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I have read (here) and heard (on astronomy podcast) that radio waves decay or are indistinguishable the more distance they travel.

Do any of man made satellites communicate with Earth solely by laser?

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    $\begingroup$ Here you find some answer: en.wikipedia.org/wiki/Laser_Communications_Relay_Demonstration $\endgroup$ – Gerald Jan 31 '16 at 12:20
  • $\begingroup$ The main disadvantage no one seems to have mentions of a laser is that they are friggin inefficient. This causes lots of problems, from energy budgets to heat dissipation to simply weak signals. $\endgroup$ – Aron Feb 2 '16 at 6:31
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    $\begingroup$ A "high efficiency" diode laser η~0.6 whereas an efficient microwave rectenna can have η>0.9. That might not seem a lot, but we are talking about over 5 times the heat dissipation requirements. $\endgroup$ – Aron Feb 2 '16 at 6:37
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Lasers have two advantages over radio waves for communication

  1. They operate at a higher frequency, allowing for higher data rate transfer.
  2. They are more focused, not dispersing the energy as much as radio waves, making their effective distance higher.

There are also a few disadvantages:

  1. The beam width is so narrow that exact pointing is required, both for the satellite and the receiver. It is said that from Mars, you not only have to point at the Earth, but point at an area a few US states wide to maintain contact.
  2. Clouds will obscure the laser communication, while they don't affect radio waves the same.

No spacecraft has used exclusively Laser Communications, but there have been a number of tests. There is at least one satellite that uses lasers for a relay communication, using lasers on a LEO satellite to communicate with a satellite in GEO orbit, although the communication with the ground still occurs over RF. See European Laser Communication Satellite.

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  • $\begingroup$ As of January 2016 at least one operational, rather than experimental system exists on Eutelsat 9B: spaceflight101.com/… though it has conventional r.f. transponders as its main payload. $\endgroup$ – Puffin Feb 1 '16 at 15:00
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The main reason for reduced received signal with radio communication in space is the spreading loss, that is intensity declines with the square of the distance from source to receiver. This is true even with beamed radio signals.

Laser communications beams suffer from exactly the same loss once the cross-sectional area of the beam exceeds size of the receiving sensor. Which for practical purposes will always be the case for space communications.

The comms. system you choose is more related to bandwidth, absorption losses, immunity to interference, the practicality of the technology, ... but not spreading losses.

For SETI laser communications signals will be more difficult to detect unless they are deliberated attempts to contact others by an alien civilisation own SETI program, as lasers usually have much narrower beam widths than radio transmitters.

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  • $\begingroup$ Is it the case that beam intensity per unit area decreases at the same rate with distance for lasers (past a certain point) as for radio? $\endgroup$ – Nathan Tuggy Feb 1 '16 at 17:56
  • $\begingroup$ Only problem I have with this answer is that it assumes a Q=1 laser with perfect focusing/columnated beam. $\endgroup$ – Aron Feb 2 '16 at 6:23
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It's not the case that radio waves decay. Radio waves are a very good choice for most long distance communication, owing to their penetration of the atmosphere and the relative ease with which even distant, low-power sources can be received. In these ways, radio waves are superior to visible laser light. Visible light is blocked by clouds, and low intensity sources are swamped by the brightness of reflected sunlight.

However for high-bandwidth communication lasers do have a place. No spacecraft has been entirely dependent on lasers for communication though.

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  • $\begingroup$ Why would SETI guys search for laser (optical) signals? While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. (from wikipedia) I am not answering my own question; just that if laser signal is blocked by clouds etc. Why do SETI guys look for laser signals if its so bad as you mention. Isn't it just pointless? $\endgroup$ – Kyslik Jan 31 '16 at 13:36

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