A Comparative Analysis of the Iridium and Globalstar Satellite Transmission Paths

This is a AIR FORCE INST OF TECH WRIGHT-PATTERSONAFB OH report procured by the Pentagon and made available for public release. It has been reproduced in the best form available to the Pentagon. It is not spiral-bound, but rather assembled with Velobinding in a soft, white linen cover. The Storming Media report number is A217163. The abstract provided by the Pentagon follows: Globalstar and Iridium will provide valuable global communications assets for business, humanitarian aid and military operations. However, the level of coverage and the quality of the transmission path of these systems are strongly dependent on the latitude of the user and, due to their orbital characteristics, both systems provide reduced levels of coverage at low latitudes. Additionally, the L- and S-Band frequencies utilized by these systems are prone to ionospheric interference at low latitudes. The results of extensive simulation analysis indicate that the Globalstar constellation architecture provides a considerably better transmission path than Iridium's in several important areas, including path elevation angles, satellite visibility and susceptibility to ionospheric effects. To assist future study, a unique set of equations has been developed which describe the distribution of Iridium and Globalstar path elevation angles entirely as a function of the user's latitude. In addition to the differences in path elevation angles, modeling indicates that ionospheric scintillation is a potentially serious problem for both systems. However, Globalstar is expected to suffer lower fade levels than Iridium due to its higher downlink frequency and multiple path availability. The research concludes that, within the scope of the analysis, Globalstar provides a higher quality transmission path for low to mid-latitude users.