Moore

							  Toward Correcting InSAR
Images for Tropospheric Delay
 A.W. Moore, S.L. Granger, S.E. Owen, F.H. Webb,
       E.J. Fetzer, E.J. Fielding, E.F. Fishbein
       Jet Propulsion Laboratory, California Institute of Technology
                    C.F. Bjorndahl, J. Lofgren
                     Chalmers University of Technology


1. Motivation & Basic Info: InSAR, AIRS, GPS
2. Intercomparison of AIRS and GPS
3. Stretched Boundary Layer algorithm combining ECMWF and DEM
4. Selecting time periods conducive to tropospheric correction
                          Motivation: InSAR Basics
InSAR: differencing two radar images to find the phase difference, and therefore
ground displacement. Spatial density can reach 20m.
But atmospheric differences between the two images can yield up to 20cm of
differential tropospheric delay in the interferogram, obscuring the true signal.

 Idea: produce a tropo
 correction map to remove
 the atmospheric
 differences from the
 interferogram.
 Bock & Williams (1997)
 suggested use of GPS;
 limited coverage (few km+)
 has been a factor.
 Z. Li & colleagues more
 recently have investigated
 use of MERIS and MODIS
 data – 1kmX1km, but not
 at night and not in
 presence of clouds.
    AIRS: Atmospheric Infrared Sounder
• High spectral resolution IR
  sounder with ~2400 channels.
• Atmospheric profiles at high
  vertical resolution.
   – 2 km vertical resolution for water
     vapor.
• Nighttime retrievals, + in the
  presence of up to 70% cloud
  cover.
• Twice daily retrievals (ascending
  and descending nodes)
• 45km (horizontal) IWV product
• Highest-quality results not
  available in heavy rainfall
                           GPS tropo
• Temporally near-continuous
  (5min)
• Measures actual delay at L-band
• Spacing few km – 10s of km in S.
  Calif

Zenith delay is formed from all rays passing through
an inverted cone centered on the antenna. Most
variability comes from the lowest 2km of
                                                       32km
atmosphere, which implies a cone of R=16km at its
top for a cutoff angle of 7 degrees.
Most authors treat the zenith delay as a point                2km
solution directly above the antenna.
  AIRS/GPS intercomparison methodology

GPS processed with GIPSY-OASIS II in PPP mode, using JPL’s Flinn final
  precise orbit. We used a 7 degree elevation cutoff and the GMF mapping
  function. Total tropospheric zenith delay and 2 gradients were estimated as
  stochastic parameters, updated at 5-minute intervals.

GPS PWV calculated from total delay by method of Bevis, et.al. (1992) with
  surface pressure & temp from either NCEP(50km) or ECMWF (25km).




AIRS products were generated at the Goddard DAAC using the v5.0 processing
algorithm.

Comparison points limited to measurements within 25km horizontally, 30m
height, and 30minutes.
                        AIRS/GPS PWV Intercomparison

    (a)


                        (b)




  (a)
                                                                         GPS, AIRS and
(a) GPS approximate ZWD estimated from                               ECMWF daily water
GEONET GPS over Japan, January 3, 2005 (b)
                                                                      vapor over Japan,
AIRS PWV over Japan on January 3 2005
                                                                     January 2005 using
                                                                        ECMWF surface
                                                                      pressure to derive
                                                                             GPS PWV

Japan GEONET with NCEP:
•0.75 correlation coefficient over Jan 05
•Bias evident
    •Consistent with previous study Fetzer 2006
    •May change with use of absolute antenna calibrations

          To be completed with data from all seasons, and other geographic areas.
                  ECMWF + DEM + GPS
•With Chalmers students J. Lofgren & F.
Bjorndahl, JPL AIRS, GPS & InSAR                  Los Angeles
investigators                                     area
•Modulates ECMWF (25kmx25km) weather              interpolated
data by 2 arcsecond (60m) USGS National           total PWV
Elevation Database (NED) topographic data         map, 2006-
in an interpolation algorithm to form 60m-        07-08 18:00
resolution TWV maps, in the “stretched
boundary layer approach” of E. Fishbein
•Will compare the interpolated ZWD with                    ECMWF
results from available Southern California                         Boundary
GPS sites                                                          layer
                                             Boundary              contracted
•Will test a differential map as an InSAR
                                             layer
correction product                           stretched
•May use GPS data as a correction
                                                                    ECMWF
                                                                    el. over
                                                                    this
                                                                    “terrain”
        Quiet atmosphere predictor
• GPS trop+range
  differences overlaid on
  interferogram generally
  correllate
• but are too sparse to
  sample short-wavelength
  variations between
  satellite measurements
• Can we select days
  without high-frequency
  variations?
      What is noisy atmosphere

• Short-spatial-wavelength activity is associated with a
  passing front, which moves through the area
   – Other causes include
      • Winds
      • Boundary layer convection
      • Evaporation from local sources of water
• A front passing a single station will leave some
  signature in that station’s trop time series
• What can we look at in a single station’s time series
  that would indicate a passing front?
       Quiet atmosphere predictor
• Range of ZWD over the day?
• Max abs(dZWD/dt) over the
  day?
• Std. Dev. of ZWD over the
  day?
• Correlated, and vary from day
  to day
• Look at “most different day”
  time series, 11 Jan 05
            11 Jan 2005 Trop Behavior
                          After 3.5 days of
                          continuous rain, the
                          weather cleared on Jan 11




Jan 10
was La
Conchita
landslide
Jan 19 – more favorable atmosphere




         Jan 11

                      Has potential to select days without
                      quickly varying (temporally) and/or
                      short-wavelength (spatially)
             Jan 19
                      atmosphere features, such that
                      GPS can be used to run between
                      satellite data collection more
                      effectively
           Ahead/summary
• Complete GPS-AIRS comparisons using
  ECMWF pressure & temp data
• Evaluate effect of absolute antenna calibrations
  on GPS-AIRS bias
• Evaluate usage of gradients and/or slant delays
  in GPS
• GPS+MERIS+MODIS+AIRS+ECMWF+DEM=InSAR
  correction map???