as much data as you can. The launch was really the trigger for us to shift into high gear." --BH


Radiosondes--the instrument packages sent upward by balloon twice each day at approximately 1,000 locations worldwide--have endured through more than 50 years as the primary way to sense the atmosphere's vertical structure. GPS/MET will give the venerable instrument a run for its money, though. The system performs each of its vertical scans in only a minute or two, compared to around 100 minutes for a radiosonde. The latter's ascent ends at 25 to 30 kilometers while GPS/MET may glean useful data from as high as 60 kilometers. The GPS/MET temperature errors should run less than 1 degree C over most of a vertical profile, about half the potential error of radiosondes. And GPS/MET is unhindered by oceans or other settings where regular balloon launches are hard to implement. Although radiosondes have the advantage of making direct temperature and humidity measurements, some studies show that the indirect readings derived from GPS/MET may work as well as radiosonde data within the gridded data bases that are fed into computer models. If projections prove accurate, a constellation of GPS/MET satellites could provide global coverage for a fraction of the cost of today's radiosonde network.

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Edited by Bob Henson, bhenson@ucar.edu
Last revised: Wed Mar 29 15:37:06 MST 2000