Interferometric synthetic aperture radar (InSAR) is a powerful tool for mapping the Earth's land, ice and even the sea surface topography. It is based on processing of the pair of images to map out the differences in the reflected signals over the area (typically 100 km x 100 km). By bouncing signals from a radar satellite off the ground in successive orbits and looking at the differences between the images, interferometric synthetic aperture radar can detect small differences in the distance between its position and the ground as the land surface moves - whether up or down. A digital SAR image can be seen as a mosaic of pixels. Each pixel gives a complex number that carries amplitude and phase information about the microwave field backscattered by all the scatterers (such as rocks, buildings, vegetation) within the corresponding resolution cell projected on the ground. The amplitude depends on the roughness and typically, exposed rocks and urban areas show strong amplitudes, whereas smooth flat surfaces (like quiet water basins) show low amplitudes. The phase is directly linked to the distance between the observed terrain and the satellite sensor. By calculating the differences in phases (interferogram) between two sets of data, one can determine ground displacements that have occurred in the time between the data acquisitions. One of the popular open source and free program called Doris (The Delft object-oriented Radar Interferometric software) is developed to process data obtained from SAR systems. Doris basic input are SLC (Single Look Complex) images. Due to modular structure of program (processing is performed in steps/blocks), it is possible to write own steps of processing the data. Program is distributed with some helpful scripts and additional programs developed by community, which are compatible and ready to use with Doris. On every step of processing could be used other programs to: improve attributes of data (e.g. extra filtering), plot charts to check processed data or draw a maps. There are many possibilities for scripts wrote in Matlab or similar programs. SAGA GIS or GMT (The Generic Mapping Tools) could be used to view images or merge parts of them. All the time new programs are being developed, which may be used with Doris or its result files. There are few other programs developed for InSAR processing like ROI_PAC or PHOTOMOD Radar, but in this article processing only in Doris is presented.The processing is not simple and may take few hours. Processing was made using data from European Space Agency before and after earthquake in Bam (Iran). At the beginning of processing, data need to be read and attached with orbits of satellite from day, when images were taken. In next step data could be connected with Digital Elevation Model (DEM) and be resampled if needed. Next, master and slave images are computed to take offsets between images. After this interferogram is created. From interferogram reference phase and reference DEM are being subtracted. At the end are being created unwrapped interferogram, coherence map and geocoded interferogram, which is presented in geographical known reference system. Obtained products from Doris like unwrapped phase map could be helpful in earth science. Maps of terrain after unwrapping shows the terrain deformation with high accuracy of cm in resulution. The best results are shown using images which are taken before and after earthquake, but this is not the only use. Here should be again mentioned that data could be obtained even at night and through the clouds or snow caps, which cause problems in some areas of Earth. Plenty of data needed to interferometry processing is available for free.