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New Scientific Applications with Existing CGPS Capabilities


New Scientific Applications with Existing CGPS Capabilities 

Earthquakes, Soil Moisture, and Environmental Imaging 

Andria Bilich

Geosciences Research Division

National Geodetic Survey



  • 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 

Short period

(seconds to days) 


Long period

(days to years) 

6-8 satellites 

Satellites in solution 

28+ satellites 

Every sample 

Position estimates 

1 per day 



5 minutes 

1 Hz or higher 

Sample rate 

30 seconds 

High-rate GPS 

Traditional Geodetic GPS


Denali Earthquake 
2002 November 3 

USGS fact sheet 

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





Power Spectral Maps 



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)


  • 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

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