Communication Satellites

---

In the 1950s and early 1960s people tried to set up communication systems by bouncing signals off metallized weather balloons. Unfortunately, the received signals were too weak to be of any practical use. Then the U.S. Navy noticed a kind of permanent weather balloon in the sky - the moon, and built an operational system for ship-to-shore communication by bouncing signals off it.

Further progress in the celestial communication field had to wait until the first communication satellite was launched in 1962. The key difference between an artificial satellite and a real one is that the artificial one can amplify the signals before sending them back, turning it into a powerful communication system.

Communication satellites have some interesting properties that make them attractive for many applications. A communication satellite can be thought of as a big microwave repeater in the sky. It contains several transponders, each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency, to avoid interference with the incoming signal. The downward beams can be broad, covering a substantial fraction of the earth's surface or narrow, covering an area only hundreds of kilometers in diameter.

Geosynchronous Satellites

According to Kepler's law, the orbital period of a satellite varies as the orbital radius to the 3/2 power. Near the surface of the earth, the period is about 90 min. Communication satellites at such low altitudes are problematic because they are within sight of any given ground station for only a short time interval.

However, at an altitude of approximately 36,000 km above the equator, the satellite period is 24 hours, so it revolves at the same rate as the earth under it. An observer looking at a satellite in a circular equatorial orbit sees the satellite hang in a fixed spot in the sky, apparently motionless. Having the satellite fixed in the sky is extremely desirable, because otherwise an expensive steerable antenna would be needed to track it.

With current technology, it is unwise to have satellites spaced much closer than 2 degrees in the 360-degree equatorial plane, to avoid interference. With a spacing of 2 degrees, there can only be 360/2 = 180 geosynchronous communication satellites in the sky at once. Some of these orbit slots are reserved for other classes of users (e.g., television broadcasting, government and military use, etc.).

Fortunately, satellites using different parts of the spectrum do not compete, so each of the 180 possible satellites could have several data streams going up and down simultaneously. Alternatively, two or more satellites could occupy one orbit slot if they operate at different frequencies.

To prevent total chaos in the sky, there have been international agreements about who may use which orbit slots and frequencies. The main commercial bands are listed in table below.

Band	Frequencies	Downlink (GHz)	Uplink (GHz)	Problems
C	4/6	3.7-4.2	5.925-6.425	Terrestrial interference
Ku	11/14	11.7-12.2	14.0-14.5	Rain
Ka	20/30	17.7-21.7	27.5-30.5	Rain, equipment cost

The C band was the first to be designated for commercial satellite traffic. Two frequency ranges are assigned in it, the lower one for downlink traffic (from the satellite) and the upper one for uplink traffic (to the satellite). For a full-duplex connection one channel each way is required. These bands are already over-crowded because they are also used by the common carriers for terrestrial microwave links.

The next highest band available to commercial telecommunication carriers is the Ku band. This band is not yet congested, and at these frequencies satellites can be spaced as close as 1 degree. However, another problem exists: rain. Water is an excellent absorber of these short microwaves. Fortunately, heavy storms are usually localized, so by using several widely separated ground stations instead of just one, the problem can be circumvented at the price of extra antennas, extra cables, and extra electronics to switch rapidly between stations.

Bandwidth has also been allocated in the Ka band for commercial satellite traffic, but the equipment needed to use them is still expensive. In addition to these commercial bands, many government and military bands also exist.

A typical satellite has 12-20 transponders, each with a 36-50 MHz bandwidth. A 50-Mbps transponder can be used to encode a single 50-Mbps data stream, 800 64-kbps digital voice channels, or various other combinations. Furthermore, two transponders can use different polarizations of the signal, so they can use the same frequency range without interfering. In the earliest satellites, the division of the transponders into channels was static, by splitting the bandwidth up into fixed frequency bands (FDM). Nowadays, time division multiplexing is also used due to its greater flexibility.

