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
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
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
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 earth.
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 the earth.
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.
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.
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:-
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
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
The field strength in the equatorial plane of a λ/2 dipole in free space at a distance of d
e = 6.85 √P / d volts/metre, where P is the power radiated in watts.
as 42,000 km (effectively it would be less), we have
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 directly.
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 service.
Fig. 4, Solar radiation would be cut off for a short period each day at the
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.
(2) 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
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
is increased to
= 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."3Epilogue - 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.)
3 "Das Problem der Befahrung des Weltraums," Hermann
4 "Frequency Modulation," A. Hund. (McGraw Hill).
5 "London Television Service," MacNamara
amd Birkinshaw. J.I.E.E., Dec. 1938.
6 "The Sun," C. G. Abbot. (Appleton-Century Co.).
7 "Journal of
the British Interplanetary Society," Jan., 1939.Posted 7/5/2011