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 of 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.
Unsatisfactory 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 study 1
has shown that such a system would require repeaters at intervals of fifty
miles or less. A system of this 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
farfetched to be taken very 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 for ever with no expenditure of
power--a second moon, in fact. The German transatlantic rocket A10 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.
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
Although such an undertaking may seem fantastic, it requires for its
fulfilment 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.
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).
Fig. 2. Typical extra-terrestrial relay services. Transmission from A
being relayed to point B and area C; transmission from D being relayed to
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
utilised. Fig. 3 shows the simplest arrangement. The stations would be
arranged approximately equidistantly around the earth, and the following
longitudes appear to be suitable :--
||-- Africa and Europe.
||-- China and Oceana.
||-- The Americas.
Fig 3. Three satellite stations would ensure complete coverage of the
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 clear 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
/2 dipole in free space at a
distance of d metres is 4
|e = 6.85
||where P is the power radiated in watts.
Taking d as 42,000 km (effectively it would be less), we have P
= 37.6 e 2 watts. (e now in
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.
When it is remembered that these figures relate to the broadcast
service, the efficiency of the system will be realised. The point-to-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 non-uniform 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.
Thermo-electric 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.
Fig. 4. Solar radiation would be cut off for a short period each day at
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.
Briefly summarised, the advantages of the space station are as
- (1) It is the only way in which true world coverage can 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 small 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.
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 loss (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 = log e
where V is the final velocity of the rocket,
the exhaust velocity and R the
ratio of initial mass to final mass (payload plus structure). So far
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
to be 3.3 km/sec. R will be 20 to
I. However, owing to its finite acceleration, the rocket loses velocity as
a result of gravitational retardation. If its acceleration (assumed
constant) is a metres/sec. 2. then the necessary ratio Rg
is increased to
For an automatically controlled rocket a would be about 5g and so the
necessary R would be 37 to I. 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 i) can be achieved by the principle of
``cellular construction'' 3.
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 as 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 Noordung.
- 4 ``Frequency Modulation,'' A. Hund. (McGraw Hill:)
- 5 ``London Television Service,'' MacNamara and 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.
A project which goes part of the way towards the goal envisaged in this
article has been put forward by Westinghouse in collaboration with 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.
EUROPEAN FREQUENCY ALLOCATIONS
THE Postmaster-General is understood to be planning an early Conference
of interested parties to consider the allocation of frequency channels for
the liberated countries of Europe. No detailed information on the scope of
the Conference was available up to the time of going to press.