In the context of the ESA BIOMASS mission in which for the first time, a P-Band SAR sensor isgoing to be mounted into a spaceborne system. With its penetration capability, it will contributeto the measurement of the biomass and carbon content in the Earth’s forests. An autofocusalgorithm is needed for the correction of phase errors introduced by the changing diffraction indexin the ionosphere. Because of the quickly changing nature of the ionosphere, defocusing has to bemeasured and corrected locally over several sections of a SAR capture.In this thesis, a deep introduction into phase errors is made having in mind that the ionosphereis expected to introduce time varying low frequency errors that can be constructed as a series ofquadratic curves. These quadratic phase errors introduce defocusing that is seen as blur and lossof contrast. An algorithm is proposed and tested for measuring this defocusing, while its strengthsand weaknesses are discussed.The idea of measuring defocusing is to try to recover the temporal phase function that introduceddefocusing in the first place. Here a method to recover this temporal phase function is introduced,and a thorough performance assessment of this retrieval is carried out. The variables involved thequality and reliability of this retrieval are studied one by one.

Measurements of soil moisture are a crucial component for understanding the global water and carbon cycle, weather forecasting, climate models, drought prediction, and agriculture production. Active and passive microwave radar instruments are currently in use for remote sensing of soil moisture. Signals of Opportunity (SoOp) based remote sensing has recently emerged as a complementary method for soil moisture remote sensing. SoOp reuses general digital communication signals allowing the reuse of allocated wireless communication signal bands for science measurements. This thesis developed a tower based SoOp instrument implementing frequencies in the P-Band and S-Band. Two field campaigns were conducted using this new instrument during the summers of 2017 and 2018 at Purdue's Agronomy Center for Research and Education.