Reach Out and Touch Someone
By Jim Sparks
Making a telephone call has been for years a very simple task. With new technology, telephone lines now provide much more than the ability to say hello to Aunt Edna on a Sunday evening. In fact, in today"s fast-paced business world, telephone access is a necessity.
It used to be that travel via ship, rail, car, or aircraft provided a rest from the stresses of the business world. But, as time is money, the communications industry began providing means of "staying in touch," even though the world might be passing by at a high rate of speed.
Commercial airlines, along with many business aircraft, are equipped with telephone systems that not only provide voice communication, but allow the transfer of electronic media through data links. When considering that many of today"s aircraft can travel thousands of miles at speeds approaching — and in some cases — exceeding that of sound, it is rather unrealistic to think of an airborne telephone system on the order of the pay phone down at the corner. Radio waves now take the place of wires and poles.
While aircraft technicians are responsible for safe and timely flights, now they also are required to ensure that passengers can conduct business whenever they need to.
Telephone communications from aircraft are categorized as either "Air to Ground" or "Air to Air." In either case, specialized technology and equipment are required. Just like with home long distance service, the aviation telecommunications market now has numerous players, and industry giants AT&T and GTE each has a significant share.
Flitefone,™ or now Claircom,™ are the AT&T systems with over 150 ground stations that provide coverage over the continental United States, Alaska, Hawaii, the Caribbean, and Canada. MagnaStar™ is the system used by GTE and has over 100 ground stations with similar coverage areas. Earth-orbiting satellites provide for "Air to Air" communications. Commonly referred to as "SATCOM," these satellites are connected to ground stations and can presently deliver communications coverage over about 85 percent of the planet with a new system on the horizon promising close to 100 percent coverage.
In the "Air to Ground" arena, the MagnaStar and Flitefone systems are functionally similar though the components are not interchangeable. In both systems, an Airborne Radio Telecommunications Unit (ARTU) provides the medium to connect the aircraft with the ground stations. The ARTU is both a receiver and a transmitter utilizing 290 channels. Frequency range during transmission is 894 to 896 MHz and the receive side is in the 849 to 851 MHz spectrum. The ten watts of transmitter power is quite adequate to provide contact with the vast array of ground stations. "Always Listening" is one of the features of the ARTU receiver that enables determinations of which ground station to use and when to hand off to another cell. It also offers tuner control and even power management along with encoding and decoding of both audio and digital signals, plus provides the protocol to interface with the ground network. An ARTU will also include Built-In Test (BIT) during power up and performs continuous system monitoring. Provisions are also made on the face of the ARTU to connect a patch cable to a laptop computer where a diagnostic software program can be utilized for fault isolation.
A Duplexer is used to connect the separate receiver and transmitter outputs from the ARTU to the common antenna. This device is a signal filter and combiner, that is a switching unit and is designed for a minimum signal loss during both the receive and transmit operations.
A 900MHz quarter wave antenna is electrically connected to the Duplexer through a low-loss coax cable. The minimum distance between the Telecomm antenna and other antenna for equipment operating in the 800 to 1200 MHz including DME, Transponder, and TCAS should be at least three feet. In the event multiple ARTUs are used, the antenna spacing is at least four feet between like units, and a minimum of two feet should be observed from all other antennas. If the antenna is to be installed on a composite skin, a metal surface extending one foot radially from the antenna connector is required to provide a ground plane. There should be less than 2.5 milliohms of resistance between the antenna and the airframe. Corrosion or paint under the antenna can result in a significant signal loss. Problems with the various co-ax cables and cable connectors are other common causes of difficulty with airborne telecommunication systems.
The Local Area Network (LAN) within the aircraft can consist of one or more telephone handsets, fax machine, and portable computer connections. A System Interface Unit (SIU) used by the Claircom system, or a Cabin Distribution Bus Repeater (CDBR) used by MagnaStar, are the components used to connect all the user equipment to the ARTU. This will include the cabin and flight deck handsets, fax machine, computer modems, and even includes SATCOM or HF Radios. In addition, a CDBR provides select calling to a specific telephone handset in the aircraft as well as managing available phone lines and connection to the usually more expensive SATCOM when all Air to Ground lines are in use or unavailable.
Most handsets in use today are digital devices and are used like a traditional telephone. They include a microphone and a small speaker, a keypad, and in some cases, a jack to allow connection of an external modem. The keypad may also be used as a device to program the entire communication system, but usually requires some type of password to get beyond all but the basic keypad functions. It is important to refer to the specific system user"s manual prior to entering the programming functions.
The means of operation of Air to Ground communications is much like a pager system. When someone on the ground wishes to contact a person in an aircraft, the appropriate telephone number is entered and a corresponding code is broadcast by all of the specific service provider"s ground stations. After the signal is received by the airborne receiver transmitter, an analysis is made as to which ground cell is providing the best signal. The ARTU will then call back to the best signal provider. It is this call back that initiates the communication link. As the aircraft continues on its course, the signal from the in-use ground station will begin to deteriorate. Another function of the ARTU is to listen to all available ground stations and make a decision based on signal condition as to which cell to handoff the phone call. This decision is based on aircraft direction and speed as well as ground station location and proper function. The handoff, when it occurs, should not be noticed by the system consumers.
In addition to voice communication, facsimile and PC modems are common users of in-flight telephone systems. Transfer of electronic data provides some interesting challenges. First of all, computer modems have gotten faster. Unfortunately, some in-flight telecommunication systems may not keep up. Presently, the data transfer or baud rate can be from 2,400 Bits Per Second (BPS) to 19,200 BPS. Also, when the initial "handshake" is made between the two communicating computers, the modems have to synchronize. As the aircraft system is limited, the ground system has to adjust its speed. This has to occur before the connection timeout, otherwise disconnection is imminent.
The most frequent culprit causing failure of in-flight data communications is from improperly matched modems. It is always best to contact the system manufacturer and determine modem compatibility prior to sending the aircraft on an extended mission with a group of executives who otherwise will be unable to retrieve their e-mail. The second most common problem is lack of knowledge on the part of the passengers as to proper operation of the system. In the event trouble is reported, it is important to find out if the problem was encountered by only one passenger or by all passengers. If only one person reports improper operation, the probability is high that the trouble is with the user and not the system.
Another important question to ask in the event of reported problem is aircraft location and if the voice communication was operational. Lack of voice communication accompanying loss of data communication could point to a system malfunction or even an antenna problem. The question of location might be important if the aircraft had traveled out of the service area or had managed to cross one of the few areas out of the range of a ground station.
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.