The Deep Space Network antennae
dish at Goldstone, California, USA
The prime objectives of mans exploration of space are the quest for life amongst the stars and the search for information. Any object put into space must retain a link with Earth. Probes sent out to the frontiers of space require a method of sending telemetry back to Mission Control. In the case of a manned missions it is essential that communication are maintained between the spacecraft and Mission Control for reasons of safety and to expand the facilities available to the crew. For unmanned missions it is often important that control can be exerted from the ground. Communication is the most important and efficient factor in space exploration and imposes some of the biggest problems when mankind tries to explore space.
The mysteries of space have always provided a distinct lure to mankind.
Since the earliest days of the space program communication has helped and
limited space exploration. This report is an attempt to understand the
role of space communication and the problems and any possible solutions
to those same problems. The first fifty years of the space program has
seen the first man in space, and the first lunar landings. However, the
future of space exploration lies beyond the moon as seen by the recent
missions to Mars and the probes sent to explore the planets further from
Earth. The first man-made objects to reach the outer planets and soon the
outer limits of our solar system have required great advancements by man,
in the area of communication. Man has explored space from the Earth for
thousands of years. Since the 17th century, we have used telescopes to
explore more and more of outer space. Advances in astronomy and especially
radio astronomy, have given mankind a greater understanding of space, without
the need to leave the Earth.
Mankind has dreamed of travelling through the stars for millennia before it became reality. Evidence of this desire exists in myth and fiction as far back as Babylonian texts of 4000 BC. The dream of man to fly and to travel to new places is represented in ancient Greek myths such of that of Icarus. As far back as the 2nd century AD, Lucian wrote about an imaginary voyage to the moon. The French writer and philosopher Voltaire, in the book "Micromégas" (1752), told of the travels of certain inhabitants of Sirius and Saturn; and in 1865 the famous author Jules Verne depicted space travel in one the most popular novels ever written "From the Earth to the Moon". The dream of flight into space expressed through fiction continued unabated into the 20th century, most notably in the works of the British writer H. G. Wells, who authored "The War of the Worlds" in 1898 and "The First Men in the Moon" in 1901. Now the ancient dream of space travel is fulfilled with an almost endless stream of science fiction.
During the many centuries when space travel was a mere fantasy, scientists
in the areas of astronomy, chemistry, mathematics, meteorology, and physics
developed an understanding of the solar system, the stellar universe, the
atmosphere of the earth, and the probable environment in space. Not until
some 1400 years after the Greek scientists discovered that the Earth was
a sphere and that the Earth moved around the sun the astronomer Nicolaus
Copernicus systematically explained that all the planets, including the
Earth, revolve about the sun. In the following centuries the observations
of astronomers changed the way in which mankind understood the laws of
the Earth and of space. Amongst the astronomers involved in this movement
were Tycho Brahe, Johannes Kepler, Galileo, Edmund Halley, Sir William
Herschel, and Sir James Jeans. Their contributions became the basis of
the new science of astronautics. It was not just astronomers but physicists
and mathematicians who also helped to lay the foundations of astronautics.
In the mid-sixteenth century the German physicist von Guericke proved that
a vacuum could be maintained. In the late 17th century English mathematician
and physicist Sir Isaac Newton formulated the laws of universal gravitation
and motion. Newton's laws of motion established the basic principles governing
the propulsion and orbital motion of the spacecraft, which would be used
to send first of all unmanned and then manned objects into space. Despite
the scientific foundations laid in earlier ages, to allow man into space,
space travel did not become possible until the advances of the 20th century.
These advances in rocket propulsion grew mostly from the Second World War
and the aftermath of it provided the actual means of rocket propulsion,
guidance, and control needed for space vehicles.