The first satellites had a single spatial beam that illuminated the entire earth. With the enormous decline in the price, size, and power requirements of microelectronics, a much more sophisticated broadcasting strategy has become possible. Each satellite is equipped with multiple antennas and multiple transponders. Each downward beam can be focused on a small geographical area, so multiple upward and downward transmissions can take place simultaneously. These so-called spot beams are typically elliptically shaped, and can be as small as a few hundred km in diameter. For example: A communication satellite for the United States would typically have one wide beam for the contiguous 48 states, plus spot beams for Alaska and Hawaii, which are geographically away.

A new development in the communication satellite world is the development of low-cost micro-stations, sometimes called VSATs (Very Small Aperture Terminals). These tiny terminals have 1-meter antennas and can put out about 1 watt of power. The uplink is generally good for 19.2 kbps, but the downIink is more, often 512 kbps. In many VSAT systems, the micro-stations do not have enough power to communicate directly with one another (via the satellite). Instead, a special ground station, the hub, with a large, high-gain antenna is needed to relay traffic between VSATs, as shown in figure below. In this mode of operation, either the sender or the receiver has a large antenna and a powerful amplifier. The trade-off is a longer delay in return for having cheaper end-user stations.

Communication satellites have several properties that are radically different from terrestrial point-to-point links. To begin with, even though signals to and from a satellite travel at the speed of light (nearly 300,000 km/sec), the large round-trip distance introduces a substantial delay. Depending on the distance between the user and the ground station, and the elevation of the satellite above the horizon, the end-to-end transit time is between 250 and 300 msec. A typical value is 270 msec (540 msec for a VSAT system with a hub).

For comparison purposes, terrestrial microwave links have a propagation delay of roughly 3 µsec/km and coaxial cable or fiber optic links have a delay of approximately 5 µsec/km (electromagnetic signals travel faster in air than in solid materials).

Another important property of satellites is that they are inherently broadcast media. It does not cost more to send a message to thousands of stations within a transponders footprint than it does to one. For some applications, this property is very useful. Even when broadcasting can be simulated using point-to-point line, satellite broadcasting may be much cheaper. On the other hand, from a security and privacy point of view, satellites are a complete disaster - everybody can hear everything. Encryption is essential when security is required.

Satellites also have the property that the cost of transmitting a message is independent of the distance traversed. A call across the ocean costs no more to service than a call across the street. Satellites also have excellent error rates and can be deployed almost instantly, a major consideration for military communication.

Low-Orbit Satellites

For the first 30 years of the satellite era, low-orbit satellites were rarely used for communication because they zip into and out of view so quickly. In 1990, Motorola broke new ground by launching 66 low-orbit satellites for the Iridium project. The idea was that as soon as one satellite went out of view, another would replace it. This proposal set off a feeding frenzy among other communication companies. All of a sudden everyone wanted to launch a chain of low-orbit satellites. We will briefly describe the Iridium system here, but the others are similar.

The basic goal of Iridium is to provide worldwide telecommunication service using hand-held devices that communicate directly with the Iridium satellites. It provides voice, data, paging, fax, and navigation service everywhere on earth.

It uses ideas from cellular radio, but with a twist. Normally, the cells are fixed, but the users are mobile. Here, each satellite has a substantial number of spot beams that scan the earth as the satellite moves. Thus both the cells and the users are mobile in this system, but the handover techniques used for cellular radio are equally applicable to the case of the cell leaving the user as to the case of the user leaving the cell.

The satellites are to be positioned at an altitude of 750 km, in circular polar orbits. They would be arranged in north-south necklaces, with one satellite every 32 degrees of latitude. With six satellite necklaces, the entire earth would be covered as shown in figure (a) below.

Each satellite would have a maximum of 48 spot beams, with a total of 1628 cells over the surface of the earth, as shown in figure (b). Frequencies could be reused two cells away, as with conventional cellular radio. Each cell would have 174 full-duplex channels, for a total of 283,272 channels worldwide. Some of these would be for paging and navigation, which require hardly any bandwidth at all.

