Ask and ye shall receive... at least sometimes. Last week I posted a request for an article by science fiction writer
Arthur C. Clarke, of 2001: A Space Odyssey
fame, describing a geostationary satellite system was published in the October 1945 edition of Wireless World magazine.
Thanks to RF Cafe visitor Terry W., from the great state of Oklahoma, it is now available for everyone to enjoy. Clarke
was not just a sci-fi writer, but also an educated visionary and card-carrying member of the
British Interplanetary Society, who proposed many technological solutions to issues of his day. In this instance,
the challenge was developing an efficient means to distribute TV signals across Europe and the world. Clarke's calculations
for the necessary number of repeater towers proved that concept impractical, so he proposed using modified surplus
German V2 rockets to launch Earth-orbiting "artificial satellites," powered by batteries or solar panels, to distribute
signals globally. He calculated power densities, orbit altitudes and periods, S/N ratios, and antenna sizes. You will
want to read this article; it only takes about 15-20 minutes.
Peacetime Uses for V2
Can Rocket Stations Give World-Wide Radio Coverage?
By Arthur C. Clarke
ALTHOUGH it is possible, by a suitable choice of frequencies and routes, to provide telephony circuits between
any two points or regions of the earth for a large part of the time, long-distance communication is greatly hampered
by the peculiarities or the ionosphere, and there are even occasions when it may be impossible. A true broadcast service,
giving constant field strength at all times over the whole globe would be invaluable, not to say indispensable, in
a world society.
though the telephony and telegraph position is, that of television is far worse, since ionospheric transmission cannot
be employed at all. The service area of a television station, even on a very good site, is only about a hundred miles
across. To cover a small country such as Great Britain would require a network of transmitters, connected by coaxial
lines, waveguides or VHF relay links. A recent theoretical study1 has shown that such a system would require
repeaters at intervals of fifty miles or less. A system of that kind could provide television coverage, at a very
considerable cost, over the whole of a small country. It would be out of .the question to provide a large continent
with such a service, and only the main centres of population could be included in the network.
The problem is equally serious when an attempt is made to link television services in different parts of the globe.
A relay chain several thousand miles long would cost millions, and transoceanic services would still be impossible.
Similar considerations apply to the provision of wide-band frequency modulation and other services, such as high-speed
facsimile which are by their nature restricted to the ultra-high-frequencies.
Many may consider the solution proposed in this discussion too far-fetched to be taken seriously. Such an attitude
is unreasonable, as everything envisaged here is a logical extension of developments in the last ten years - in particular
the perfection of the long-range rocket of which V2
was the prototype. While this article was being written, it was announced that the Germans were considering a similar
project, which they believed possible within fifty to a hundred years.
Before proceeding further, it is necessary to discuss briefly certain fundamental laws of rocket propulsion and
"astronautics." A rocket which achieved a. sufficiently great speed in flight outside the earth's atmosphere would
never return. This "orbital" velocity is 8 km per sec, (5 miles per sec), and a rocket which attained it would become
an artificial satellite, circling the world forever with no expenditure of power - a second moon, in fact. The German
transatlantic rocket Aro to would. have reached more than half this velocity.
It will be possible in a few more years to build radio controlled rockets which can be steered into such orbits
beyond the limits of the atmosphere and left to broadcast scientific information back to the earth. A little later,
manned rockets will be able to make similar flights with sufficient excess power to break the orbit and return to
There are an infinite number of possible stable orbits, circular and elliptical, in which a rocket would remain
if the initial conditions were correct. The velocity of 8 km/sec. applies only to the closest possible orbit, one
just outside the atmosphere, and the period of revolution would be about 90 minutes. As the radius of the orbit increases
the velocity decreases, since gravity is diminishing and less centrifugal force is needed to balance it. Fig. 1 shows
this graphically. The moon, of course, is a particular case and would lie on the curves of Fig. 1 if they were produced.
The proposed German space-stations would have a period of about four and a half hours.
Fig. 1, Variation of orbital period and velocity with distance from the centre of
It will be observed that one orbit, with a radius of 42,000 km, has a period of exactly 24 hours. A body in such
an orbit, if its plane coincided with that of the earth's equator, would revolve with the earth and would thus be
stationary above the same spot on the planet. It would remain fixed in the sky of a whole hemisphere and unlike all
other heavenly bodies would neither rise nor set. A body in a smaller orbit would revolve more quickly than the earth
and so would rise in the west, as indeed happens with the inner moon of Mars.
Using material ferried up by rockets, it would be possible to construct a "space-station" in such an orbit. The
station could be provided with living quarters, laboratories and everything needed for the comfort of its crew, who
would be relieved and provisioned by a regular rocket service. This project might be undertaken for purely scientific
reasons as it would contribute enormously to our knowledge of astronomy, physics and meteorology. A good deal of literature
has already been written on the subject.2
Although such an undertaking may seem Fantastic, it requires for its fulfillment rockets only twice as fast as
those already in the design stage. Since the gravitational stresses involved in the structure are negligible, only
the very lightest materials would be necessary and the station could be as large as required.
