SATELLITE COMMUNICATIONS NETWORKS

by John Nordlie

In 1957 Russia launched the worlds first orbital satellite: Sputnik. About the size of a soccer ball, the only instrumentation carried by sputnik was a battery powered radio transmitter. Sputnik created quite a stir when it was launched, and prompted the United States into action, and the space race was born. Sputnik was hardly a sophisticated spacecraft by today's standards, but it did provide some important information: electrical devices could operate in the space environment, and radio waves from a spacecraft in orbit could penetrate the earths atmosphere. When Sputnik's batteries gave out a few days later, few could imagine what these few pieces of information could mean to society.

One person who could was Arthur C. Clarke. Years earlier, in 1945, Clarke published an article in Wireless World (a british radio journal) describing a global communications system using three satellites orbiting the earth in a geostationary orbit. Geostationary orbit is a special circular orbit at a ceratin distance (22,300 miles above the earth's surface) at which the period of rotation (time it takes the satellite to make one orbit) is equal to the time it takes the earth to make one rotation around its axis. This meant that from the earth, the satellite would appear to stand still in the sky, making antenna pointing an easy task. The three satellites could be accessed from anywhere on the ground that they were visible from. Radio signals could be passed from ground to satellite to ground, or ground to satellite to another satellite to ground, making communications with any other point on the surface of the earth possible without the need for land lines. Since Clarke was a science fiction writer, scientists at the time thought of Clarke's idea as an interesting fantasy. When Sputnik was launched in 1957, that all changed. As many scientists scrambled to play catch-up with the Russians by building and launching Explorer 1 a few months later, others were rediscovering Clarke's idea and planning for the future. [1]

Today, almost all major communications satellites are parked in "geosync" or the "Clarke belt". There are some satellites in low lever polar orbit though, most notably the ham radio or "OSCAR" (Orbiting Satellite Carrying Amateur Radio) satellites from the United States, Britain, Japan, and Europe, and their Russian counterparts: Radio Sputnik. Being in low-level polar orbit means that these small, low power satellites are accessible at a certain site for only a short time each day (typically 30 minutes). Since the satellites move with regard to the user during the pass, the uplink/downlink antennas must be mounted on adjustable mounts to allow them to be pointed at the satellite. Aside from the problems of antenna tracking and short pass times, the ham radio satellites see a lot of use by amateur radio operators world wide, and have made significant contributions to what is known about satellite orbits and operation. In fact, two ham radio satellites built at the University of Surrey in England have no communications transponders at all. These two satellites (UoSAT-1 and UoSAT-2) carry an array of scientific instruments that measure temperature, radiation, plasma and magnetic fields, solar light output, and even a CCD (Charge Coupled Device) television camera to take pictures of the earth. Data gathered by these satellites have been used as the basis of several scientific papers submitted to professional journals! Not bad for a bunch of "hams" tinkering with a box full of batteries and semiconductors. No ham radio satellites to date have been launched as the primary payload of a rocket, they have all flown as "ballast" (launch rockets are designed to lift a specific mass to a specific speed. When the main payload is lighter than this mass, it is cheaper and easier to add dead weight to the vehicle than to "throttle it down"). This does not mean that the launches were free, but the reduced price has made it possible for many amateur radio organizations to launch communications satellites. Ham radio satellites have even been launched by the space shuttle from "GAS" (Get Away Special) cans modified with flip-top lids. Ham radio satellites promise to continue to offer experience and data to advance the science of space communications. [2]

