Adventures in Geodesy, Seismology, and Hydrology

Abstract:

Over a decade ago I began working on developing methods so that GPS could be used to measure ground displacements during large earthquakes. At the time, geodesists estimated receiver coordinates once per day, as this is entirely adequate for tectonic applications. Along with everyone else, I ignored multipath - the error caused by signals that reflect off the land surface. My group quickly realized that multipath was the largest error source in GPS seismology and developed tools to mitigate its impact. At some point I decided to try and measure those reflections on purpose – leading to my more recent work in hydrogeodesy, where GPS reflection data are used to measure soil moisture, snow depth, and vegetation water content. I had the good fortune to start this initiative at about the same time that the NSF EarthScope Plate Boundary Observatory (PBO) was built, meaning the GPS reflection technique could be economically applied on a large scale. PBO H2O (http://xenon.colorado.edu/portal) was subsequently developed so that water cycle products developed from PBO data could be made available to the public in a timely manner. Thanks to excellent archive centers and the support of many station operators, we are now able to provide 10-year time series of environmental parameters for many PBO sites. We have since extended the reflection technique to measure sea level and the cryosphere. As GPS reflections are based on data collected by geodetic-quality receivers, a GPS tide gauge record can be directly linked to ITRF. Recently John Wahr showed that combining GPS coordinate solutions and reflections provides sensitivity to firn density of the Greenland Ice Sheet. Given the expansion of continuously-operating GPS (and GNSS) networks around the world and the cost-effectiveness of using this infrastructure in lieu of purpose-built networks, I see great opportunities for this methodology in the future.