Climate has been changing dramatically over the past several decades. Terrestrial snow and polar ice sheets have been studied intensively as indicators of climate change. The following research supports two major objectives. The first objective is to use a new microwave remote sensing technique, P-band GNSS-SAR interferometry, to characterize the Snow Water Equivalent of Terrestrial. The second objective is to assist in estimating polar ice sheet mass balance using active and passive microwave remote sensing data. To support the GNSS-SAR remote sensing of terrestrial snow, my research focused on simulating the P-band near specular bistatic scattering coefficients of mountainous areas. Given that reliable measurement of the near specular scattering coefficients of land surface in the P-band Signal of Opportunity concept will only be available in the future, simulation work is currently the only way to understand the near specular bistatic scattering in the P-band. The bistatic scattering coefficient of variance fields, denoted by γ_v, is calculated at various scattering azimuth angles. Simulations using AKS show that the γ_v can exceed 10 dB across a range of azimuth angles, ϕ_s. The values are much larger than those of radar backscattering, suggesting potential support for employing a Synthetic Aperture Radar (SAR) concept based on Signals of Opportunity, particularly with data acquisition near the forward direction. The much stronger surface scattering ability loosens the requirements of receiving antenna gain. Large swath sensing of terrestrial snow is thus possible. Two subtopics are covered in my research to support the mass balance study. The first subtopic involved the density variation properties in the dry zone, while the second subtopic focused on the modeling work for the perennial firn aquifer. Fluctuation of firn density near the surface is a major uncertainty in characterizing mass balance. Previous research has shown that firn density profiles can be represented using three processes: “long” and “short” length scale density variations and “refrozen layers”. My research shows that the short and long-scale firn processes can be modeled as 3D continuous random medium with finite vertical and horizontal correlation lengths. I also showed that there are refrozen layers in the firn, the number of which can be determined by radar echograms. The density parameters used for the long-scale profile to match the UWBRAD brightness temperature measurements are consistent with those from CFM modeling. Our model predictions also explain SMOS's V and H-pol multi-angle measurements at Dome-C, Antarctica. This work demonstrates that co-located active and passive microwave measurements can infer polar firn properties, which are important in characterizing the mass balance of the polar ice sheet. In my research, a Full wave simulation approach at the L-band was used to characterize the effective permittivity as a function of liquid water content. At the same time, a radiative transfer model was implemented to relate the brightness temperature observed by SMAP with the liquid water content in the firn aquifer. Bi-continuous media-modeled aquifer structures show a different permittivity prediction from the classical mixing formulas. A radiative transfer model based on 3D density characterization explains the V/H pol data with a single set of parameters. The modeling work will help characterize liquid water content in firn aquifer and the hydrology study in the polar ice sheets. Eventually, the research will benefit the evaluation of the effects of aquifers on ice sheet mass balance.