New Scientific Applications with Existing CGPS Capabilities Earthquakes, Soil Moisture, and Environmental Imaging Andria Bilich Geosciences Research Division National Geodetic Survey

Overview New uses of existing geodetic networks and stations (CORS, IGS, etc.) Earthquakes / seismograms High-rate GPS Example: 2002 Denali Fault event Soil moisture Near-field multipath Example: Uzbekistan Environmental imaging Near- and far-field multipath Examples: Mauna Kea and Canada

Earthquakes with High-Rate GPS GPS/GNSS positioning No upper limit to amplitude No preset ‘frequency response’ Positions can be computed at every data epoch, independently Precise and accurate displacements Well-defined reference frame Earthquakes Static and transient deformations Potentially large magnitude Frequencies = seconds to hours

GPS Data Rates and Analysis Strategies Traditional Geodetic GPS High-rate GPS Long period (days to years) Signal Short period (seconds to days) 30 secondsSample rate1 Hz or higher 5 minutesDecimationNone 1 per day Position estimates Every sample 28+ satellites Satellites in solution 6-8 satellites

Denali Earthquake 2002 November 3 USGS fact sheet Long strike- slip rupture Magnitude 7.9 Shallow SE directivity Large surface waves

Clipped Seismometers + 1-Hz GPS Many broadbands in western North America went off scale… … and high-rate GPS fills in the gaps

Denali GPS Seismograms 25 GPS stations 1 sample per second Different azimuths and distances

GPS-Seismometer Comparison

Take-home lessons: High-rate GPS/GNSS GPS and seismometers have complementary strengths/weaknesses Noisy GPS Off-scale seismometers Possible only through GNSS technology advances: data storage, chipsets, firmware, etc. Existing HR GPS networks expanding…

And now for something completely different…

Multipath Background What is multipath? Site-specific Time-varying Sensitive to environmental changes How can we measure multipath? Pseudorange data combination Solution residuals Signal-to-noise ratio

Signal-to-Noise Ratio (SNR) Measure of signal strength Total SNR = direct plus reflected signal(s) Direct amplitude = dominant trend Multipath signal = superimposed on direct

Soil Moisture from Near- Field Multipath Existing GPS stations! Ground reflections Amplitude attenuation at ground Soil moisture affects attenuation (reflection coefficient) Method = monitor SNR amplitude changes over time Larson et al., GPS Solutions, 2007.

Take-home lessons: Soil Moisture Possible to use existing CGPS monuments and receivers SNR always computed, sometimes reported S1,S2 = archived in RINEX Challenges and issues: SNR data quality Antenna gain pattern effects Satellite power Vegetation, temperature effects Sensing depth and footprint

Environmental Imaging with Near- & Far-field Multipath Extension of soil moisture principles… SNR data Reflection strength from multipath amplitude … plus frequency content of SNR Satellite motion creates time-varying signature   h (fast = far; slow = close) Power spectral maps Frequency and amplitude with respect to satellite position (elevation/azimuth) Projected onto map of antenna environment

Mauna Kea (MKEA), Hawaii

MKEA Power Maps Long periods at low satellite elevation angles Shorter periods at high elevation angles High power returns from cinder cones 60-90s 30-60s10-30s

Dual-Frequency Power Spectral Maps S1S2 Reflection from distant object (building?) Reflection from nearby object (rock outcrops?) Churchill (CHUR), Manitoba, Canada

Take-home lessons: Environmental Imaging Assess multipath environment Frequency: distance to object Amplitude: magnitude of errors due to object Consider position errors at different frequencies (think high-rate GPS positioning) No new equipment SNR routinely recorded … but need precise and accurate SNR related to multipath model (not always possible)

Summary Existing CGPS networks extended to unforeseen science applications Sensing soil moisture Understanding reflections and potential sources of error Measuring displacements from short-period, transient phenomena