Update Readme.md

Readme.md

 # Rugged 
 
-**A sensor-to-terrain mapping tool**
+_**A sensor-to-terrain mapping tool**_
 
-Rugged is a free java library for geolocation and used for satellite imagery.
+**Rugged is a free java library for geolocation and used for satellite imagery.**
 
 ![rugged-logo.png](src/site/resources/images/rugged-logo.png)
 
 
+Rugged is an add-on for [Orekit](https://www.orekit.org/) handling Digital Elevation Models contribution to 
+line of sight computation. It is a free software intermediate-level library written in Java.
+
+It mainly provides direct and inverse location, i.e. it allows to compute accurately 
+which ground point is looked at from a specific pixel in a spacecraft instrument, 
+and conversely which pixel will see a specified ground point. This mapping between 
+ground and sensor is computed with a viewing model taking into account:
+* ground Digital Elevation Model (DEM), 
+* Earth rotation will all its tiny irregularities, 
+* on-board sensor pixels individual line-of-sights, 
+* spacecraft motion and attitude,
+* several physical effects.
+
+Direct and inverse location can be used to perform full ortho-rectification of 
+images and correlation between sensors observing the same area.
+
+Homepage: [https://www.orekit.org/rugged/](https://www.orekit.org/rugged/)
+
+
+
+
 * [Overview](src/site/markdown/index.md)  
 * [Getting the sources](src/site/markdown/sources.md)
 * [Building](src/site/markdown/building.md)