London headquartered Inmarsat, the International Maritime Satellite Organisation, has narrowed the choice of system for its proposed global hand-held satellite telephone service, Inmarsat P. Inmarsat has rejected the low earth orbit option favoured by its arch-rival in the embryonic business, Motorola Inc’s Iridium Inc, and will now choose between a geostationary or intermediate orbit system. According to the director general of Inmarsat, the low earth orbit alternative has been dropped because of a combination of costs, the degree of phone user co-operation required with different orbit configurations, and the relative complexities and risks relating to the production, implementation and on-orbit management of a large number of satellites – Motorola originally thought it would need 77 for global coverage, hence the Iridium name, but has now decided it can get by with 66. Inmarsat says that its decision was based on market, technical and financial appraisals of the system concepts and spacecraft. These incorporated technical studies by spacecraft manufacturers under contract to the organisation. Of the remaining choices, a geostationary satellite system would be the conservative’s choice. Essentially an enhanced version of the systems currently used by Inmarsat and most other satellite operators, this would require a small number of large satellites to be placed in orbit over the equator at an altitude of about 22,500 miles to provide global coverage. The intermediate orbit system would require a constellation of satellites, possibly up to 15, moving in inclined orbital planes to provide the desired global coverage. These satellites would be at an altitude of around 6,250 miles. The maritime satellite body says it is now embarking on a final selection phase between the two options. According to a spokeswoman, a decision will be made in February 1994.
