A number of industry groups are planning to construct satellite systems that will provide personal communications services (PCS's) (typically voice, data, and fax) to users over small, handheld, cellular-type telephones. Some of these systems (e.g., Iridium and Globalstar) will be constructed using satellites in low earth orbit. Two (Ellipso and ICO) will employ medium earth orbit (3-8-h period), and still others (ACeS and Agrani) will operate from geostationary altitude. The origin and

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... n of mobile satellite communications is discussed first, including the Inmarsat system and several land-based mobile satellite systems currently in operation. The rival approaches to the PCS market are then described, and some of the technical challenges inherent in each are indicated. This paper concludes with speculation on the factors influencing the likelihood of business success for the various PCS systems.