Graham Warwick/WASHINTON DC

Iridium's mobile communications network went live on 1 November, but aviation users will have a wait a few months before they can access the satellite system.

AlliedSignal Aerospace plans to begin shipments of its Iridium-based Airsat-1 aeronautical satellite-communications (satcom) system on 1 March next year. Deliveries of provisioning kits will begin on 15 December, allowing customers to begin using the Iridium system as soon as they receive the equipment.

Redmond, Oregon-based AlliedSignal is the exclusive provider of the single channel Airsat system for the business and general aviation, air transport and military markets. The company is also the exclusive service provider for the business and general aviation market.

"We are making satcom affordable for the first time," says AlliedSignal's Iridium marketing manager, Micel Gelinas. "The 15lb [6.8kg] system will fit in any aircraft." The company sees a chance to equip some 17,000 turboprops and jets, and Gelinas sees markets for low-cost satcom in Latin America and Africa, as well as in the USA.

Iridium provides the first competition for the Inmarsat aeronautical satcom system, which is well established with international airlines, providing passenger, airline operational and air traffic control communications. Inmarsat uses a few geostationary satellites, but Iridium is a constellation of 66 spacecraft in low earth orbit. The closer proximity of the Iridium L-band satellites allows the use of lighter equipment with less power.

The Iridium Airsat system, therefore, is lighter and cheaper than the Inmarsat Aero-H equipment carried by airliners and larger business jets. AlliedSignal says it will be competitive with the new Aero-I system, which exploits the spot-beam capability of the latest Inmarsat 3 satellites to allow use of smaller and cheaper satcom antennas and avionics. The first Aero-I systems have just been certificated.

Airsat-1 is a voice-only system initially, says AlliedSignal, as the Iridium network will not be able to support data communications until the middle of next year. The company still has to develop an interface unit, and expects to be able to offer 2,400-baud PC data and facsimile service by the end of 1999.

The single channel Airsat-1 costs $29,500, and AlliedSignal has advance orders for over 740 systems from distributors. Call costs are expected initially to be $3.50/min for domestic calls (to/from a number in the country over which the aircraft is flying), and $5.50/min for international calls. This is more expensive than terrestrial flight-telephone system services, which now cost around $2/min, but the Iridium system is available globally - even over the poles. AlliedSignal expects call costs to come down as utilisation and competition increases.

The company is also developing multi-channel systems, which are expected to be available by early 2000. Whereas the Airsat-1 is aimed mainly at general aviation and smaller business aircraft, to complement existing flight-telephone systems, the five and eight channel systems are planned to replace the heavy Aero-H equipment now installed in airliners. Prices for the smaller Iridium-based systems are expected to be lower, starting around $100,000.

The multi-channel systems, which will use the same compact blade antenna as the Airsat-1, will introduce additional features, including the ability to use Iridium for air traffic control (ATC) communications. This requires multiple channels and pre-emption schemes to ensure that safety-related messages always get through. Iridium's continuous global coverage, including polar regions, offers advantages over Inmarsat for ATC communications, the operator says.

Multichannel systems will use a standard interface allowing the avionics to be connected to any cabin telecommunications unit. This will allow use of the system with existing handsets. Multichannel Airsat systems will also be the first to accept Iridium's "smart card", allowing subscribers to use handheld and airborne telephones and receive one consolidated monthly bill.

The single-channel Airsat-1 presently comes with its own handset, but AlliedSignal plans to develop an interface between the unit and other flight telephone systems used in the business and general aviation markets, including its own flightfone 800 and possibly Teledyne Controls' Magnastar system. This would allow use of a single handset for seamless terrestrial and satellite communications.

Extensive tests of the Iridium system preceded its operational debut at the start of this month. The spacecraft orbit the earth every 100min, each covering a 3,500km-diameter footprint with 48 spot beams, so a user changes beams every 60-90sec. Automatic handover between beams works well, says AlliedSignal.

Calls go from the user equipment to a satellite then either direct to a ground station or via intersatellite links to a more distant gateway, where the call is connected to the terrestrial telephone system. There are 12 gateway ground stations worldwide.

It is not clear for how long AlliedSignal will keep its exclusive status with Iridium. It expects to continue to be the sole provider and hardware and service for the business and general aviation markets for the foreseeable future, but Iridium may approve extra equipment and service providers for the air transport market. Honeywell/ Racal says its next aeronautical satcom system will be compatible with Inmarsat and Iridium. Then there are the other operators planning constellations in low or medium earth orbit: GlobalStar and ICO (an Inmarsat spinoff). Both are expected to offer aeronautical services and are in discussions with potential hardware suppliers. Further ahead, there is the Teledesic "internet in the sky" network. That may give a hint about the direction the next generation of aeronautical satcoms may take.

Source: Flight International