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Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent aliens. Active SETI messages are usually in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message. Active SETI is also known as METI (Messaging to Extra-Terrestrial Intelligence), or positive SETI. Active SETI is contrasted to passive SETI, which only searches for signals, without any attempt to send them.
The science known as SETI deals with searching for messages from aliens. METI science deals with the creation of messages to aliens. Thus, SETI and METI proponents have quite different perspectives. SETI scientists are in a position to address only the local question “does Active SETI make sense?” In other words, would it be reasonable, for SETI success, to transmit with the object of attracting ETI’s attention? In contrast to Active SETI, METI pursues not a local and lucrative impulse, but a more global and unselfish one – to overcome the Great Silence in the Universe, bringing to our extraterrestrial neighbors the long-expected annunciation “You are not alone!”
Rationale for METI 
In the paper Rationale for METI transmission of the information into the Cosmos is treated as one of the pressing needs of an advanced civilization. This view is not universally accepted, and it does not agree to those who are against the transmission of interstellar radio messages.
Radio Message construction 
The lack of an established communications protocol is a challenge for METI.
First of all, while trying to synthesize an Interstellar Radio Message (IRM), we should bear in mind that Extraterrestrials will first deal with a physical phenomenon and, only after that, perceive the information. At first, ET's receiving system will detect the radio signal; then, the issue of extraction of the received information and comprehension of the obtained message will arise. Therefore, above all, the Constructor of an IRM should be concerned about the ease of signal determination. In other words, the signal should have maximum openness, which is understood here as an antonym of the term security. This branch of signal synthesis can be named anticryptography.
Also characteristics of the radio signal such as wavelength, type of polarization, and modulation have to be considered.
Over galactic distances, the interstellar medium induces some scintillation effects and artificial modulation of electromagnetic signals. This modulation is higher at lower frequencies and is a function of the sky direction. Over large distances, the depth of the modulation can exceed 100%, making any METI signal very difficult to decode.
Error correction 
In METI research it is implied that any message must have some redundancy, although the exact amount of redundancy and message formats are still in great dispute.
Using ideograms instead of binary sequence already offers some improvement against noise resistance. In faxlike transmissions ideograms will be spread on many lines. This increases its resistance against short bursts of noise like radio frequency interference or interstellar scintillation.
One format approach proposed for interstellar messages was to use the product of two prime numbers to construct an image. Unfortunately, this method works only if all the bits are present. As an example, the message sent by Frank Drake from the Arecibo Observatory in 1974 did not have any feature to support mechanisms to cope with the inevitable noise degradation of the interstellar medium.
Error correction tolerance rates for previous METI messages
- Arecibo Message (1974) : 8.9% (one page)
- Yevpatoria message (1999) : 44% (23 separate pages)
- Yevpatoria message (2003) : 46% (one page, estimated)
The 1999 Cosmic Call transmission was far from being optimal (from our terrestrial point of view) as it was essentially a monochromatic signal spiced with a supplementary information. Additionally, the message had a very small modulation index overall, a condition not viewed as being optimal for interstellar communication.
- Over the 370,967 bits (46,371 bytes) sent, some 314,239 were “1” and 56,768 were “0”, creating a ratio of 5.54 times more “1” than “0”.
- Since frequency shift keying modulation scheme was used, most of the time the signal was on the “0” frequency.
- In addition, “0” tended to be sent in long stretches (white lines in the message).
Realized projects 
- The Morse Message (1962)
- Arecibo message (1974)
- Cosmic Call 1 (1999)
- Teen Age Message (2001)
- Cosmic Call 2 (2003)
- Across the Universe (2008)
- A Message From Earth (2008)
- Hello From Earth (2009)
- RuBisCo Stars (2009)
- Wow! Reply (2012) 
|Name||Designation||Constellation||Date sent||Arrival date||Message|
|Messier 13||NGC 6205||Hercules||November 16, 1974||approx. 27000||Arecibo Message|
|Altair||Alpha Aql||Aquila||August 15 1983||1999||Altair (Morimoto - Hirabayashi) Message|
|Spica||Alpha Vir||Virgo||August 1997||2247||NASDA Cosmic-College|
|16 Cyg A||HD 186408||Cygnus||May 24, 1999||November 2069||Cosmic Call 1|
|15 Sge||HD 190406||Sagitta||June 30, 1999||February 2057||Cosmic Call 1|
|HD 178428||Sagitta||June 30, 1999||October 2067||Cosmic Call 1|
|Gl 777||HD 190360||Cygnus||July 1, 1999||April 2051||Cosmic Call 1|
|HD 197076||Delphinus||August 29, 2001||February 2070||Teen Age Message|
