Many C3I systems rely on satellites in geostationary and other near-earth orbits that are largely within the earth's magnetosphere. It has only recently been observed that the ionosphere plays a major role in determining the hot plasma environment in which these satellites must operate. Spacecraft charging, radiation effects, and degradation of sensitive surfaces used in optical and thermal control systems are attributal to the interaction of spacecraft with this ambient hot plasma, and these effects are especially serious for the newer satellite systems designed to function in orbit for a decade or longer. Recent energetic ion mass spectrometer observations have shown that the hot magnetospheric plasma in the energy range from about 100 eV to about 30 keV contains a large and variable O+ fraction which is essentially all of ionospheric origin. The other principal constituent is H+ which can be either of ionospheric or solar wind origin. The O+/H+ ratio of the plasma is an important parameter in modelling the spacecraft-plasma interactions to attempt to predict the magnitude of the above described effects. This report reviews the mass spectrometer observations by the International Sun Earth Explorer (ISEE 1) and the Spacecraft Charging at High Altitudes (SCATHA) satellites and examines the data for various signatures of the plasma sources. The relative contributions of the ionosphere and the solar wind to the plasma density are estimated in the altitude range from about 25,000 to 70,000 km on the basis of the ion composition. It is found that the ionosphere is an important or dominant source during both quiet and storm time conditions.