Experiments flown on U.S. Air Force Satellites from 1961 until 1976 have measured the 55-MeV proton flux at low altitudes, between 275 km and 600 km. The analysis of these data shows that in spite of all the uncertainties involved, the agreement obtained between the theoretical calculations and the data is quite good. We conclude that the major determining factors of the 55-MeV proton fluxes in the inner zone are a nearly constant source coupled with a solar cycle varying atmospheric ionization loss process.