# Presentation: 227595

## 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

- Motivation & Basic Info: InSAR, AIRS, GPS
- Intercomparison of AIRS and GPS
- Stretched Boundary Layer algorithm combining ECMWF and DEM
- 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

- 32km

- 2km

- Zenith delay is formed from all rays passing through an inverted cone centered on the antenna.  Most variability comes from the lowest 2km of 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 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

- GPS, AIRS and ECMWF daily water vapor over Japan, January 2005 using ECMWF surface pressure to derive GPS PWV

- (a)

- (a) GPS approximate ZWD estimated from GEONET GPS over Japan, January 3, 2005 (b) AIRS PWV over Japan on January 3 2005

- (b)

- (a)

- 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 investigators
- Modulates ECMWF (25kmx25km) weather data by 2 arcsecond (60m) USGS National Elevation Database (NED) topographic data in an interpolation algorithm to form 60m-resolution TWV maps, in the “stretched boundary layer approach” of E. Fishbein
- Will compare the interpolated ZWD with results from available Southern California GPS sites
- Will test a differential map as an InSAR correction product
- May use GPS data as a correction

*Los Angeles area interpolated total PWV map, 2006-07-08 18:00*

- ECMWF el. over this “terrain”

- ECMWF

## 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

- Has potential to select days without quickly varying (temporally) and/or short-wavelength (spatially) atmosphere features, such that GPS can be used to run between satellite data collection more effectively

- Jan 11

- Jan 19

## 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???