Deep Space Communication: Problems And Solutions

Abstract
Introduction
History of Mankind and Space
Deep Space Communication
Possible Solutions
Conclusions
References
Bibliography

Abstract

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.


Introduction

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.
 

History of Mankind and Space

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.
 

Deep Space Communication

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
3.75 - 2.4 8 -12
K  2.4 - 0.75 12 - 40
  ("Fig 1: The bands, wavelengths and frequencies of Space communication." Deep Space Network)

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 Maximum approximate timings for signals to travel to planets in our solar system. http://eee2proj/dnb97/Comms/commsdoc.html)

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.

Possible Solutions

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.

Conclusion

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.

References

[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)
 

Bibliography

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

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