Decoding Weather and Climate by Using Global Positioning System Technology
Knox, John, National Forum
The U.S. military created it decades ago, but now it is exploding onto the civilian scene with seemingly limitless applications. The Internet? No, it is global positioning system technology, better known simply as GPS. If you have heard of GPS, it is probably because you have seen a newspaper item on how it will guide cars of the future to the nearest gas station, or point recalcitrant husbands to the laundry room (see comic strip below). In this column, we will examine a more exotic, but potentially even more revolutionary application of GPS: the improvement of weather forecasts and climate observations.
WHAT IS GPS?
GPS works like this: the U.S. military has placed about two dozen satellites in orbit around the earth, roughly halfway between the surface and where weather satellites typically reside. This network of satellites routinely checks each other's positions and then transmits this information to receivers via radio waves. If a receiver intercepts signals from four or more satellites, then it can calculate its own location very accurately. How accurately? Amazingly, to within about a foot and a half, or just the size of the little saucer-shaped antenna! Little wonder, then, that everyone from aviators to seismologists to foresters is snapping up this technology; wherever precise locations matter, GPS is the wave of the future.
GPS AND WEATHER
The application of GPS to weather forecasting, like weather radar, transforms a fly-in-the-ointment into a scientific bonanza.
Radar was invented to detect enemy aircraft during World War II. However, the military soon discovered that weather conditions interfered with aircraft detection. But this wartime drawback could pay peacetime dividends, because severe weather is a mortal foe, too. Weather radar was born, and with it came the improved detection of thunderstorms and tornadoes.
In the case of GPS, the accuracy of an estimate of location depends - just as with radar - on the transmission of radio waves through the earth's atmosphere. Thunderstorms and tornadoes are invisible to GPS, but the continuous changes in atmospheric temperature, pressure, and especially moisture delay GPS signals on their route to Earth by tiny fractions of a second - just enough to reduce the accuracy of navigation.
This navigator's headache is the meteorologist's "Eureka." The delay in GPS signals can be "inverted" to infer the conditions of the atmosphere around the globe. This information is the Holy Grail of weather forecasting, especially when it comes to moisture, which is the energy source for storms. Many (if not most) of the glitches in weather forecasting today result from our imperfect knowledge of what the atmosphere is doing overhead, over the entire globe. Currently our best sources of such information are weather balloons; but few of them are launched over the oceans, and even in the United States only one balloon is launched every twelve hours at about one site per state. GPS technology promises much more continuous and global information.
GPS AND CLIMATE
The debate over "global warming" pivots on a few key issues: Is the atmosphere really warming globally? If so, will atmospheric moisture increase and wreak havoc via more severe storms? Can we trust our data sources, which often are based on a variety of incompatible methods? And do weather and climate phenomena get in the way of the reliability of the data-gathering methods?
GPS can make a solid contribution to this debate through a technique called "radio occultation," in which the radio signals are received by another satellite in a low orbit over Earth. As a global data source, GPS can avoid the bias of observing temperature only in populated regions over land. GPS is especially sensitive to water-vapor concentrations in the atmosphere, so changes can be detected rapidly and reliably. …