The vast cost of placing a man into space made manned space flights unattractive to the public and to the government facing an economic drain from the war in Vietnam. Therefore the development of satellites was as a cheaper alternative to manned space flights. Probes were designed to collect information from the outer planets of our solar system, such as Neptune, Uranus and Jupiter. These probes contained some computer technology on board, but were mainly controlled by mission control, and mission control received information back from these probes, millions of kilometres away. The challenge of maintaining communication over millions of kilometres led to the creation of the NASA Deep Space Network (DSN). The Deep Space Network consists of three antennae placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert, near Madrid, Spain, and near Canberra, Australia. The 120-degree angle is necessary to communicate with deep space probes at all times of the day and night. When a probe such as Galileo sends a picture back to Earth getting the picture is the task of Deep Space Network. The digital data bitstream can be transmitted from a spacecraft in various frequencies. Galileo, for example, transmits in "S-band," at rates up to 160 bits per second. At this rate, one 800 x 800-pixel image, compressed at 10:1, is received on Earth every 53 minutes (5,120,000 ÷ 160 ÷ 60 ÷ 10). The bitstream is received by huge antenna receivers at one of the three DSN sites around the globe. [2]
The Deep Space Network antennae
dish at Goldstone, California, USA
When communicating across deep space communication the frequencies used are very important. Electromagnetic radiation with frequencies between about 10 kHz and 100 GHz are referred to as radio frequencies (RF). These radio frequencies are divided into groups which have similar characteristics, called "bands," such as "S-band," "X-band," etc. These bands are then further divided into small ranges of frequencies called "channels," some of which are allocated for the use of deep space telecommunications. Many deep-space vehicles use S-band and X-band frequencies, which are in the region of 2 to 10 GHz. These frequencies are among those referred to as microwaves, because their wavelength is short, on the order of centimetres. Deep space telecommunications systems are being developed for use on the even higher frequency Kband. [2]
Fig. 1 below shows the radio band frequencies and the range of the various
wavelengths
| Band Range | Wavelengths (cm) | Frequency (GHz) |
| L | 30 -15 | 1 -2 |
| S | 15 - 7.5 | 2 -4 |
| C | 7.5 - 3.75 | 4 - 8 |
| X | 3.75 - 2.4 | 8 -12 |
| K | 2.4 - 0.75 | 12 - 40 |
The Problems of Space Communication
Two-way communication with spacecraft over such a large distance poses many problems. When communicating with probes or craft in space the time taken for a signal to travel to and from the craft becomes an important factor. Einstein's theory of relativity states that nothing may travel faster than the speed of light, which means that the fastest signal that could ever be sent is 3x108ms-1. Also all radio-based communication must cope with spreading loss. [3] Spreading loss is an inverse-square relationship with distance and so the further a spacecraft goes away from Earth, the weaker its signal received becomes. [2] By the time a spacecraft reaches the limits of our solar system it will have very little power especially using solar power and being so far away from the sun, their available power for communication will be much less.
The table below gives maximum approximate timings for signals to travel
to planets in our solar system.
| Planet | Mean Distance From Earth (AUs) | Propagation Time (sec) |
| Mercury | 0.61 | 30 |
| Venus | 0.28 | 14 |
| Mars | 0.52 | 26 |
| Jupiter | 4.20 | 210 = 3.5 minutes |
| Saturn | 8.54 | 427 = 7 minutes |
| Uranus | 18.18 | 910 = 15 minutes |
| Neptune | 29.06 | 1454 = 24 minutes |
| Pluto | 28.44 | 1423 = 23.5 minutes |
Fig 2 shows that the further away from Earth a spacecraft travels the longer it takes for a signal to be received. Therefore if it takes 24 minutes for a message to be received it makes control of the spacecraft from million of kilometres away a slow and inefficient process. The process of sending a signal, receiving it and then responded to it takes close to an hour without any errors or interference. Under more difficult circumstances the process could take a significantly longer time.
