The satellite mix

by Chuck Husick

Right now, there several choices in satellite communications for mariners. And while the systems presently available tend to be on the large and expensive side, in the next half decade that could all change with the introduction of not one, but two worldwide satellite systems that will employ handheld transceivers similar to today’s cellular phones.

A system from Motorola, called Iridium (“World phone” Issue No. 52), will compete with a service scheduled to be offered by Inmarsat, called Inmarsat P.

Not many years ago, a mobile telephone in a vehicle was a rarity. Mobile transceivers had to share a very limited number of available radio channels, so only a few sets could be licensed in any one area. Users of the system often had to wait for a clear channel before placing or receiving a call.

All of that changed when the Bell System developed the first cellular radio telephone system. The key to the cellular concept is limiting the range of the mobile transceivers. This technique allows each frequency to be reused only a few miles away. Cellular systems require a large number of fixed site transceivers and a complex computer switching system to oversee the smooth transfer of a mobile user from one fixed site transceiver to another as the mobile unit moves.

The prototype system worked. In fact, it worked so well that the original estimates of the number of subscribers was proven woefully low. Once service began in a given area everyone wanted a mobile phone. With progress in miniaturization, the mobile phone became the transportable phone, which in turn became the pocket phone in use today.

The cellular approach is appealing to both users and to the communication companies that install the fixed site transceivers and computer switching networks that oversee the total operation. The cost of installing a conventional wired system with either copper wires or optical fibers to each subscriber is very high and generally becoming more expensive all the time. From the standpoint of economics, radio links are becoming less expensive, especially with the advent of digital modulation and frequency agility which greatly increases the channel capacity of each fixed transceiver site. (The economics are particularly appealing in those parts of the world where no extensive wired system exists. In many developing countries the only available telephone service is likely to be cellular, until traffic density justifies installation of wire or optical fiber links.)

History shows that once communication is made easy and reasonable in cost, people will use it more than was originally estimated. (An increase in international calling that followed the introduction of direct dialing is a good example.)

The development of satellite communications began with experiments in aiming radio signals at orbiting satellites and then receiving echoes. The earliest communications satellite was called Telstar and was launched by the Bell System into a low Earth orbit (LEO). As an improvement on this passive approach, later satellites had transponders on board and when they received signals, they rebroadcast them back to Earth.

This was followed by the development and launch of satellites whose orbital altitude is such that they appear to remain stationary over a point on Earth’s surface—geostationary satellites. Most geostationary satellites are used for telephone company traffic, radio, TV, and computer data transfer between users at various land-based locations, but access by mobile users, those on ships and aircraft, was also foreseen. Today, there are about 23 million terrestrial mobile phones in use. By the year 2000, this number is expected to exceed 100 million. Compared to that, Inmarsat’s mobile customer accounts number some 30,000. Inmarsat services

In 1979 Inmarsat was established, with its headquarters in London, to provide global communications service to the maritime and aviation community. Currently, there are 71 member countries in the organization and services available include direct dial telephone, telex, facsimile, electronic mail, and data transmission. Inmarsat also provides safety and emergency communications. The services presently available include A, which began in 1982; B and M, which became available in 1993; and C, which started in 1991. Service A, the oldest, uses analog modulation techniques. Service B, M, and C are digital signals. All systems are based on geostationary satellites

The distance to a geostationary satellite is considerable, more then 19,000 nautical miles. In order to handle data at the high rates required for voice communications, the signal must be strong (to provide a favorable signal-to-noise ratio) and relatively large, highly directional antennas must be used. On ships or planes, these antennas must be gyrostabilized so as to remain accurately aimed at the distant satellite. Due to the need for a stabilized antenna, the use of these three services is usually restricted to vessels at least 40 feet or more in length. The real limitation, especially for service M, which uses a reasonably small antenna, is not so much the size of the equipment as the cost. The shipboard service M unit costs $15,000 to 20,000. The M system supports digital voice at 4.8 kbit/s and data and facsimile at 2.4 kbit/s.

Service C is digital and sends data at a very low rate, 600 bits per second. This low data rate allows the use of smaller, fixed, non-directional antennas. When the data is digital and is transmitted and received slowly, it is possible to work successfully with much weaker signals relative to the background noise level. This is similar to the use of low-powered Morse code by radio amateurs, who routinely communicate over thousands of miles using transmitters whose output is often less than one watt. The advantage of the relatively low cost for service C is offset by the fact that it can only handle digital data (no voice) and that the transmission and reception is carried out in a store and forward mode: Data received by satellites is stored and assembled with other service C messages until a convenient time for retransmission. The return link operates similarly. Service C is not intended to serve as a form of worldwide cellular telephone.

