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====== ROI_PAC Overview ======
Here is a list of procedures you will need to perform as you produce your interferograms.
Each section corresponds to a major step involved in processing your data.
Follow the links in each numbered step to view more detailed instructions. If
you are using ROI_PAC for the first time, go to the [[install|installation
page]] to install and test the software.
===== Choosing a Region of Study =====
You first, obviously, need to locate your area of interest. You must make sure that it
is in an area that will be coherent (e.g. not a lot of vegetation or atmospheric disturbance). It is helpful to
research the geology of the area as well as what type of natural and man-made
processes have happened in the past that could have produced ground displacement. For example,
if you want to map out a landslide in an area that is tectonically active, you will want to
make sure you understand the earthquake history in the area so that you don't confuse
coseismic surface displacements with landslide displacements.
===== Obtaining SAR data =====
Next, it is time to actually download your SAR data. You will likely
need to produce many interferograms in order to fully investigate an area, and for each interferogram you will need two SAR scenes.
There are various [[sar|online catalogues]] where you can search for interferometric
pairs based on the location, date, and baseline. If your institution is a
member of the
[[sar#WInSAR|WInSAR Consortium]], you can
download SAR data for the western U.S. from their website.
===== Downloading Orbit Files =====
There exist two sources for obtaining accurate orbits for the ERS satellites. The [[http://www.deos.tudelft.nl/|Delft
Institute for Space-Oriented Space Research]] (DEOS) provides orbit
information for both [[http://www.deos.tudelft.nl/ers/precorbs/|ERS and
ENVISAT]]. An alternative source is the
[[http://earth1.esrin.esa.it/f/eeo2.400/pgersorbprc|Precise Orbit Product]]
through the [[http://www.esa.int/|European Space Agency]] (ESA). Download
instructions are posted [[orbits|here]]. This step does not have to be repeated once the orbit files are downloaded for a given scene.
===== DEM Preparation =====
In order to use the two-pass method, a Digital Elevation Model (DEM) is
necessary to subtract out the topographic contribution from the
interferogram. Global DEM data can be obtained from the [[http://srtm.usgs.gov/|Shuttle Radar Topography Mission (SRTM)]].
It can also be obtained indirectly from the SAR scenes using the three-pass
method. A detailed list of procedures that explain
how to use SRTM data can be found here: [[srtm|Using SRTM data]]. This step does not have to be repeated once the DEM is created.
===== ROI_PAC Processing =====
You are finally ready to use the program ROI_PAC to produce interferograms.
You will need to be familiar with [[:links#Unix|Unix commands]] in order to
navigate through the file system and perform tasks.
**1)** After you have downloaded your SAR data and
[[:roipac:process:process#File Management|renamed]] the files
appropriately, you will have to pre-process
that data using the [[:roipac:process:process#make_raw.pl|make_raw.pl]] script.
**2)** Create a [[:roipac:process:process#Parameter File Setup|parameter file]] that tells ROI_PAC
the location of the SAR data
files, the various processing parameters, and where to place the output files.
**3)** Finally, start the processing by running the script
[[:roipac:process:process#process_2pass.pl|process_2pass.pl]].
This perl script runs a sequence of programs
that eventually produces your differential interferogram.
===== Viewing Your Results =====
It's now time to interpret your data! ROI_PAC produces a long list of
intermediate and final [[:output|output files]]. Typically, the final product
is a geocoded unwrapped interferogram (geo_//date1-date2//.unw). An image
viewer is not distributed along with
ROI_PAC. You can
use a variety of external programs to view your results (i.e. mdx, dgx,
[[:matlab|Matlab]], GMT, ENVI, or other program that is flexible enough to read a binary
image file).
After you view your results, you may choose to re-process your data
with different initial [[:roipac:process:parameter|parameters]] or experiment with
some [[:advanced|advanced techniques]].