COSMIC GPS ionospheric sensing and space weather

As our civilization becomes more dependent on space based technologies, we become more vulnerable to conditions in space weather. Accurate space weath er specification and forecasting require proper modeling which account for the coupling between the sun, the magnetosphere, the thermosphere, the iono sphere and the mesosphere. In spite of the tremendous advances that have be en made in understanding the physics behind different space weather phenome na, the ability to specify or predict space weather is limited due to the l ack of continuous and extensive observations in these regions. Placing a co nstellation of GPS receivers in low-Earth orbit, such as the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC), provide s an extremely powerful system for continuously and extensively measuring o ne of these regions, the ionosphere. COSMIC, by use of GPS occultations, wi ll make it possible to obtain continuous and global 3-dimensional images of electron density, irregularities and TIDs in the ionosphere and plasmasphe re. COSMIC would provide nearly 5600 globally distributed occultations per day suitable for ionospheric sensing. Occultations can be processed individ ually to obtain vertical profiles of electron density, with vertical resolu tion of similar to 1km, or collectively by means of tomography or data assi milation 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 assimilat ion. In each of these techniques, the accuracy is assessed either via exami nation of real data from GPS/MET or via simulation. Brief discussions of me asuring ionospheric irregularities and TIDs are given.