Different Types of Satellite Phone Systems: Advantages and Disadvantages

The Need for Satellite Phone Systems

There are only two ways to hook up two telephones in different parts of the world: they can be connected by a cable which runs on the surface of the earth and under the sea, or they can be connected by radio. Not every place on the earth is served by a cable connection to the Public Switched Telephone Network (PSTN), and thus some locations must either have no telephone service or be connected to the PSTN by a radio link.

Modern cellular phones in the industrial countries mostly work in the cities and on the major highways. This is because each cell is a radio link from the cell phone to the PSTN; and if a cell phone user is located where there are no cells, there is no service.

This was brought out very humorously in the movie "Six Days Seven Nights" with Harrison Ford and Anne Heche. After their plane crashed on an isolated Pacific island, Anne Heche said "I'll save us." and pulled her cell phone out of her purse intending to call for help. Imagine her surprise when the phone didn't work. (A Globalstar or Iridium cell phone, however, might have worked. See below.)

Because the earth is round, and because most radio waves travel in straight lines, it is not possible to form a direct radio link to a telephone or a gateway to the PSTN which is located over the horizon. An exception to this rule are the low frequency bands which can follow the curvature of the earth, but these frequencies were all allocated for other purposes decades ago and are not generally available for telephone service. Also it is possible to have a line of radio relay stations that stretch along the earth, but these are only practical for a large number of circuits carried by the PSTN. They play a similar role to the surface cables of the PSTN and are not generally available to individual subscribers.

Because of the problems with Geostationary satellites, to be described below, almost all of the world-wide telephone voice traffic is currently carried by surface and under-sea cables. This has now changed because Globalstar and Iridium have now become operational.

Geostationary or 24-Hour Satellites

The higher a satellite is above the surface of the earth, the longer it takes to make one revolution around the earth. For example, a satellite at 200 miles (322 km) circles the earth in 84.3 minutes, a Globalstar satellite at 870 miles (1400 km) requires 114 minutes, and a satellite at 22,283 miles (35,870 km) requires exactly 24 hours to circle the earth.

This last case, a satellite at 35,870 kilometers, is of great utility because it requires exactly the same time to circle the earth as the earth does to make one revolution. The consequence of this is that if such a satellite is also located on the equator, it always appears to stay in the same spot.

These satellites are known as geostationary, GEO, or 24-Hour satellites. Television broadcast satellites are GEOs, and they have the advantage that the user must only point his dish (antenna) at one point in the sky. In contrast a satellite at 200 miles altitude would quickly pass overhead and be out of sight. The biggest use of geostationary satellites are the Direct Broadcast Satellites (DBS) sending TV programs.

Geostationary satellites, however, are also used for telephone traffic, both voice and data. Their big advantage is that one satellite can cover a large area, and three or four satellites can cover the whole earth.

They have, however, two great disadvantages for telephone voice traffic: 1) there is a very long noticeable delay in the path and 2) their great distance from the earth means that they need more power for the link. This usually means that the telephone antenna is a dish about the size of an inverted umbrella.

The delay is about 0.5 seconds for a round trip. This means that if one caller tells a joke, it is 0.5 seconds before the respondent laughs; and the impression is created that the respondent is slow-witted, dumb, or otherwise impaired. It isn't just jokes where this matters. When two people carry on a conversation, the person doing the listening is continually responding with "yes", "I see", "uh-huh", etc. If these responses are delayed 0.5 seconds, again this generates the impression that the respondent is not quite with it. For this reason GEO telephone voice traffic has been almost completely replaced with cables.

There is no way out of this delay because it is determined by the speed of light. Since a round trip is four times 35,870 km which is 143,480 km, and since the speed of light is about 300,000 km/second; it takes about 0.5 seconds for the voice signal to travel the 143,480 km.

The primary companies offering GEO telephone service are Intelsat (which primarily serves phone companies) and Inmarsat which serves individuals and companies. Inmarsat is used by companies such as the BBC to report on fighting in some very remote part of the world, by Construction Companies working at very isolated sites, ships at sea, etc.

Constellations of Low-Earth-Orbiting (LEO) Satellites

The problem of the GEOs, the need for high power and the 0.5 second delay, can be solved by using Low-Earth-Orbiting (LEO) satellites where the satellites are very close to the user. The problem of the LEOs, that they move so fast that they are soon out of sight, can be solved by using a constellation of satellites. When one satellite disappears over the horizon, its place is taken by the next satellite which is just coming up. In fact, in most constellations, 3 or 4 satellites can be visible at one time.

At the present time, there are two companies which have a constellation of low altitude satellites, Globalstar and Iridium.

The Iridium System

Iridium uses the digital phone system, TDMA, and they have duplicated the whole terrestrial telephone network in the sky by having satellite to satellite connections. TDMA and satellite-satellite connections are prone to dropping calls, having delays, and having poor voice quality. To give Iridium credit, they have worked very hard on these problems; and they are still operational.

Thus Iridium handsets work with the satellites and not with the existing terrestrial cellular phone networks. This means that the Iridium phones do not have the existing cell network to fall back on if, for example, they were inside of a building where the satellite reception is very poor or nonexistent. On the other hand, they have world-wide coverage whereas Globalstar does not have coverage in Africa, India, and much of the Pacific ocean.

The Globalstar System

In contrast to Iridium, Globalstar uses the newer digital technology CDMA and is only a "bent pipe". The "bent pipe" concept means that a Globalstar satellite is only a relay station that connects the user's handset to a gateway on the earth which is connected to the PSTN (i.e. there are no satellite-satellite links). Incidentally, the "bent pipe" concept means that Globalstar can easily add wide band data links to meet the new requirements of the market since no processing is done in the satellites. If there were processing in the satellites, the system would be "cast in concrete" and could not be augmented with wide-band equipment.

Globalstar is combined with the existing cell-phone networks. If normal cellular service is available, the Globalstar phone automatically uses the normal cell service. The Globalstar handsets are tripple-mode in North America, satellite CDMA, terrestrial CDMA, and terrestrial Analog. In Europe, the hand sets are dual mode, satellite CDMA and terrestrial GSM. Globalstar is a wholesale company which has partnered the cell phone service providers in each land. This in turn means that in those areas which have standard cell-phone service, the Globalstar handsets will still work in a building even if no satellite connection is available. Only in areas where there is no telephone service will the satellites be used.

In addition CDMA can talk to more than one satellite at a time which gives a more reliable connection and insures that one satellite's problems will not result in dropped calls.

Globalstar also sells special accessories to aid their service: car kits, roof and window antenna repeaters, fixed-location conventional telephones with roof antennas, and pay phones with roof antennas for third-world villages. One version of the pay phone uses solar arrays for its power, thus it can be installed in villages with no electricity.

Bankruptcy!!

Both Iridium and Globalstar have filed for Chapter 11 Bankruptcy protection, but they are both still operational. The problem was that the standard cell phone market expanded so fast that almost no market remained for Iridium and Globalstar.

A question arises as to the long term viability of these two companies. The satellites have a design life time of about 10 years, and sometime about 2010 will begin failing. Replacing the satellites represents an expenditure of one or two billion dollars, and it is not clear where this money will come from.