|47 UMa||HD 95128||Ursa Major||September 3, 2001||July 2047||Teen Age Message|
|37 Gem||HD 50692||Gemini||September 3, 2001||December 2057||Teen Age Message|
|HD 126053||Virgo||September 3, 2001||January 2059||Teen Age Message|
|HD 76151||Hydra||September 4, 2001||May 2057||Teen Age Message|
|HD 193664||Draco||September 4, 2001||January 2059||Teen Age Message|
|HIP 4872||Cassiopeia||July 6, 2003||April 2036||Cosmic Call 2|
|HD 245409||Orion||July 6, 2003||August 2040||Cosmic Call 2|
|55 Cnc||HD 75732||Cancer||July 6, 2003||May 2044||Cosmic Call 2|
|HD 10307||Andromeda||July 6, 2003||September 2044||Cosmic Call 2|
|47 UMa||HD 95128||Ursa Major||July 6, 2003||May 2049||Cosmic Call 2|
|Polaris||HIP 11767||Ursa Minor||February 4, 2008||2439||Across the Universe|
|Gliese 581||HIP 74995||Libra||October 9, 2008||2029||A Message From Earth|
|Gliese 581||HIP 74995||Libra||August 28, 2009||2030||Hello From Earth|
|GJ 83.1||GJ 83.1||Aries||November 7, 2009||2024||RuBisCo Stars|
|Teegarden's Star||SO J025300.5+165258||Aries||November 7, 2009||2022||RuBisCo Stars|
|Kappa1 Ceti||GJ 137||Cetus||November 7, 2009||2039||RuBisCo Stars|
|HIP 34511||Gemini||August 15, 2012||2163||Wow! Reply|
|37 Gem||HD 50692||Gemini||August 15, 2012||2069||Wow! Reply|
|55 Cnc||HD 75732||Cancer||August 15, 2012||2053||Wow! Reply|
Potential risk 
Active SETI has been heavily criticized due to the perceived risk of revealing the location of the Earth to alien civilizations, without some process of prior international consultation. Notable among its critics is scientist and science fiction author David Brin, particularly in his article/"expose" Shouting at the Cosmos.
However, Russian and Soviet radio engineer and astronomer Alexander L. Zaitsev has argued against these fears: see Sending and Searching for Interstellar Messages and Detection Probability of Terrestrial Radio Signals by a Hostile Super-civilization. Indeed, Zaitsev argues that we should consider the risks of NOT reaching out to extraterrestrial civilizations: see Making a Case for METI.
To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics  adopted in 2007 a new analytical tool, the San Marino Scale . Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the San Marino Scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are created equal, thus each must be evaluated on a case-by-case basis before establishing blanket international policy regarding Active SETI.
Beacon proposal 
One proposal for a 10 billion watt interstellar SETI beacon was dismissed by Robert A. Freitas Jr. to be infeasible for a pre-Type I civilization on the Kardashev scale. As a result it has been suggested that civilizations must advance into Type I before mustering the energy required for reliable contact with other civilizations.
However, this 1980s technical argument assumes omni-directional beacons which may not be the best way to proceed on many technical grounds. Advances in consumer electronics have made possible transmitters that simultaneously transmit many narrow beams, covering the million or so nearest stars but not the spaces between. This multibeam approach can reduce the power and cost to levels that are reasonable with current mid-2000s Earth technology.
Once civilizations have discovered each others' locations, the energy requirements for maintaining contact and exchanging information can be significantly reduced through the use of highly directional transmission technologies.
In 1974, the Arecibo Observatory transmitted a message toward the M13 globular cluster about 25,000 light-years away, for example, and the use of larger antennas or shorter wavelengths would allow transmissions of the same energy to be focused on even more remote targets, such as those attempted by Active SETI.
See also 
- Messaging to Extra-Terrestrial Intelligence, Alexander Zaitsev, IRE, Russia
- An Awkward History of Our Space Transmissions
- "RuBisCo Stars" and the Riddle of Life
- (Russian) Transmission and retrieval of intelligent signals in the universe, А. Л. Зайцев AL Zaitsev, РАН Institute of Radio Engineering and Electronics, (Keynote Address at the National Astronomical Conference VAK-2004 "Horizons of Universe", Moscow, MSU, June 7, 2004)
- (English) interstellar radio message (IRM), Encyclopedia of Science
- Is anybody listening out there?, BBC News, October 9, 2008; accessed on line November 11, 2008.
- Word MIR (it signifies both "peace" and "world" in Russian) was transmitted from the EPR on 19 November 1962, and words LENIN and SSSR (the Russian acronym for the Soviet Union) – on 24 November 1962, respectively were sent to the direction near the star HD131336 in the Libra constellation.
- Freitas Jr., Robert A. (1980). "INTERSTELLAR PROBES: A NEW APPROACH TO SETI". Journal of the British Interplanetary Society 33: 95–100. Bibcode:1980JBIS...33...95F.
- Scheffer, L. (2005) A scheme for a high-power, low-cost transmitter for deep space applications, Radio Science, , 40, RS5012.
- Interstellar Radio Messages
- Making a Case for METI
- Should We Shout Into the Darkness?
- Error Correction Schemes In Active SETI
- The Evpatoria Messages
- Encounter 2001 Message
- METI: Messaging to Extra-Terrestrial Intelligence
- The Pros and Cons of METI from Centauri Dreams
- Classification of interstellar radio messages