Fig. 2, Typical extra-terrestrial relay services. Transmission from A being relayed
to points B
and area C; transmission from D being relayed to whole hemisphere.
Let us now suppose that such a station were built in this orbit. It could be provided with receiving and transmitting
equipment (the problem of power will be discussed later) and could act as a repeater to relay transmissions between
any two points on the hemisphere beneath, using any frequency which will penetrate the ionosphere. If directive arrays
were used, the power requirements would be very small, as direct line of sight transmission would be used, There is
the further important point that arrays on the earth, once set up, could remain fixed indefinitely.
Moreover, a transmission received. from any point on the hemisphere could be broadcast to the whole of the visible
face of the globe, and thus the requirements of all possible services would be met (Fig. 2)
It may be argued that we have as yet no direct evidence of radio waves passing between the surface of the earth
and outer space; all we can say with certainty is that the shorter wavelengths are not reflected back to the earth.
Direct evidence of field strength above the earth's atmosphere could be obtained by V2 rocket technique, and it is
to be hoped that someone will do something about this soon as there must be quite a surplus stock somewhere! Alternatively,
given sufficient transmitting power, we might obtain the necessary evidence by exploring for echoes from the moon.
In the meantime we have visual evidence that frequencies at the optical end of the spectrum pass through with little
absorption except at certain frequencies at which resonance effects occur. Medium high frequencies go through the
E layer twice to be reflected from the F layer and echoes have been received from meteors in or above the F layer.
It seems fairly certain that frequencies from, say, 50 Mc/s to 100,000 Mc/s could be used without undue absorption
in the atmosphere or the ionosphere.
A single station could only provide coverage to half the globe, and for a world service three would be required,
though more could be readily utilized. Fig. 3 shows the simplest arrangement. The stations would be arranged approximately
equidistantly around the earth, and the following longitudes appear to be suitable:-
30 E - Africa. and Europe.
150 E - China and Oceana.
90 W - The Americas.
Fig. 3, Three satellite stations would ensure complete coverage of the globe.
The stations in the chain would be linked by radio or optical beams, and thus any conceivable beam or broadcast
service could be provided.
The technical problems involved in the design of such stations are extremely interesting,3 but only
a few can be gone into here, Batteries of parabolic reflectors would be provided, of apertures depending on the frequencies
employed. Assuming the use of 3,000 Mc/s waves, mirrors about a metre across would beam almost all the power on to
the earth. Larger reflectors could be used to illuminate single countries or regions for the more restricted services,
with consequent economy of .power. On the higher frequencies it is not difficult to produce beams less than a degree
in width, and, as mentioned before, there would be no physical limitations on the size of the mirrors. (From the space
station, the disc of the earth would be a little over 17 degrees across). The same mirrors could be used for many
different transmissions if precautions were taken to avoid cross modulation.
It is dear from the nature of the system that the power needed will be much less than that required for any other
arrangement, since all the energy radiated can be uniformly distributed over the service area, and none is wasted.
An approximate estimate of the power required for the broadcast service from a single station can be made as follows;-
The field strength in the equatorial plane of a λ/2 dipole in free space at a distance of d metres is4
e = 6.85 √P / d volts/metre, where P is the power radiated in watts.
Taking d as 42,000 km (effectively it would be less), we have
P=37.6 e2 watts. (e now in μV/metre.)
If we assume e to be 50 microvolts/metre, which is the F.C.C. standard for frequency modulation, P will be 94
kw. This is the power required for a single dipole, and not an array which would concentrate all the power on the
earth, Such an array would have a gain over a simple dipole of about 80. The power required for the broadcast service
would thus be about 1.2 kW.
Ridiculously small though it is, this figure is probably much too generous. Small parabolas about a foot in diameter
would be used for receiving at the earth end and would give a very good signal/noise ratio. There would be very little
interference, partly because of the frequency used and partly because the mirrors would be pointing towards the sky
which could contain no other source of signal. A field strength of 10 microvolts/metre might well be ample, and this
would require a transmitter output of only 50 watts.
A project which goes part of the way towards the goal envisaged in this article has been put
Westinghouse in collaboration with the Glen L. Martin Co. of America. The radius of coverage would
be increased from 50 to 211 miles by beamed radiation from an aircraft flying at a height of
30,000 ft., and equipped
with television and FM transmitters.
When it is remembered that these figures relate to the broadcast service, the efficiency of the system will be
realised. The pointto-point beam transmissions might need powers of only 10 watts or so. These figures, of course,
would need correction for ionospheric and atmospheric absorption, but that would be quite small over most of the band.
The slight falling off in field strength due to this cause towards the edge of the service area could be readily corrected
by a nonuniform radiator.
