On 5/5/16 12:22 PM, Hal Murray wrote:
jim...@earthlink.net said:
Well, in deep space optical comm, we send many photons with a laser, and we
use pulse position modulation at the receiver detecting single photons (or
"few photons"), by which we can send "many bits/photon" (e.g. if you have
256 possible time slots in which the photon can arrive, you have 8 bits/
photon)
Neat. Could you please say a bit more.
I'll look for a handy summary presentation
Google for "deep space optical communications" and you'll find quite a lot.
What sort of distance?
Many AU - Mars to start (0.5 to 2.5 AU), but really, Europa and
Enceledaus would be more interesting.
Bandwidth? Error rate?
Big, low. - you're basically competing against Ka-band RF, where Mbits
at 1E-6 rates is easy.
How big is the laser and telescope?
I don't recall exact numbers off hand: on the spacecraft side, "smaller
than a breadbox" is a good description for the prototypes I've seen.
What sort of optics on the receiver?
How hard is it to point the receiver in the right direction? How hard is it
to point the transmitter telescope? ...
Well, for space to earth, you point at the earth, allowing for the time
delay - so you have to point to where earth will be. That's pretty easy;
For earth to space - you know where the spacecraft is, so you point to it.
Beyond that, the receivers on both ends have array sensors so you can
tell if you're pointed correctly.
How does the receiver get timing?
I don't know. I recall seeing that time slots were 100 picoseconds or
something like that.
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