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1.History of Spacecraft Communications

1.1 Introduction

The first spacecraft Sputnik was launched by Soviet Union in 1957. It was followed later by the first manned flight and first American space flight. After this a great number of spacecrafts has been sent to explore the space. All this required communication between spacecraft and ground station. In space the distances are much greater than on earth and spacecrafts depend on communications much more than vehicles used on earth. Because of that, special care and special ways were needed when communication was designed, implemented and used. The information of Soviet spacecraft systems is not easy to find so this text will concentrate only in American space communication systems.

Information from spacecraft to the ground station, including flight performance data and prelaunch data was transmitted by telemetry.

Mercury was the first space program of the United States. The first worldwide tracking network was built for Mercury program. This was one of the most expensive parts of the project, over 177 000 miles of communications lines were built for it. For the case of emergency, there was a system in the spacecraft that allowed the ground station to get the spacecraft back on earth. This information was transmitted on secret frequency [1].
 

1.2 Apollo program communications

The Apollo program allowed human being to step on the moon. This was achieved by Apollo 11 flight in 1969 and after this there were yet many other Apollo landings until 1975.

The functional telemetry system was essiential for Apollo flights. Communication was done through data link or by radio transmission. 22 telemetry links were placed also on launch vehicle (Saturn V).

Three ground networks were used for telemetry receiving and  the most important was Manned Space Flight Network, which was controlled by Goddard Space Flight Center in Greenbelt, Maryland, USA.

To control both command and lunar modules succesfully with the great distance, unified S-band system was used. It used the band between  1550 and 5220 MHz, the name was invented during the Second World War. Jet Propulsion Laboratory had developed a system with this frequency for unmanned probes and it was used also with Apollo flights. System functions included tracking, ranging, measurement telemetry and command, voice and television communications [2].

For the tracking system over 100 locations all over the world were connected by high-speed communications implemented by cables, radio communications and satellite links and included also ships and aeroplanes. Three spaceflight centres were connected by links. Also US Air Force equipment was used. For the communications while near moon, three communication stations were constructed: one in Goldstone, California, one in Australia near Canberra and one in Spain near Madrid, each with two 26 m antennas. The locations chosen made possible the communications on every moment because at least one of them was always facing the moon. In Earth orbit the Mercury tracking system with 9 m antennas was used but their power was not enought when the spacecraft moved farther away from the Earth. Later, one big 64 m antenna was constructed in Goldstone [3].
 

1.3 Space Shuttle communications

The space shuttle was the first spacecraft that was designed to be used many times. It's most important part is the orbiter which carries the crew. After the launch the orbiter will land on earth like aeroplanes so it an be used again. The first space shuttle was launched in 1981 and after that
almost 100 shuttle flights have been made and many new fights have been planned in future.

Space shuttle has been using two systems for communication and tracking:

Tracking and Data Relay Satellite System (TDRSS) uses three geosynchronous satellites, one launched in 1983 ane another in 1988. One has not yet been launched. The system works with S-band, 4 to 6 GHz C-band and 12 to 14 GHz Ku-band. Satellite data is received at White Sands ground terminal. Three biggest antennas at the center have almost 20 feet diameter and they ar used in Ku-band transmission. Various smaller antennas are used for the other bands. Also a second ground station will be builded

Space flight tracking and data network is the traditional ground network which will be closed when
TDRSS is completed. It's main centre is at Goddard Space Center and it has tracking stations all over the world.

The orbiter communication system is divided in the following parts

• S-band frequency and phase modulation, 1700 - 2300 MHz is used for ground communications together with Ku-band
• Ku-band system, 15250 -17250 MHz.
• Ultra-high frequency system, backup for S- and Ku-band systems, 296,8 or 259,7 MHz
• Payload communication system
• Audio system
• Close-circuit television system

The data is encrypted and decrypted aboard.[4]

1.4 Unmanned spacecraft communications

1.4.1 Pioneer

Pioneer 10 was the first spacecraft sent to the outer solar system in 1972. Pioneer 11 was sent a year later.

Two methods of communication were used. One was an omnidirectional antenna set which was
connected to one receiver. Also a high-gain antenna was used for receiving. Two transmitters with the power of 8W were used at the frequency of 2292 MHz. The data tranfsfer rate was 2048 bps until Jupiter After that it was decreased to 1024 and after reaching Saturn it was only 16 bps. [5]
 

1.4.2 Voyager

Voyager 1 and 2 were sent in 1977 to explore the outer part of the solar system.

Voyager communication system used one high gain antenna and low gain antenna which was used as a backup. X-band telemetry was used first time and also S-band was used. [6]
 
 

1.5 References

[1] This New Ocean: A History of Project Mercury,  Loyd S. Swenson Jr., James M. Grimwood and Charles C. Alexander, 1989, http://www.hq.nasa.gov/office/pao/History/SP-4201/toc.htm

[2] Moonport: A History of Apollo Launch Facilities and Operations, Charles D. Benson and William Barnaby Faherty, 1978, http://www.hq.nasa.gov/office/pao/History/SP-4204/contents.html

[3] Chariots for Apollo: A History of Manned Lunar Spacecraft, Courtney G. Brooks. James M. Grimwood and Loyd S. Swenson, Jr., 1979
http://www.hq.nasa.gov/office/pao/History/SP-4205/contents.html

[4] Space Shuttle News Reference Manual, Information content from NSTS Shuttle Reference Manual,1988,  http://www.ksc.nasa.gov/shuttle/technology/sts-newsref/stsref-toc.html

[5] Pioneer 10, J.H. King 1998, http://nssdc.gsfc.nasa.gov/cgi-bin/database/www-nmc?72-012A

[6] Voyager Project Information, J.H. King 1999, http://nssdc.gsfc.nasa.gov/planetary/voyager.html

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