The planned Inmarsat fourth-generation satellite system, for broadband traffic at up to 431Kbit/s, has been the subject of Requests for Proposals (for manufacturing) issued earlier this month.

Inmarsat says that companies receiving the RFPs "will certainly include" those which responded earlier to the former Horizons proposal. These were Hughes Space & Communications, Lockheed Martin and Matra Marconi Space. It is not impossible that these are the only companies involved. All have worked for Inmarsat on earlier satellite generations, and have "kept in touch" during the deliberations on Horizons, which was finally sidelined during the Inmarsat privatisation process earlier this year.

Inmarsat-4, as the new system will be known. will be complementary to the present Inmarsat-3 and will operate alongside it. Initially it will comprise two active satellites at 54 degrees.West and 64 degrees East (plus a ground spare); these will be co-located with the present INM-3 Atlantic Ocean West and Indian Ocean satellites.

The new system is to be designed as an overlay to the G3, IMT-2000 cellular telephony system now emerging from the ITU. It will cater for those regions outside G3 coverage. Typical user equipment will consist of laptop computers with an Internet-ready satellite modem. The satellites will offer three communication modes, unlike INM-3s: global beams at 39dBW receive power; 19 overlay wide spotbeams at 56dBW; and 200 narrow spotbeams at 67dBW. Data rates obtainable will vary as a function of the beamwidth and receive power; the lowest (presumably global) rate will be 144Kbit/s. By contrast, the INM-3 spotbeam rate is 64Kbit/s.