Facsimile operations can be every bit as finicky as PC modems. Compatible fax machines are a prerequisite along with proper programming of the ground-based fax as well as the CDBR. Timeout of the Ground Based station has to be set to take into account the "call back" feature of the Airborne System. If the programming of the system within the aircraft is not accomplished properly, the CDBR of SIU may not direct the incoming call to the port where the airborne fax machine is connected.
Satellite Communication is one means of staying in touch even though the aircraft may venture out of the Air to Ground service area. The Inmarsat system was put in operation in 1978 and has worked very well over the past two decades. Four satellites orbiting 22,000 miles above the earth"s equator provide the targets for aircraft making use of their capabilities, and enables coverage of about 85 percent of the earth. The areas of limited or no coverage are found around the Polar Regions. Before any communications can take place, the aircraft has to find a satellite. This is accomplished by a sophisticated antenna system, which can utilize large fixed plate antennas or a movable platform antenna incorporating gimbals and servo- motors. Satellite positions relative to time are recorded in a computer memory that supplies this information to the servo- motors, which control antenna position. This position is based on aircraft location as determined by an Inertial Reference System. If an attempt is made to initiate satellite communications without first initiating the aircraft attitude and heading systems a link with the appropriate satellite will not occur. The operating frequency of this system is 1,530 to 1,660 MHz and incorporates a time delay of about one to two seconds due to the high altitude of the satellites. Should someone on the aircraft initiate a telephone call, which for whatever reason can not be handled by the usual Air to Ground method, an automatic transfer to SATCOM could occur. With the aircraft antenna fixed on an appropriate satellite, communication can now take place. The satellite sends the signal to the predetermined ground station where landlines can be used to connect the aircraft transmission to the appropriate destination.
In September 1998, a new cellular system in the sky was turned on and although not fully operational as yet, the Iridium Network could be a major player in the aviation telecommunications market. This system utilizes its own constellation called Leo consisting of 66 satellites operating at 422 nautical miles above the earth. Spot Beam technology is implemented, which defeats the need for a satellite finding system on the aircraft. In this case, each satellite will transmit a cone shaped signal toward earth. As the signal gets farther away from the satellite, the diameter or coverage area gets larger. This equates to a 350 nautical mile Foot Print on the ground. The satellite orbiting positions enable an overlap between most of the transmitted cones thereby minimizing dead zones. In fact, coverage of this type system is expected to be close to 100 percent of the earth"s surface. At present, only voice capability exists, but future plans call for data transmission.
Iridium telephones are also a reality and can be used in any corner of the globe. This system, like Inmarasat, utilizes a number of ground stations where satellite calls are switched over to land lines and operates on a frequency of 1,616 to 1,626MHz. The CDBR or SIU is programmed via the handset as to when Satellite Communications are used. This is an important choice as present SATCOM calls are significantly more expensive than Air to Ground calls.
Who knows, maybe the next time you are winging your way over some remote corner of the globe, if it just happens to be a Sunday evening, you can still reach out and touch Aunt Edna and maybe even send her an e-mail. But, chances are, it will cost more than 35 cents.
By reducing antenna size and increasing affordability, this new system will make satellite-based voice and data connectivity practical for a whole new segment of the industry.
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