As our civilization becomes more dependent on space based technologies, we become more vulnerable to conditions in space weather. A ccurate space weather specification and forecasting require proper modeling which account for the coupling between the sun, the magnetosphere, the thermosphere, the ionosphere and the mesosphere. In spite of the tremendous advances that have been made in understanding the physics behind different space weather phenomena, the ability to specify or predict space weather is limited due to the lack of continuous and extensive observations in these regions. Placing a constellation of GPS receivers in low-Earth orbit, such as the Constellation Observing System for Meterology, Ionosphere, and Climate (COSMIC), provides an extremely powerful system for continuously and extensively measuring one of these regions, the ionosphere. COSMIC, by use of GPS occultations, will make it possible to obtain continuous and global 3-dimensional images of electron density, irregularities and TIDs in the ionosphere and plasmasphere. COSMIC would provide nearly 5600 globally distributed occultations per day suitable for ionospheric sensing. Occultations can be processed individually to obtain vertical profiles of electron density, with vertical resolution of ~1km, or collectively by means of tomography or data assimilation to obtain 3-D images of electron density or irregularity structure. In this paper we describe the GPS observables for ionospheric sensing and the occultation geometry. Our presentation evolves from discussing simple to more complicated inversion techniques starting with the Abel inversion, gradient-constrained Abel inversion, tomography, and finally data assimilation. In each of these techniques, the accuracy is assessed either via examination of real data from GPS/MET or via simulation. Brief discussions of measuring ionospheric irregularities and TIDs are given.