METEOROLOGISTS take "the broad view" on any new development: "How will this affect us," they tend to ask. So it was that when the Global Positioning System, or the GPS, came into widespread use, they wondered how it might be used for weather forecasting. And it looks like being a very handy tool indeed.
The GPS is based on a global array of 21 low flying satellites, and allows anyone equipped with a suitable receiver to read off his or her position anywhere in the world to within an inch or two. The system is based on the normally correct assumption that each of the 21 satellites, in continuous contact with base, knows exactly where it is in space at any instant.
Moreover, each satellite transmits frequent radio pulses at precisely predetermined times, so a receiver equipped with an accurate clock can calculate how long it takes each pulse to reach it. Since the speed of the signal is known, elementary algebra provides the distance of the receiver from the satellite, and simultaneous readings from several different spacecraft allow the receiver to pinpoint its position on the surface of the earth exactly.
Now, it is known that the earth's atmosphere delays the radio signals very slightly on their journey. There are three main components to this delay those which occur as the signal passes through the electrically charged ionosphere in the very high atmosphere, retardation by the "dry" atmosphere itself, and a delay caused by the water vapour in the lower atmosphere, or troposphere.
When the GPS is used in standard mode as a positioning system, these three sources of error are corrected for, using standard techniques. But some meteorologist somewhere had the bright idea of not correcting for the moisture in the troposphere. A GPS receiver on the ground used in this fashion will therefore read a position that is false by several centimetres, with the size of the error being a measure of the amount of water vapour in the troposphere above it. And since the receiver's true location is already known, this error can be identified precisely.
It is hoped that a network of GPS receivers used like this can estimate the "precipitable water vapour" over a large area - what might be called "the potential amount of rain". If this information is then fed into the computer models for predicting the weather, a significant improvement in the accuracy of the resulting forecast is expected.