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As everybody knows, the pioneering work of The Professor and his serendipitous collaborator Gilligan demonstrated the reality of ground to space optical communications back in 1964. In that case information was encoded in the incandescent radtaion of soot particles via spatial modulation1 rather than the more modern temporal modulation.

The launch of this crewed and remarkable mission can be seen here.

Questinon: Since that "fateful trip", how many independent demonstrations of optical communications between ground and space have been successful?


1 https://www.imdb.com/title/tt0588095/mediaviewer/rm2807151617

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Probably the first and only ground to space optical communications experiment with an object on the lunar surface was a laser communication line with Lunokhod-2.

"Эта же наземная аппаратура была использована для эксперимента по исследованию возможности передачи информации методом время-импульсной модуляции по оптическому каналу связи Алма-Ата - "Луноход-2". Объект использовался для ретрансляции сигналов на Землю по радиоканалу. Пропускная способность данной линии связи равна 15 дв.зн/сек."

The same ground-based equipment was used for an experiment to study the possibility of transmitting information by the method of time-pulse modulation over the optical communication channel Alma-Ata - Lunokhod-2. The object was used to relay signals to Earth via a radio channel. The throughput of this communication line is 15 two-digit characters per second. http://russianspacesystems.ru/wp-content/uploads/2018/01/1973_Radiotekhnicheskiy_kompleks_Luna21_Lunokhod_2.pdf

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  • $\begingroup$ Thanks, this is absolutely awesome!!! $\endgroup$
    – uhoh
    Nov 17 '20 at 18:14
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    $\begingroup$ "дв.зн." probably means "двоичных знаков", "binary digits" (i.e., bits). $\endgroup$
    – Litho
    Mar 3 at 19:30
  • $\begingroup$ I'm still hoping for a complete answer but this is so important (and since it's expiring) I'll award the bounty here. Thanks! $\endgroup$
    – uhoh
    Mar 10 at 6:12
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Laser communication in space is the use of free-space optical communication in outer space. Communication may be fully in space (an inter-satellite laser link) or in a ground-to-satellite or satellite-to-ground application.

The main advantage of using laser communication over radio waves is increased bandwidth, enabling the transfer of more data in less time.

In November 2014, the first-ever use of gigabit laser-based communication as part of the European Data Relay System (EDRS) was carried out. Further system and operational service demonstrations were carried out in 2014. Data from the EU Sentinel-1A satellite in LEO was transmitted via an optical link to the ESA-Inmarsat Alphasat in GEO and then relayed to a ground station using a conventional Ka-band downlink. The new system can offer speeds up to 7.2 Gbit/s.The Laser terminal on Alphasat is called TDP-1 and is still regularly used for tests. The first EDRS terminal (EDRS-A) for productive use has been launched as a payload on the Eutelsat EB9B spacecraft and became active in December 2016. It routinely downloads high-volume data from the Sentinel 1A/B and Sentinel 2A/B spacecraft to the ground.

So far (April 2019) more than 20000 links (11 PBit) have been performed.

This Wikipedia gives more answers: Laser Communication in Space

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  • $\begingroup$ Thank you for your answer! It is true that for spacecraft in low Earth orbit radio is limited in bandwidth compared to optical. But there are other advantages to using optical for LEO besides bandwidth. With a wavelength roughly 10,000 times smaller, the "antennas" are now small telescopes of 5 to 50 cim in diameter rather than meter to tens of meters, and fast pointing and tracking is a lot easier because they are both compact and well, they are telescopes and so can more easily track signals by imaging the transmitted spot and in some cases other things as well. $\endgroup$
    – uhoh
    Mar 6 at 15:00
  • $\begingroup$ But for deep space applications, let's say to Mars or Jupiter or even the Kuiper belt, bandwidths are far lower than radio is capable, and that's because the wavelengths are centimeters. Antenna gains on the ground are 40 or 50 dB only, and those big dishes on spacecraft are 10 to 20 dB. Where optical communications really "shines* (pardon the pun) is for deep space. A 30 cm telescope will have a gain of 120 dB, but one at each end and you will have a much stronger signal. This is not because the bandwidth per se is bigger, but because the signal is larger than thermal noise over wider freq. $\endgroup$
    – uhoh
    Mar 6 at 15:05
  • $\begingroup$ Also, the question asks "How many independent demonstrations...?" not how many "links". To answer you'll need to count the number of independent demonstrations that were successful. I think that the answer is a half-dozen or less, not "20,000". $\endgroup$
    – uhoh
    Mar 6 at 15:06
  • $\begingroup$ Here's an example of a link budget calculation where those gains can be used to calculate a data rate: space.stackexchange.com/a/50575/12102 $\endgroup$
    – uhoh
    Mar 6 at 15:12

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