In August, a group of scientists at the Scripps Institution of Oceanography reported that the severe drought gripping the western United States in recent years is causing a “uplift” in the western United States.
About the same time, governmental agencies were reporting widespread cases of land subsidence in California’s central San Joaquin Valley caused by overpumping of water from wells there.
According to the Scripps researchers, the largest uplift of the tectonic plate — that piece of the Earth’s crust on which the western states rest — amounted to only 15 millimeters (about half an inch) in California’s mountains and an average of four millimeters (0.15 of an inch) across the West.
How did they do it? How did they detect so precisely such a small movement over thousands of square miles of terrain?
From analysis of massive sets of data accumulated over the past 11 years from high-precision GNSS monitoring stations scattered across the region. Indeed, GNSS technology has created a figurative common ground where space science meets Earth science, which in its larger scope encompasses the field of geodesy: the measuring and monitoring of the size and shape of our planet.
Finding out how that has come about led us to Gerald Mader, chief of the Geosciences Research Division of the National Geodetic Survey (NGS), an office of the U.S. National Oceanic and Atmospheric Administration (NOAA). Mader joined the NGS in 1983 and has seen GNSS grow from an obscure military program into, among other things, a crucial tool for modern geodesy.
Mader is a coauthor of the original RINEX format, a cofounder of the International GNSS Service, and the developer of NGS’ antenna calibration program. He has also written and supervised NGS’s GPS software for precise static and kinematic positioning.
NGS is the oldest scientific agency in the nation, descended from the Survey of the Coast, founded in 1807 by President Jefferson. Through both research and development, NGS focuses on defining and delivering to the public the National Spatial Reference System (NSRS). A good example of its activities is OPUS, NGS’s Online Positioning User Service, now referred to as OPUS-S. Mader promoted development of this web-based service that provides users submitting GPS static positioning data with NSRS coordinates accurate to a few centimeters with which to improve their positions.
A few years ago, he launched the Kinematic GPS Challenge, an effort to attract volunteers to help NGS facilitate development of its GPS processing method for GRAV-D (Gravity for the Redefinition of the American Vertical Datum). Interested researchers and companies from around the world were invited to compute and submit position solutions from samples of actual GRAV-D data.
High-precision GNSS accuracy has made possible, or greatly enhanced, a variety of remote sensing applications, including photogrammetry, LIDAR mapping, synthetic aperture radar, and airborne gravity. Road grading, precision agriculture, and monitoring the stability of buildings, bridges, and dams are part of a very long list of applications facilitated by GNSS.
“Just about any type of platform that moves, whether on the surface of the earth, in air or in orbit, has had GNSS attached to it to precisely measure its motion,” says Mader.