The uplinks and downlinks would operate in the L band, at 1.6 GHz, making it possible to communicate with a satellite using a small battery-powered device.

Messages received by one satellite but destined for a remote one would be relayed among the satellites in the Ka band. Sufficient bandwidth is available in outer space for the inter-satellite links. The limiting factor would be the uplink/ downlink segments. Motorola estimates that 200 MHz would be sufficient for the whole system.

The projected cost to the end user is about 3 dollars per minute. If this technology can provide universal service anywhere on earth for that price, it is unlikely that the project will die for lack of customers. Business and other travelers, who want to be in touch all the time, even in undeveloped areas, will sign up in droves.

Satellites versus Fiber

A comparison between satellite communication and terrestrial communication is instructive. As recently as 40 years ago, a case could be made that the future of communication lay with communication satellites. After all, the telephone system had changed little in the past 100 years and showed no signs of changing in the next 100 years. This glacial movement was caused in no small part by the regulatory environment in which the telephone companies were expected to provide good voice service at reasonable prices (which they did), and in return got a guaranteed profit on their investment. For people with data to transmit, l200-bps modems were available. That was pretty much all there was.

The introduction of competition in 1984 in the United States and somewhat later in Europe changed all that radically. Telephone companies began replacing their long-haul networks with fiber and introduced high-bandwidth services like SMDS (Switched Multi megabit Data Service) and B-ISDN. They also stopped their long-time practice of charging artificially high prices to long-distance users to subsidize local service.

All of a sudden, terrestrial fiber connections looked like the long-term winner. Nevertheless, communication satellites have some major niche markets that fiber does not (and sometimes, cannot) address. We will now look at a few of these.

While a single fiber has, in principle, more potential bandwidth than all the satellites ever launched, this bandwidth is not available to most users. The fibers that were being installed 20 years back were used within the telephone system to handle many long distance calls at once, not to provide individual users with high bandwidth. Furthermore, few users even had access to a fiber channel because the trusty old twisted pair local loop was in the way. Calling up the local telephone company end office at 28.8 kbps will never give more bandwidth than 28.8 kbps, no matter how wide the intermediate link is. With satellites, it is practical for a user to erect an antenna on the roof of the building and completely bypass the telephone system. For many users, bypassing the local loop is a substantial motivation.

For users who (sometimes) need 40 or 50 Mbps, an option is leasing a (44.736 Mbps) T3 carrier. However, this is an expensive undertaking. If that bandwidth is only needed intermittently, SMDS may be a suitable solution, but it is not available everywhere, and satellite service is.

A second niche is for mobile communication. Many people nowadays want to communicate while jogging, driving, sailing, and flying. Terrestrial fiber optic links are of no use to them, but satellite links potentially are. It is possible, however, that a combination of cellular radio and fiber will do an adequate job for most users (but probably not for those airborne or at sea).

A third niche is for situations in which broadcasting is essential. A message sent by satellite can be received by thousands of ground stations at once. For example, an organization transmitting a stream of stock, bond, or commodity prices to thousands of dealers might find a satellite system much cheaper than simulating broadcasting on the ground.

A fourth niche is for communication in places with hostile terrain or a poorly developed terrestrial infrastructure. Indonesia, for example, has its own satellite for domestic telephone traffic. Launching one satellite was much easier than stringing thousands of undersea cables among all the islands in the archipelago.

A fifth niche market for satellites is where obtaining the right of way for laying fiber is difficult or unduly expensive.

Sixth, when rapid deployment is critical, as in military communication systems in time of war, satellites win easily.

In short, it looks like the mainstream communication of the future will be terrestrial fiber optics combined with cellular radio, but for some specialized uses, satellites are better. However, there is one thing that applies to all of this - economics. Although fiber offers more bandwidth, it is certainly possible that terrestrial and satellite communication will compete aggressively on price. If advances in technology radically reduce the cost of deploying a satellite (e.g., some future space shuttle can toss out dozens of satellites on one launch), or low-orbit satellites catch on, it is not certain that fiber will win in all markets.

---