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GCN : February 2014
WHILE NECESSITY may be the mother of invention, tight budgets are the mother of innovative repurposing of technologies. Scientists at NASA s Jet Propulsion Lab, the National Oceanic and Atmospheric Ad- ministration and the Scripps Institution of Oceanogra- phy have teamed up to take advantage of a network of hundreds of GPS stations in Southern California, origi- nally installed to measure tectonic movements and to monitor and predict hazard- ous events such as earth- quakes and flash floods. While the scientific-level GPS stations bounce signals o satellites to record very small changes in location, those signals can also be used to measure water vapor in the atmosphere. "A GPS receiver fundamen- tally is measuring the amount of time it takes signals to travel from the GPS satel- lites to the receiving antenna on the ground," explained Angelyn W. Moore, a scientist on JPL s Geodynamics and Space Geodesy Group. "That travel time is modified by the amount of water vapor in the atmosphere. The upshot is that whenever we measure a geodetic-quality GPS station s position, we are also measur- ing the delay due to water va- por. That delay can be related to precipitable water vapor with a surface pressure and temperature measurement." When this moisture data is combined with data from barometers and thermom- eters, it can give forecasters greater accuracy in predicting rainfall and flash floods. "We are evaluating hardware to provide water vapor [data] at 5 minutes or less at a test site, and plan to install that at approximately 25 sites," she added. While forecasters are cur- rently accessing the system s water-vapor estimates via a Web interface, said Moore, "We are pursuing integration into their standard forecaster displays." According to Moore, the network in Southern Califor- nia consists of 475 GPS sta- tions, about 175 of which are operating in real time and 17 of which have been equipped with accelerometers. While GPS measurements can be used to measure large move- ments during an earthquake, accelerometers can measure smaller movements. More importantly, accelerometers can measure primary waves, or P-waves, which can be help seismologists predict the arrival of the secondary waves, which signal the phase of violent shaking during an earthquake. Since P-waves move through the earth faster than S-waves, data from P- waves could be used to provide an early-warning system. While Moore doesn t know of imminent plans to expand the system, she says doing so would o er important data. "Certainly the spatial extent can be extended," she said. "Existing real-time GPS sta- tions tend to be located in Cali- fornia more than in Arizona, so north is the most obvious opportunistic direction at present." That would enable use of the GPS water vapor [data] for other weather conditions such as atmospheric rivers." Atmospheric rivers are rela- tively narrow regions in the atmosphere in which relatively large amounts of water vapor are transported horizontally. According to NOAA, "While ARs come in many shapes and sizes, those that contain the largest amounts of water va- por, the strongest winds, and stall over watersheds vulner- able to flooding, can create extreme rainfall and floods." In fact, NOAA already has several hundred GPS- equipped weather stations scattered throughout the na- tion. Although the density of that network is not su cient for the kinds of system being developed by the JPL/NOAA/ Scripps team, they could be incorporated into an expand- ing network. More traditional applica- tions of GPS tracking are also being considered for predict- ing weather-related disasters. At the University of Hawaii and NOAA s Pacific Tsunami Warning Center, scientists are exploring the possibility of setting up a GPS tracking network on commercial ship- ping fleets. One of the Center s research vessels apparently detected an earthquake re- lated tsunami o the Chilean Coast in 2010. Now some scientists believe a network of shipboard GPS sensors could have led to earlier warnings of the devastating Indian Ocean tsunami of 2014, according to GPS News. • REPURPOSING TECHNOLOGY: CAN GPS BE USED TO FORECAST RAINFALL AND FLASH FLOODS? BY PATRICK MARSHALL EMERGING TECH GPS signal delays can be related to precipitable water vapor. Data on that delay, when combined with information from barometers and thermometers, can give forecasters greater accuracy in predicting rainfall. 34 GCN FEBRUARY 2014 • GCN.COM