Update README.md

Author: AngeliqueBenoit

README.md

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-# pyaps3
+# pyaps3
+
+PYAPS3
+
+Python 3 Atmospheric Phase Screen 
+
+This python 3 module estimates differential phase delay maps due to the stratified atmosphere for correcting radar interferograms. It is rewritten in Python 3 language from PYAPS source code and adapted for ECMWF's ERA-5 corrections. The metholody and validation of DelayPackage can be found in:
+Jolivet, R., R. Grandin, C. Lasserre, M.-P. Doin and G. Peltzer (2011), Systematic InSAR tropospheric phase delay corrections from global meteorological reanalysis data, Geophys. Res. Lett., 38, L17311, doi:10.1029/2011GL048757.
+
+Warning: PyAPS3 could not work with NARR and MERRA weather models. Feel free to modify these parts of the code.
+
+Examples in the example directory.