For the past few years a consortium of companies, led by Motorola, has been developing a worldwide cellular system called Iridium. This system will be based on handheld telephones (or more properly, communicators, since they will be more than just telephones). Inmarsat’s response to this is their proposed Service P – a handheld satellite telephone service to be introduced in the period 1998 to 2000.

During 1992 and 1993, Inmarsat studied three possible satellite system approaches for system P: a high-powered geostationary orbit (GSO) system, an intermediate circular orbit (ICO) system, and a LEO system. In the summer of 1993, Inmarsat’s ruling council decided to focus on the ICO and GSO approaches, dropping the LEO option. For the GSO system, the satellites would have to orbit at 19,391 nautical miles, making the path length to and from the satellites quite long. The satellites would carry more powerful transmitters and larger, higher gain antennas than the existing Inmarsat satellites. The ICO system would use between nine and 15 satellites, orbiting in near-polar orbits at altitudes between approximately 5,400 and 8,200 nautical miles. Inmarsat states that their satellite system choice will be made in February, 1994. Global handheld phones

Inmarsat refers to the user terminal for service P as a global handheld phone. It is planned to provide digital phone quality, two-way voice communication and, in addition, support group III facsimile and data service at 2.4 kbit/s. It will also offer paging capability. The handset is planned to incorporate ports for connection to a computer and a printer.

When a user is communicating with a satellite, the handset will need a clear view of the satellite, so the P units won’t function as satellite communicators while indoors. Because of this, the P communicator will offer a second operating mode, functioning as a conventional cellular phone where such service is available. For maritime use, the line of sight requirement should not impose a burden, except when used belowdecks in a metal or graphite-decked boat. In such cases, some form of remote antenna will be needed. Current estimates place the price of a P handset at approximately $1,500. The handset may incorporate a built-in GPS receiver. Depending on the satellite system chosen by Inmarsat, handset location information from a GPS module may be a requirement for system operation. A satellite-only version of the handset (no local cellular network capability) may be offered at a slightly lower price.

Inmarsat P service signals will go from the P handset to a satellite and from the satellite to an Earth station. Traffic would then flow via the public switched telecommunications networks (land lines). The choice of a satellite system will, in part, depend on the trade-off between the higher power required to communicate with GSO satellites and the potential for service interruption that exists when ICO satellites are used. Since the ICO satellites will move across the observer’s sky, it is possible for objects to block radio signals during a satellite pass. For an ICO-based system to work, more than one satellite will likely have to be in view of a handset at all times. For use at sea, blocked signals should be minimal.

Another consideration is the effect the long transmission path has in causing delay in two-way voice communication. Even though the radio frequency energy is moving at close to the speed of light, noticeable delays are present, especially with GSO systems. For a GSO system, the total delay will be on the order of 360 to 390 milliseconds. When using an ICO system, the delay will be 190 to 220 milliseconds. By comparison, the LEO-based Iridium system delays should be on the order of 130 to 150 milliseconds. All of these delays will be increased somewhat by the necessary use of either landbased telephone networks or satellite-to-satellite communication as employed by Iridium. In any event, users will have to get used to some amount of delay – much like that on some present-day international phone calls. Wide corporate support

The Inmarsat P program is supported by 15 key signatories. The companies collaborating with Inmarsat in the services portion of the P study include Hughes Aircraft (U.S.); MM Astro (U.S.); Matra Marconi (U.K./France); Alcatel (France); ISRO (India); British Aerospace (U.K.); TRW (U.S.); Aerospatiale (France); Alenia (Italy); and Deutsche Aerospace (Germany). Work in the features’ area is being done by Nokia (Finland); Ericsson (Sweden and U.S.); NEC (Japan); HNS (U.S.); and Alcatel (France). As with its likely competitor, Iridium, the project is so large as to require international participation.

Comparing the Inmarsat P system with Motorola’s Iridium shows that Iridium, with 66 LEO satellites, and using satellite-to-satellite communication rather than immediate resort to landbased telephone networks, is probably the more challenging from a technical standpoint. The large number of satellites and the possibly shorter service life of satellites at their lower orbital altitude may add to the cost of the system. At the same time, each satellite can be less expensive than either of Inmarsat’s satellite choices. Another possibly significant factor is the cost savings that can result from building a large number of identical satellites. In the history of satellite construction an order for more than a few identical birds has been very rare. Assuming Inmarsat chooses the ICO route, it is likely that the casual maritime user wouldn’t notice a great difference in the operation of either system.

The good news is that with the prospect of two telephone companies competing for business, the cost of service should be quite reasonable. Iridium has estimated the cost of a one minute call at $3.00. Inmarsat has released no estimates at this time, however their service M cost is approximately $5.50 per minute. Now, if it were only possible to complete a call in only a minute!

Contributing editor Chuck Husick is sailor and pilot who often writes on technical topics.

By Ocean Navigator