IEEE Antennas and Propagotion Society Newsletter, April 1 9 8 4

Uplink, Downlink: A Tribute to C. Arthur Clark

John Kraus Ohio State University

Satellites are transforming our society. Voice, TV and data now go worldwide by satel l i te . Yet , the waves that carry them were unknown 100 years ago.

In 1884, the theories of electricity and magnetism had been unified by James Clerk Maxwell, but experimental verification of his ideas had not yet been accomplished. It was not until 1888 that Heinrich Rudolph Hertz constructed the f i rs t radio t ransmit ter and receiver and demonstrated that except for their much greater length, radio waves were one with light. So in 1884, radio waves were unknown, x-rays had not been discovered, relativity had not been proposed, neither had quantum theory. Cosmic rays were unknown, there were no airplanes, blood letting with leeches was a standard medical cure-all, Edison's incandescent light was making slow headway against the entrenched gas illumination industry. In the United States, controversy raged over whether the e lec t r ic s t ree t car or the horsedrawn car was better, and in England, the Red Flag Act, not repealed until 1896, required that any self-propelled highway vehicle be preceded by a man carrying a red f lag by day and a red lantern by night. This was the situation only 100 years ago!

Figure 1. Arthur C. Clarke in his Royal Air Force uniform at the t ime he wrote his classic art icle on "Extra- Terrestrial Relays."

In 1934, o r 50 years ago, physics was undergoing a revolution with the discovery of the neutron, the positron and deuterium. Only four years earlier, Ernest Lawrence had built the first cyclotron, ushering in a n e r a of high-energy physics. But penicillin had not been discovered, transistors were unknown, television was only a toy, and jet a i rc raf t and atomic bombs were yet in the future.

Then at war's end in 1945, Arthuc C. Clarke published an a r t ic le in "Wireless World" entitled "Extra-Terrestrial

Relays.'' With equations and graphs, Clarke proposed the use of ar t i f ic ia l satellites in geostationary orbit as a solution to the world's communication problem. He discussed many pract ical aspects of the system:

1. How th ree satellites could provide complete coverage of the ear th .

2. How frequencies in the VHF and UHF range could provide a vast number of channels.

3. How ground stations would experience little interfer- ence from terrestrial sources, since their antennas would be directed towards the sky.

4. How a modest power could provide broadcast service for one hemisphere.

5. How at some frequencies, earth station dishes could be as small as 30 cm (1 foot) in diameter.

But Clarke anticipated skepticism for his idea, stating, "Many may consider this proposal too far fetched to be t aken seriously." However, he added, "Such an a t t i t ude is unreason- able, since everything envisaged here is a logical extension of developments in the last ten years.!'

Clarke went on to point out that if the German t rans- Atlantic rocket had become operational, it would have attained half the velocity required to g e t into low ear th orbit. He further indicated that the higher orbit for his satellite would not require an unreasonable increase in the launching capabilities of rockets.

In 1945, when Clarke% article appeared, he was an off icer in the Royal Air Force. Now he is world famous as

ing "2001" and "2010." the author of many science and science fiction books, includ-

Twelve years to the month after Clarke's proposal was published, Sputnik I went up, launching us into the space age. And only six years after that, in 1963, the first successful geostationary satellite, Syncom 2, was put into orbit and Clarke's proposal became a reality. Now the re are hundreds of such satell i tes forming a Clarke r ing around the ear th at a height of 36,000 ki lometers above the equator with more going up at frequent intervals. The satellites ro t a t e w i th t he ear th , remaining at a fixed point above the ear th 's equator , their transponders relaying the uplink signals back on down- links. With a transponder op a Clarke orbit satell i te, it is like having your antenna on a tower 36,000 km high, with half t h e world spread out below.

Although the U. S. has more satellites in the Clarke orbit than the Soviets, the Russians make up for this with their Molniya (lightning) satellites which serve the same purpose better at high latitudes where Clarke orbit satellites graze the southern horizon.

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IEEE Antennas and Propagalion Sociely Newsleiter, April 1984

Figure 2. The father of the communication satell i te, Arthur C. Clarke, with his earth-station antenna in Sri Lanka.

In orbits inclined at about 65 degrees to the equator , one group of these Molniya satell i tes oeprates over Siberia, and another group over North America. Typically, a North American Molniya satellite will %hang up" near i ts apogee 40,000 km over Hudson's Bay, remaining quasi-stationary for about six hours. Its orbi t then carr ies i t south, where i t z ips around the southern hemisphere rapidly at a low perigee. A s soon as one Molniya moves south and away from Hudson's Bay, another one comes up to take i ts p lace so tha t there i s always a Molniya satell i te up there. One of its transponders operates on Channel 8 of t h e 4 GHz C hand. Another transponder provides the hot-line link between the Kremlin and the White House.

In his book "Megatrends," John Naisbitt tells how we have become a global information-based society with the communication satell i te a key link in the exchange and dissemination of information. We talk to others via satel l i te ; we get news, information and entertainment via satell i te TV; we exchange data and link our computers by satel l i te , and we do i t as easi ly with someone on a dis tant cont inent as with someone next door. Although communication satellites already play a vast role, it is as nothing compared to their potential for the future. International teleconferencing, electronic mail, direct-to-home TV, instantaneous personal communication wherever you go, links to l ibraries and data banks worldwide, these are but a few of the possibilities.

In speaking of the future of communications satellites at the signing of the Intelsat agreements in Washington in 1971, Arthur Clarke recal led an incident that occurred in England about 100 years before, when news came from the Uni ted S ta tes tha t a Mr. Bell had invented something called a telephone. A parliamentary commission convened to consider the evidence, and reported reassuringly that nothing further would be heard of this "impractical Yankee invention."

The chief engineer of the Brit ish Post Office, responsi- ble for operating the telegraph l ines in Great Britain, was one

of the witnesses called to testify. Asked whether he thought that th is device which t ransmits human speech would be of use in Great Britain, he replied, "No. The Americans may have need of the telephone, but we do not. Here in Britain we have plenty of messenger boys."

This very competent engineer completely failed to recognize the value of the t e lephone , bu t as Clarke pu ts i t , "Who could blame him? Could anyone, back in ISSrJ, have imagined that the t ime would come when every home would have a telephone, and business and social life would depend on it almost completely?"

Fig. 3. Earth with communication satellites in Clarke orbit (upper right) and typical footpr int of a North American satellite downlink (lower left) with ERP (effective radiated power) contours in dB above 1 W isotropic. (From John Kraus, Electromagnetics, McGraw-Hill, 1984; used by permission of the author.)

The eventual impact of the communications satel1it.e upon a l l of mankind, according to Clarke, will be at least as g r e a t as t h a t of the telephone. The technology is straight- forward, but will its application be in the best interests of all? Clarke is optimistic and believes that communication satellites can unite mankind. The development of the United States one hundred years ago became possible because of two inventions, the railroad and the telegraph. Now, according t o Clarke, what the railroads and telegraph did here a century ago, the jets and communciation satell i tes are doing to t h e whole world.

As Clarke has said, "For thousands of years, men have sought their future in the starry sky. Now this old astro- logical superstition has at last come true, for our destinies do indeed depend on celestial bodies--those that we have c rea ted ourselves."

The satell i tes cannot convey goods or materials, but they transmit radio photons which carry our new wealth: information and know-how.

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