Deep Space Communication between the Earth and a probe or satellite
can also be affected by a condition known as space weather. The term space
weather describes conditions in space that affect a wide variety of technological
systems including satellites. Space weather is an integral part of the
space age and, like traditional weather, is most noticeable when it causes
problems. Communications at all frequencies are affected by space weather.
This is especially true of high frequency radiowave communications, which
rely on ionospheric reflection to carry signals great distances. Trans-ionospheric
signals passing through these irregularities can experience variations
in signal strength called scintillations, in the VHF and UHF frequency
bands (30MHz to 3GHz). This can introduce delays in transmission, and sometimes
complete disruption of the signal. Ionospheric disturbances not only affect
telecommunications companies, who are increasingly dependent on higher
frequencies to penetrate the ionosphere to relay communications via satellite,
but also users of the Global Positioning Satellite (GPS) system which loses
accuracy because of time delays and signal refraction errors. [4]
Certain conditions in space can cause high-energy particles to penetrate
deep into satellites and adversely affect electronic components including
computer memory chips. Another problem is the accumulation of charge on
the satellite due to deep dielectric charging and surface charging. Subsequent
discharges cause both material damage and electrical transients on the
spacecraft. [4] The electrical transients can then masquerade as phantom
commands appearing to spacecraft’s onboard systems to be orders from mission
control. These phantom commands can cause the malfunction of instruments,
power and propulsion systems.
There are no easy solutions to the problem of communicating through
deep space. The only way at this time to communicate over long distances
is to keep building bigger and bigger radio telescopes with higher gain
and more sensitive receivers for a more efficient signal to noise ratio.
Despite the building these improved antennae the problems of spread loss
and the low power of the probes are still to be solved.
One possible solution may be to build relay stations, which would boost
the signals being sent and received, eliminating the problem of the probes
having low power. The construction of the relay stations would be a huge
engineering problem. If a network of relay stations could be built it would
improve quality of the data being sent. The relay network would work in
a similar fashion to a standard computer network where a device is placed
to boost the signal when it reaches it’s optimal distance from it’s source
or the last booster device.
The use of more efficient data compression and communication techniques
on Earth may provide the answers to improving deep space communication
much as VHF technology allowed the Soviets in the 1950’s to communicate
with Vostok 1, the first manned spacecraft.
It is clear that the future of man’s exploration of space is reliant
on mankind finding a way to communicate across the vastness of the solar
system.
The problems facing the scientists and engineers who must find a way
to allow Earth to communicate with the spacecraft sent to explore the depths
of space are enormous. The connection between mankind and space continues
to be as strong with every generation breeding new scientists, engineers
and astronauts all with their own ideas on how to solve these problems.
The technology required allowing communication with spacecraft may have
to break or in the least bend the Law of Relativity as Einstein clearly
stated that nothing can travel faster than the speed of light. A signal
moving faster than the speed of light may be what is necessary to allow
instantaneous communication between Earth and an object millions of kilometres
away.
The work of NASA and in particular the Jet Propulsion Lab and the Deep
Space Network are an intricate part of solving the problems of Deep Space
Communication and allowing man to travel into space, even establish colonies
on other planets and retain contact with Earth.
[1] Microsoft Encarta 97: “Space Exploration”
[2] http://eee2proj/dnb97/Comms/commsdoc.html (Unknown author)
[3] http://deepspace.jpl.nasa.gov/dsn/index.html (Unknown author)
[4] http://osprey.itd.sterling.com/spacecast/about_sw.html
(Unknown author)
http://eee2proj/dnb97/Comms/commsdoc.html
http://deepspace.jpl.nasa.gov/dsn/index.html
http://osprey.itd.sterling.com/spacecast/about_sw.html
http://hubble.gsfc.nasa.gov/discussion.html
http://www.jpl.nasa.gov/radioastronomy/
Maral G., Bousquet M. “Satellite Communications Systems: Systems, Techniques and Technology Third Edition”, John Wiley and Sons Ltd Chichester.
Picture from the Deep Space Network
This Report authored by Andrew
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