Home
Generic Atmospheric Correction Online Service for InSAR (GACOS) is a online service which provide InSAR phase delay images of D-InSAR correction. It use GPS and atmospheric data for calculate delay due to instability. For more details about GACOS go through the references. This is a workflow to use this data product for phase delay correction using this data.
To install this package from R Studio:
CRAN: install.packages("GInSARCorW") or
Github:
install.packages("devtools")
devtools::install_github("SubhadipDatta/GInSARCorW")
Sentinel-1A unwrap phase from SNAPHU is used as an example. Due to DEM error the output is not fully corrected.
GACOS product have 2 file one binary file (.ztd) contained the image and a .ztd.rsc meta file. Here we import the path of meta file.
i1m<-system.file("td","20170317.ztd.rsc",package = "GInSARCorW") # GACOS phase delay image of time one (T1).
i2m<-system.file("td","20170410.ztd.rsc",package = "GInSARCorW") # GACOS phase delay image of time two (T2).
Here ZTD images are imported from the meta files.
GACOS_ZTD_T1<-GACOS.Import(i1m,F) # Import T1 phase delay.
GACOS_ZTD_T2<-GACOS.Import(i2m,F) # Import T2 phase delay.
Sentinel 1A unwrapped & coherence image images are imported here. To avoid the CRS problem use ReadDIM package to import Sentinel 1 images from snap output. Phase delay between the time calculated in the next step.
unw_pha<-raster(system.file("td","Unw_Phase_ifg_17Mar2017_10Apr2017_VV.img",package = "GInSARCorW")) # Unwrap phase image of sentinel-1(rectified). crs(unw_pha)<-CRS("+proj=longlat +datum=WGS84 +no_defs")
coh_band<-raster(system.file("td","coh_IW2_VV_17Mar2017_10Apr2017.img",package = "GInSARCorW")) # Coherence image of sentinel-1(rectified).
crs(coh_band)<-CRS("+proj=longlat +datum=WGS84 +no_defs")
dztd<-d.ztd(GACOS_ZTD_T1,GACOS_ZTD_T2)
The GACOS phase delay images are resampled here to unwrapped image resolution and extent.\
re_dztd<-d.ztd.resample(unw_pha,dztd)
In this step we run the correction for atmospheric phase delay. We used the latitude and longitude of highest coherence value as a reference point but it need a Ground point as on the original documentation. You have to rectify unwrapped image for this process, we aren't calculating displacement in the Line of sight direction so we used 90 deg as incident angle. 0.055463 is the wavelength of the Sentinel 1 in meter.
coh_band<-setMinMax(coh_band)
mcoh<-coh_band==maxValue(coh_band)
mcoh[mcoh==0]<-NA
mcoh<-rasterToPoints(mcoh,spatial = F)
unw_phase<-GACOS.PhCor(unw_pha,re_dztd,0.055463,inc_ang=90,ref_lat=mcoh[2],ref_lon=mcoh[1])
Ground displacement from phase calculated here. you can calculate this in meter, centimetre or milometer.
dispm<-Phase.to.disp(unw_phase,0.055463,unit="m",0)
TO masked out areas with low coherence use this funtion.
cohm<-coh.mask(dispm,coh_band,threshold=0.4)
- http://ceg-research.ncl.ac.uk/v2/gacos/
- Yu, C., Li, Z., Penna, N. T., & Crippa, P. (2018). Generic atmospheric correction model for Interferometric Synthetic Aperture Radar observations. Journal of Geophysical Research: Solid Earth, 123. https://doi.org/10.1029/2017JB015305. [link]
- Yu, C., Li, Z., and Penna, N. T. (2018), Interferometric synthetic aperture radar atmospheric correction using a GPS-based iterative tropospheric decomposition model, Remote Sens. Environ., doi:10.1016/j.rse.2017.10.038. [link]
- Yu, C., Penna, N. T., and Li, Z. (2017), Generation of real-time mode high-resolution water vapor fields from GPS observations, J. Geophys. Res. Atmos., 122, 2008–2025, doi:10.1002/2016JD025753. [Link]
- https://cran.r-project.org/web/packages/GInSARCorW/GInSARCorW.pdf
** If you found any bugs in this please write in the issues section.