Of course, the traffic on the ham radio satellites is minuscule compared to the traffic carried on the major communications satellites parked in geosync orbit.[4] Unlike their ham radio cousins, these "big birds" all operate in the microwave region.[1] This is necessary to allow the enormous data rates at which these communications satellites operate (1.2 Giga(c)bit per second data rates are not uncommon).[4] A commercial communications satellite may seem very complex at first glance: Attitude control thrusters, flight computers, spun/despun sections, solar arrays, nickel-hydrogen batteries, etc. But actually, a communication satellite is surprisingly simple in concept: a radio receiver to receive signals from earth, hooked to an amplifier to boost signal lever, hooked to a radio transmitter to transmit the signal back to a different spot on the earth. All the other goodies are there to support this simple function: the thrusters and flight computer keep the package where it should be and the antennas pointed where they should be, and the solar arrays and power cells provide electricity for the radios.[3] Most of the interesting stuff is done at the ground stations. An example: The Comstar(c)2 satellite has a transponder (receiver-amplifier-transmitter) that receives on 4 GHz and retransmits on 6GHz (a satellite cannot transmit and receive on one frequency simultaneously: the signal from the transmitter would overwhelm the relatively weak signal from earth, thus "desensing" the receiver) using a 500 MHz bandwidth. This 500 MHz bandwidth is broken up by ground stations into 12 40MHz bands using Frequency Division Multiplexing. Each of these channels can support a data rate of 50 Megabits per second. The data is demultiplexed at the receiving or "downlink" station, and sent on its way. Satellites use FM (Frequency Modulated) radio systems, which require larger bandwidth than AM (Amplitude Modulated) radios, so bandwidth sizes are sometimes larger than needed. FM radios are used to minimize transmitter power requirements on board the satellite (power is at a premium on a satellite). A more efficient multiplexing system gaining popularity in satellite communication is TDM (Time Division Multiplexing). TDM is more efficient than FDM since the entire bandwidth can be used (FDM requires guard bands between data bands to prevent interference). A close cousin of TDM is TDMA (Time Division Multiple Access), which can support multiple ground station access at any one time (the other two are used for one pair of stations at a time). TDMA requires that special hardware be added to the transponder, though, and hence raises the cost. With the cost of semiconductor control systems dropping substantially in the last few years, companies have begun to build "smarter" satellites using more complex multiplexing techniques to maximize their throughput. Radio systems that operate at even higher frequencies have also helped increase the data rates of communications satellites substantially.[4] So why do we need these marvelous pieces of technology floating around the earth? Who has all this data that they need shipped? Some of the bigger users of communications satellites are the television and radio broadcasting companies (CBS, ABC, NBC, TBS) and phone companies. The new craze in phone transmission is fiber optics: transmission of your phone conversation over a "light pipe" in digital form. But there are only so many fiber optic cables laid on the earth, and laying them underwater is expensive, so communications satellites are used. Transmission in digital format eliminates much of the noise and distortion of a phone connection, making it ideal for high speed computer communication as well. Western Union, Bank of America and many other commercial banks and business use communications satellites to allow their computers to talk to each other, exchanging important information at mind-boggling speeds. In television, satellite networking has revolutionized broadcasting. Intercontinental conferencing that was once a dream of newscasters is now an everyday occurrence. Private and small commercial television users have benefited as well; TVRO (TV Receive Only) dishes dot the landscape, and small cable TV companies can now offer remote towns 35 channels or more with only a couple of 15 foot microwave dishes for downlinks.[1] With more and more cheap hardware making access to satellite communications a reality for everyone, commercial users have become increasingly concerned with security. Just a few years ago, anyone with a back yard dish could pick up the feeds for many pay TV services (ShowTime, HBO, etc.) for free. Network station feeds were also free for the viewing. The pay TV services were justifiably concerned with the potential dollars they were losing to these dish users, and the networks were concerned for some reason or other (I guess they didn't want us to see David Brinkely picking his nose). And so began another phase in the communications satellite story: scrambling. Scrambling is just a technique of encoding or adjusting your signal so that special hardware is needed to receive it correctly. Most scrambling done by the TV industry involves adding a "jamming" signal in their regular signal at a strategic location. This jamming signal will foul up any regular receiver tuned to receive that particular station. To unjam the signal, a "notch" filter is added to the line just before it enters the receiver. The notch filter blocks a specific frequency (the jamming frequency), and allows all other frequencies to pass. The escapades of "Captain Midnight", who used a satellite uplink station to jam the satellite that HBO uses to relay its programming has created quite a bit of concern among satellite users lately. Guarding against this sort of intrusion is almost impossible at the present time. The military, of course, uses much more complex methods of encoding its data, and does not care to discuss them with the public. I couldn't find much on military use of communications satellites, but I did find one interesting story: In the 1970's, A satellite engineer became interested in what happened to all the data that passed through the satellites he worked on. He was flabbergasted to find that, in typical military "Hurry-up-and-wait" style, the data that came down from his 500 Megabit/second communications satellite was then sent to 25 bit/second teletype printers for output.[3] This brings up a good point: in order to justify the cost of using a communications satellite, your equipment must all be reasonably up to date. One other government agency that uses communications satellites regularly is NASA. NASA is building their own network of satellites to enable them to communicate with the space shuttle, Hubble space telescope, and various other orbiting devices they control. This is a case where there is only one ground station. Since there are only X frequencies available, and a finite amount of space in the Clarke belt, there can only be a certain maximum number of communications satellites placed there. This is a cause for concern for many developing nations: they are afraid that the superpowers will fill the Clarke belt to capacity before they have the technology to launch communications satellites themselves. This will create a monopoly, requiring that all small, poor, or underdeveloped nations rent use of existing communications satellites from the commercial suppliers. Another issue this brings up is: who owns the space up there? Can the US park a reconnaissance satellite in geosync above Russia? Can Russia complain about this, or shoot the bird down? When space for communications satellites becomes scarce, watch for this to become a hotly debated issue.

REFERENCES:
[1] Easton, A., T. The satellite TV handbook. 435p.
Howard W. Sams & Co. Inc.

[2] Davidoff, M. The satellite experimenter's handbook. 207p.
American Radio Relay League.

[3] Jaffe, L. Satellite communications in the next decade. 175p.
American astronautical society.

[4] Stallings, W. Data and computer communications. 653p.
Macmillan publishing company.


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