Home > New Scientific Applications with Existing CGPS Capabilities

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


 
 

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 

Short period

(seconds to days) 

Signal 

Long period

(days to years) 

6-8 satellites 

Satellites in solution 

28+ satellites 

Every sample 

Position estimates 

1 per day 

None 

Decimation 

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
 

60-90s 

30-60s 

10-30s


 
 

Dual-Frequency 
Power Spectral Maps 

S1 

S2 

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

Set Home | Add to Favorites

All Rights Reserved Powered by Free Document Search and Download

Copyright © 2011
This site does not host pdf,doc,ppt,xls,rtf,txt files all document are the property of their respective owners. complaint#downhi.com
TOP