The efficiency of the system is strikingly revealed when we consider that the London Television service required
about 3 kW average power for an area less than fifty miles in radius.5
A second fundamental problem is the provision of electrical energy to run the large number of transmitters required
for the different services. In space beyond the atmosphere, a square metre normal to the solar radiation intercepts
1.35 kW of energy.6 Solar engines have already been devised for terrestrial use and are an economic proposition
in tropical countries. They employ mirrors to concentrate sunlight on the boiler of a low-pressure steam engine. Although
this arrangement is not very efficient it could be made much more so in space where the operating components are in
a vacuum, the radiation is intense and continuous, and the low-temperature end of the cycle could be not far from
absolute zero. Thermoelectric and photoelectric developments may make it possible to utilise the solar energy more
Though there is no limit to the size of the mirrors that could be built, one fifty metres in radius would intercept
over 10,000 kW and at least a quarter of this energy should be available for use.
The station would be in continuous sunlight except for some weeks around the equinoxes, when it would enter the
earth's shadow for a few minutes every day. Fig.-4 shows the state of affairs during the eclipse period. For this
calculation, it is legitimate to consider the earth as fixed and the sun as moving round it. The station would graze
the earth's shadow at A, on the last day in February. Every day, as it made its diurnal revolution, it would cut more
deeply into the shadow, undergoing its period of maximum eclipse on March 21st, on that day it would only be in darkness
for 1 hour 9 minutes From then onwards the period of eclipse would shorten, and after April 11th (B) the station would
be in continuous sunlight again until the same thing happened six months later at the autumn equinox, between September
12th and October 14th The total period of darkness would be about two days per year, and as the longest period of
eclipse would be little more than an hour there should be no difficulty in storing enough power for an uninterrupted
Fig. 4, Solar radiation would be cut off for a short period each day at the equinoxes.
Briefly summarised , the advantages of the space station are as follows:-
(1) It is the only way in which true world coverage call be achieved for all possible types of service.
It permits unrestricted use of a band at least 100,000 Mc/s wide, and with the use of beams an almost unlimited number
of channels would be available.
(3) The power requirements are extremely smart since the efficiency of "illumination"
will be almost 100 per cent. Moreover, the cost of the power would be very low.
(4) However great the initial
expense, it would only be a fraction of that required for the world networks replaced, and the running costs would
be incomparably less.
Appendix - Rocket Design
The development of rockets sufficiently powerful to reach"orbital" and even "escape" velocity is now only a matter
of years. The following figures may be of interest in this connection.
The rocket has to acquire a final velocity of 8 km/sec. Allowing 2 km/sec for navigational corrections and air
resistance less (this is legitimate as all space-rockets will be launched from very high country) gives a total velocity
needed of 10 km/sec. The fundamental equation of rocket motion is:2
V = v loge R
where V is the final velocity of the rocket, v the exhaust velocity and R the ratio of initial mass to final mass
(payload plus structure). So far v has been about 2-2.5 km/sec for liquid fuel rockets but new designs and fuels will
permit of considerably higher figures, (Oxyhydrogen fuel has a theoretical exhaust velocity of 5.2 km/sec and more
powerful combinations are known). If we assume v to be 3.3 km/sec, R will be 20 to 1. However, owing to its finite
acceleration, the rocket loses velocity as a result of gravitational retardation. If its acceleration (assumed constant)
is α meters/sec2, then the necessary ratio Rg is increased to
Rg = R (α +g) / α
For an automatically controlled rocket α would be about 5g and so the necessary R would be 37 to 1. Such ratios
cannot be realised with a single rocket but can be attained by "step-rockets"2, while very much higher
ratios (up to 1,000 to 1) can be achieved by the principle of "cellular construction."3
Epilogue - Atomic Power
The advent of atomic power has at one bound brought space travel half a century nearer. It seems unlikely that
we will have to wait much as twenty years before atomic-powered rockets are developed. and such rockets could reach
even the remoter planets with a fantastically small fuel/mass ratio ---only a few per cent. The equations developed
in the appendix still hold, but v will be increased by a factor of about. a thousand.
In view of these facts, it appears hardly worth, while to expend much effort on the building of long-distance relay
chains. Even the local networks which will soon be under construction may have a working life of only 20-30 years.
1 Radio-Relay Systems," C. W. Hansell. Proc.
I.R.E.,. Vol 33, March 1945
2 "Rockets," Willy Ley. (Viking Press, N.Y.)
Problem der Befahrung des Weltraums," Hermann Noordung.
4 "Frequency Modulation," A. Hund.
5 "London Television Service," MacNamara amd Birkinshaw. J.I.E.E., Dec.
6 "The Sun," C. G. Abbot. (Appleton-Century Co.).
7 "Journal of the British
Interplanetary Society," Jan., 1939.
Posted July 5, 2011