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01/21/92

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Method to study other planets could offer new view of EArth

Radio astronomers have constructed detailed profiles of five of the planets in the solar system, mapping temperature, density and pressure at various elevations. None of those five planets is Earth.

Now, a team of Stanford and Lockheed scientists proposes taking one of the techniques they've perfected on Venus and Jupiter and bringing it down to Earth to study the home planet.

The technique is radio occultation - the study of how radio waves, beamed from a spacecraft, travel through a planet's atmosphere, bending and changing speed and in the process revealing details about the atmosphere's composition.

Radio occultation studies of Earth's atmosphere could enhance knowledge of global climate change, provide insight into the problem of ozone depletion and improve the daily weather forecast.

Such studies of Earth are economically feasible for the first time, now that the U.S. military's satellite system, the NAVSTAR Global Positioning System is fully operational, Stanford senior research scientist David Hinson told a session of the American Geophysical Union's winter meeting in San Francisco last month.

Hinson, along with Stanford Prof. Leonard Tyler and senior research associate Ivan Linscott, and colleagues Ken Hardy and Yam Chiu at the Lockheed Palo Alto Research Laboratory, have proposed such a study to the Air Force Office of Scientific Research.

Stanford scientists have spent more than 25 years perfecting radio occultation methods in outer space. Their next challenge may be to explore this planet's unique and highly variable atmosphere.

The first planetary radio occultation discovery was made in 1965, when Stanford electrical engineering Prof. Von Eshleman measured the atmosphere of Mars, finding it to be less than 1 percent the pressure of Earth's atmosphere. Since then, radio astronomers from Stanford and elsewhere have used radio waves transmitted by spacecraft, such as the planetary explorer Voyager 1, to study the atmospheres of Venus, Jupiter, Saturn, Uranus and Neptune.

"Occult" is derived from the Latin for "to hide," and that is the idea behind radio occultation. In planetary experiments, a spacecraft transmitting radio waves to Earth flies on a trajectory that carries it into hiding behind the planet under study.

The radio waves from the spacecraft cut a slice through the planet's atmosphere - from its outer edge to the very surface atmosphere - as the spacecraft circles around and disappears behind the planet.

The signal strength and frequency of the radio waves vary as they pass through the planet's atmosphere, according to variations in the atmosphere's density. Scientists interpret these variations, then apply laws of atmospheric physics and thermodynamics to determine the distant atmosphere's density, temperature and pressure as a function of altitude.

The result is a vertical profile of a slice of the atmosphere.

Because radio occultation can measure temperature within a fraction of a degree, it could aid in understanding critical climatic and atmospheric questions confronting Earth.

While meteorologists have the tools to predict short- term weather changes in the lower atmosphere, there is no sure means so far of determining the rate, causes and consequences of global warming or ozone depletion. Radio occultation could be one of the critical tools for grappling with these questions, because it can be used to gather detailed, comprehensive information about the three-dimensional atmosphere and its behavior.

The technique could provide a vertical series of nearly simultaneous readings at different altitudes, one kilometer apart, on a global scale.

The usual global meteorological tools - radiometers and spectrometers carried by satellites - look down through the atmosphere and cannot distinguish between one layer and the one above it. They show broad aerial views, spanning hundreds of miles in all four compass directions.

Weather balloons provide accurate vertical profiles, but they are few and far between: For example, there are only 10 to 20 balloons covering the air over all the world's oceans. This leaves a virtual data desert for 70 percent of the Earth's atmosphere above the oceans.

While there is general (although not universal) consensus that the surface temperature of the Earth is increasing, there are disagreements over the cause and rate of warming. The differences of opinion are partly due to the many variables that affect surface temperature readings. For instance, many temperature records have been compiled in cities; but since cities can heat up their local environments, those records neither prove a global warming trend nor indicate its cause.

According to meteorologist Ken Hardy, though, if global warming is due to the greenhouse effect, then the warming of the lower atmosphere will be accompanied by a cooling of the upper atmosphere. Radio occultation is uniquely capable of such high altitude temperature readings, reaching up nearly 40 miles. Most standard readings reach no higher than a dozen miles.

Extensive systematic measurements taken over a sustained period, including readings at remote locations and altitudes, could "make a difference between determining trends in five years or 20 years," Hinson said.

Hinson, a specialist in atmospheric waves, said that radio occultation studies of such waves could aid our understanding of ozone depletion. The waves transport man-made chemicals from the lower atmosphere up into the stratosphere. At a certain point, the waves break into random motion, mixing the man-made chemicals with ozone. Some of these chemicals react with ozone, breaking it apart and depleting the Earth's shield against ultraviolet radiation.

These destructive chemical reactions are temperature dependent. Understanding the dynamic mixing caused by atmospheric waves in the stratosphere, as well as temperature changes at these altitudes, may provide insight into the process and causes of ozone depletion.

The measurements provided by radio occultation using the Global Positioning System (GPS) satellite network also could improve weather prediction. Forecasts often rely on computer programs that put together the physical laws for atmospheric behavior with data from satellites and weather balloons. While the program's predictions are initially accurate, they usually break down after a couple of days. With more, and more accurate, data on a global scale, weather prediction would have a longer shelf life.

In order for radio occultation to work, both the signal generator and the receiver must be outside the atmosphere being studied. To study distant planets, this means the receiver can be on Earth. However, to study Earth, the receiver itself also must be in orbit. The need for two satellites is what has made such studies discouragingly expensive. With a GPS satellite as a signal source, 13,000 miles up, half the problem is solved - all that is needed is a receiving satellite.

Hinson, Tyler and their colleagues propose placing a compact, lightweight receiver in a lower, faster orbit around Earth. This receiver could even hitch a ride on an existing communications, weather or defense satellite in low Earth-orbit. Each time the receiving satellite would "rise" or "set," as seen from any one of the GPS satellites, an occultation measurement could be made, profiling Earth's atmosphere from top to bottom in about one minute.

The 16 GPS satellites launched since 1978 provide complete global coverage, each emitting a continuous radio signal 24 hours a day. By 1993, there are plans for 24 GPS satellites in orbit.

The positioning system was conceived of as a military tool. It gave U.S. forces in the Gulf War accuracy within a fraction of an inch. On the ground, receivers the size of a cigar box picked up radio signals from four satellites simultaneously, and used the information to pinpoint location - latitude, longitude and altitude - for ground troops, ships, planes and missiles.

Industry and science have picked up quickly on peaceful applications of the system's potential. Its radio signals have already been used by surveyors, polar explorers, railroad engineers and geologists tracking earthquake activity.

The American Geophysical Union session brought together more than 40 scientists interested in radio occultation studies of Earth. Represented at the session were scientists from France and the former Soviet Union, from Lockheed's Palo Alto Research Laboratory and NASA's Jet Propulsion Laboratory.

JPL has put forward a similar proposal, which would place a receiver on a NASA Earth Observing System satellite late in the decade. Connections made and information exchanged at the AGU session may aid both efforts, Hinson and Tyler said.

Meanwhile, Stanford researchers have not abandoned their exploration of more distant planets. Tyler and senior research associate Richard Simpson are currently analyzing data from the Magellan spacecraft mapping the surface of Venus.

Tyler also heads the radio science team of NASA's upcoming "Mars Observer" mission. The Mars team, which includes Hinson, will acquire eight to 10 occultation readings a day of the Red Planet, for a two-year period beginning in 1993. These studies will provide in-depth information about Martian seasonal cycles and may provide insights about climate change on Earth.

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