[time-nuts] laser as clock source

[jimlux]

On 5/5/16 12:22 PM, Hal Murray wrote:
>
> jimlux at 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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